Logging is an integral element of the development process for all applications, including web APIs. It assists developers in monitoring and analyzing the flow of their applications, identifying issues, and resolving issues. NLog is a prominent logging framework in the ASP.NET Core ecosystem, providing a robust and flexible logging solution. This article examines how to implement monitoring in an ASP.NET Core WebAPI with NLog.

What exactly is NLog?
NLog is an adaptable and extensible platform for logging in.NET applications. It supports multiple logging destinations, including files, databases, and email. NLog is extremely configurable, enabling developers to configure logging behavior based on their particular needs.

Configuring NLog for ASP.NET Core WebAPI
Follow the steps below to get started with NLog in an ASP.NET Core WebAPI project:

Step 1. Add NLog dependencies
Add the required NLog dependencies to your project to get started. The NuGet Package Manager in Visual Studio can be used to search for and install the following packages:
NLog NLog.Web.AspNetCore

These packages provide the essential monitoring functionality and ASP.NET Core integration.

Step 2. Configure NLog
Create a nlog.config file in the project's root directory. This file defines NLog's configuration, including log targets and rules. Here is an example of a simple nlog.config file:

<?xml version="1.0" encoding="utf-8"?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    autoReload="true" internalLogLevel="Trace" internalLogFile="${basedir}\logs\internallog.log" throwConfigExceptions="true">
    <!-- enable asp.net core layout renderers -->
    <extensions>
        <add assembly="NLog.Extended"/>
    </extensions>

    <targets>
        <!-- File Target for all log messages with basic details -->
        <target xsi:type="File" name="logfile" fileName="${basedir}/logs/nlog-${date:format=yyyy-MM-dd}.log"
            layout="${longdate}|${event-properties:item=EventId_Id:whenEmpty=0}|${level:uppercase=true}|${logger}|${message} ${exception:format=tostring}" />
        <!-- Other targets like console, database, etc. can be added here -->
    </targets>
    <rules>
        <logger name="*" minlevel="Trace" writeTo="logfile" />
        <!-- Other rules can be added here -->
    </rules>
</nlog>

In this example, we define a file target called "logfile" that writes log messages to a file named nlog-YYYY-MM-DD.log within your application's logs subdirectory. You can add more targets such as console, database, or email as per your requirements. The minlevel attribute specifies the minimum log level to be captured, whereas the writeTo attribute specifies the destination(s) to which log messages should be written.

Step 3. Configure NLog within the ASP.NET Core application.
To configure NLog within your ASP.NET Core WebAPI application, open the Program.cs file and make the following modifications:
using NLog.Web;

val logger = NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger(); try
var builder = WebApplication.CreateBuilder(args); // Add services to the container. builder.Services.AddControllers(); // To learn more about configuring Swagger/OpenAPI, visit https://aka.ms/aspnetcore/swashbuckle. builder.Services.AddEndpointsApiExplorer(); builder.Services.AddSwaggerGen(); //Configure logging. build
     loggingBuilder.ClearProviders(); loggingBuilder.AddNLog(); ); var app = builder.Build(); // Configure the request pipeline for HTTP requests.
    if (app.Environment.IsDevelopment())
     app.UseSwagger(); app.UseSwaggerUI(); app.UseHttpsRedirection(); app.UseAuthorization(); app.MapControllers(); app.Run(); app.UseSwaggerUI();
rescue (Exception error)
// NLog: capture configuration error
    logger.Error(exception, "Stopped program because of exception"); throw;
finally
// Ensure internal timers/threads are flushed and terminated prior to application termination (Avoid segmentation fault on Linux).
    NLog.LogManager.Shutdown(); }

4. Inject ILogger into controllers or services using C#
Create an instance of the Logger class within your controllers or services to use NLog for logging. For instance:
using NLog;

[ApiController] [Route("api/[controller]")]
public class SampleController inherits from ControllerBase; private static read-only Logger = LogManager.GetCurrentClassLogger();

    public IActionResult Get() _logger; [HttpGet].Example: Info("SampleController: Get method called");

        // ...

        return Ok(); } }

NLog entries are written to the specified file by default. Depending on the requirements of your application, you can configure additional targets, such as databases, email, and custom targets.

To view the logs, you can either manually open the log file or use tools such as NLog Viewer, which provides a graphical interface for monitoring and analyzing the logs.

This article demonstrated how to use NLog to implement logging in an ASP.NET Core WebAPI. We installed NLog, configured it to write logs to a file, and implemented logging in WebAPI controllers. NLog simplifies the process of logging and log administration in ASP.NET Core projects, which is an essential aspect of application development.

Description of ElasticSearch
ElasticSearch is a scalable, open-source search and analytics engine developed on top of Apache Lucene. It is designed to manage vast amounts of data and provide distributed search capabilities that are both rapid and distributed. ElasticSearch is frequently used for a variety of purposes, such as full-text search, log analytics, real-time analytics, and data visualization.

Principal Elements of ElasticSearch

• ElasticSearch is designed to distribute data across multiple nodes, allowing for horizontal scalability and high availability. It employs sharding and replication techniques to distribute and replicate data throughout the cluster.
• Full-Text Search: ElasticSearch provides robust full-text search capabilities, enabling complex queries across structured and unstructured data. It allows for filtering, faceting, highlighting, and relevance scoring.
• ElasticSearch is designed to manage the ingestion and analysis of real-time data. It enables near-real-time indexing and searching of data, making it suitable for applications that require current information and prompt responses.
• ElasticSearch is schema-less, meaning documents can be indexed and searched without a predefined schema. It dynamically maps and indexes data based on its structure, providing flexibility and simplifying the process of indexing data.
• ElasticSearch provides a comprehensive RESTful API that enables interaction with the search engine via HTTP requests. This facilitates ElasticSearch's integration with numerous applications and programming languages.
• ElasticSearch provides robust aggregation capabilities for executing data analytics and generating insightful insights. Aggregations permit the grouping, filtering, and calculation of data sets, facilitating the extraction of valuable information from indexed documents.
• ElasticSearch integrates seamlessly with other Elastic Stack components, such as Logstash for data ingestion, Kibana for data visualization and dashboarding, and Beats for lightweight data shipment. This ecosystem offers a comprehensive data management and analysis solution.

Enterprise search, e-commerce, content management, log analysis, cybersecurity, and other industries utilize ElasticSearch extensively. Its scalability, adaptability, and robust search capabilities make it a popular option for applications requiring the efficient and rapid retrieval of structured and unstructured data.

Here is an example of conducting CRUD operations for managing products using ElasticSearch and the.NET Core API:

Install NEST (Elasticsearch.Net & NEST) NuGet packages.

Specify Product Model:

using Nest;

public class Product
{
    [Keyword]
    public string Id { get; set; }

    [Text]
    public string Name { get; set; }

    [Number]
    public decimal Price { get; set; }

    // Add additional properties as needed
}

Configure ElasticSearch Connection:

using Nest;

public class ElasticSearchConfig
{
    private readonly string _elasticSearchUrl = "http://localhost:9200";
    private readonly string _indexName = "products"; // Name of your index

    public ElasticClient GetClient()
    {
        var settings = new ConnectionSettings(new Uri(_elasticSearchUrl))
            .DefaultIndex(_indexName);

        return new ElasticClient(settings);
    }
}

Create the Product Service:
using Nest;

public class ProductService
{
    private readonly ElasticClient _elasticClient;

    public ProductService(ElasticClient elasticClient)
    {
        _elasticClient = elasticClient;
    }

    public async Task<bool> AddProduct(Product product)
    {
        var indexResponse = await _elasticClient.IndexDocumentAsync(product);
        return indexResponse.IsValid;
    }

    public async Task<Product> GetProduct(string id)
    {
        var searchResponse = await _elasticClient.GetAsync<Product>(id);
        return searchResponse.Source;
    }

    public async Task<bool> UpdateProduct(Product product)
    {
        var updateResponse = await _elasticClient.UpdateAsync<Product>(product.Id, u => u.Doc(product));
        return updateResponse.IsValid;
    }

    public async Task<bool> DeleteProduct(string id)
    {
        var deleteResponse = await _elasticClient.DeleteAsync<Product>(id);
        return deleteResponse.IsValid;
    }
}

Create the Product Controller:
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/products")]
public class ProductController : ControllerBase
{
    private readonly ProductService _productService;

    public ProductController(ProductService productService)
    {
        _productService = productService;
    }

    [HttpPost]
    public async Task<IActionResult> Create(Product product)
    {
        var success = await _productService.AddProduct(product);
        if (success)
        {
            return Ok();
        }
        return BadRequest("Failed to create product.");
    }

    [HttpGet("{id}")]
    public async Task<IActionResult> Get(string id)
    {
        var product = await _productService.GetProduct(id);
        if (product != null)
        {
            return Ok(product);
        }
        return NotFound();
    }

    [HttpPut]
    public async Task<IActionResult> Update(Product product)
    {
        var success = await _productService.UpdateProduct(product);
        if (success)
        {
            return Ok();
        }
        return BadRequest("Failed to update product.");
    }

    [HttpDelete("{id}")]
    public async Task<IActionResult> Delete(string id)
    {
        var success = await _productService.DeleteProduct(id);
        if (success)
        {
            return Ok();
        }
        return BadRequest("Failed to delete product.");
    }
}

Register Dependencies in Startup.cs:
using Nest;

public class Startup
{
    // ...

    public void ConfigureServices(IServiceCollection services)
    {
        // ...

        // Add ElasticSearch configuration and services
        services.AddSingleton<ElasticSearchConfig>();
        services.AddScoped<ElasticClient>(serviceProvider =>
        {
            var config = serviceProvider.GetRequiredService<ElasticSearchConfig>();
            return config.GetClient();
        });

        // Add ProductService
        services.AddScoped<ProductService>();

        // ...
    }

    // ...
}

With these code examples, you can create a .NET Core API for CRUD operations on products using ElasticSearch as the data store.

I have explained how to encrypt and decrypt text using the AES Encryption standard in this article.
Open Visual Studio, Create a new console application.

Provide a project name and choose the location to store the project information, and click next.

Choose the .Net framework based on your project requirement, then click Create.

Once the project has been created, then Right Click on the project name, choose to add, and click on the new item.
Add the class file to the existing project.

From the C# node, then choose the class definition and provide the class file name like Encrypt and decrypt file.cs.
After we added the class file to our project solution.

The class file was created successfully after we wrote the code in the class file.

Use the below code to encrypt the given text input.
public static string EncryptString(string plainText)
  {
      byte[] array;
      using (Aes aes = Aes.Create())
      {
          aes.Padding = PaddingMode.PKCS7;
          aes.KeySize = 256;
          aes.Key = new byte[32];
          aes.IV = new byte[16];
          aes.Padding = PaddingMode.PKCS7;
          ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);
          using (MemoryStream memoryStream = new MemoryStream())
          {
              using (CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, encryptor, CryptoStreamMode.Write))
              {
                  using (StreamWriter streamWriter = new StreamWriter((Stream)cryptoStream))
                  {
                      streamWriter.Write(plainText);
                  }
                  array = memoryStream.ToArray();
              }
          }
      }
      return Convert.ToBase64String(array);
  }

Use the below code to decrypt the given text input.
public static string DecryptString(string cipherText)
   {
       byte[] buffer = Convert.FromBase64String(cipherText);
       using (Aes aes = Aes.Create())
       {
           aes.Padding = PaddingMode.PKCS7;
           aes.KeySize = 256;
           aes.Key = new byte[32];
           aes.IV = new byte[16];
           ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
           using (MemoryStream memoryStream = new MemoryStream(buffer))
           {
               using (CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, decryptor, CryptoStreamMode.Read))
               {
                   using (StreamReader streamReader = new StreamReader((Stream)cryptoStream))
                   {
                       return streamReader.ReadToEnd();
                   }
               }
           }
       }
   }

Then open the "Program.cs" file to consume the encryption and decryption method.
using System;
using Encrypt__Decrypt__AES_Operation__file;

namespace EncryptionDecryptionUsingSymmetricKey
{
class Program
{

        public static void Main(string[] args)
        {

            while (true)
            {
                ProcessEncryptDecrypt();
            }
        }


        public static void ProcessEncryptDecrypt()
        {
            int iChoice = 0;
            string strPwd = string.Empty;
            var encryptedString = string.Empty;
            Console.WriteLine("Enter your choice:");
            Console.WriteLine("1.Decryption   2.Encryption  3.Exit ");
            Console.WriteLine("...............");
            iChoice = Convert.ToInt32(Console.ReadLine());

            if (iChoice == 1)
            {
                Console.WriteLine("Enter the Password:");
                strPwd = Convert.ToString(Console.ReadLine());
                encryptedString = AesOperation.EncryptString(strPwd);
                Console.WriteLine($"encrypted string : {encryptedString}");


            }
            else if (iChoice == 2)
            {
                Console.WriteLine("Enter the Password:");
                strPwd = Convert.ToString(Console.ReadLine());
                var decryptedString = AesOperation.DecryptString(strPwd);
                Console.WriteLine($"decrypted string : {decryptedString}");
            }
            else
            {
                Environment.Exit(0);
            }
        }

}
}

After the successful implementation of the above code, run the application; the output seems like the below screenshot.

In the previous article, I introduced the fundamentals of FluentValidation in .NET Core using a Console Application. In addition, I will introduce some ASP.NET Core-based applications in this article.

Suppose we have a class named Student.
public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<string> Hobbies { get; set; }
}

Now, we want to create an API to query students' hobbies.

So, we create a QueryStudentHobbiesDto class to define the request parameters.
public class QueryStudentHobbiesDto
{
    public int? Id { get; set; }
    public string Name { get; set; }
}

And let's create the validator first.
public class QueryStudentHobbiesDtoValidator: AbstractValidator<QueryStudentHobbiesDto>
{
    public QueryStudentHobbiesDtoValidator()
    {
        RuleSet("all", () =>
        {
            RuleFor(x => x.Id).Must(CheckId).WithMessage("id must greater than 0");
            RuleFor(x => x.Name).NotNull().When(x=>!x.Id.HasValue).WithMessage("name could not be null");
        });

        RuleSet("id", () =>
        {
            RuleFor(x => x.Id).NotNull().WithMessage("id could not be null")
                     .GreaterThan(0).WithMessage("id must greater than 0");
        });

        RuleSet("name", () =>
        {
            RuleFor(x => x.Name).NotNull().WithMessage("name could not be null");
        });
    }

    private bool CheckId(int? id)
    {
        return !id.HasValue || id.Value > 0;
    }
}

Let's begin with a familiar way.

Manual validation
You can regard this usage as a copy of the Console App sample that I showed you in the last article.
// GET api/values/hobbies1
[HttpGet("hobbies1")]
public ActionResult GetHobbies1([FromQuery]QueryStudentHobbiesDto dto)
{
    var validator = new QueryStudentHobbiesDtoValidator();
    var results = validator.Validate(dto, ruleSet: "all");

    return !results.IsValid
               ? Ok(new { code = -1, data = new List<string>(), msg = results.Errors.FirstOrDefault().ErrorMessage })
               : Ok(new { code = 0, data = new List<string> { "v1", "v2" }, msg = "" });

What we need to do are three steps.
    Create a new instance of the validator.
    Call the Validate method
    Return something based on the result of the Validate method.

After running up this project, we may get the following result.

Most of the time, we create a new instance directly. It is not a very good choice. We may use Dependency Injection in the next section.

Dependency Injection(DI)
There are two ways to use DI here. One is using the IValidator directly, the other one is using a middle layer to handle this, such as the BLL layer.
Use IValidator directly

What we need to do is inject IValidator<QueryStudentHobbiesDto> and call the Validate method to handle.
private readonly IValidator<QueryStudentHobbiesDto> _validator;
public ValuesController(IValidator<QueryStudentHobbiesDto> validator)
{
    this._validator = validator;
}

// GET api/values/hobbies5
[HttpGet("hobbies5")]
public ActionResult GetHobbies5([FromQuery]QueryStudentHobbiesDto dto)
{
    var res = _validator.Validate(dto, ruleSet: "all");

    return !res.IsValid
               ? Ok(new { code = -1, data = new List<string>(), msg = res.Errors.FirstOrDefault().ErrorMessage })
               : Ok(new { code = 0, data = new List<string> { "v1", "v2" }, msg = "" });
}

And don't forget to add the following code in the Startup class.
public void ConfigureServices(IServiceCollection services)
{
    //inject validator
    services.AddSingleton<IValidator<QueryStudentHobbiesDto>, QueryStudentHobbiesDtoValidator>();
}

When we run it up, we will get the same result in a manual way.

Use a middle layer
We also can create a service class to handle the business logic.
public interface IStudentService
{
    (bool flag, string msg) QueryHobbies(QueryStudentHobbiesDto dto);
}

public class StudentService : IStudentService
{
    private readonly AbstractValidator<QueryStudentHobbiesDto> _validator;
    //private readonly IValidator<QueryStudentHobbiesDto> _validator;

    public StudentService(AbstractValidator<QueryStudentHobbiesDto> validator)
    //public StudentService(IValidator<QueryStudentHobbiesDto> validator)
    {
        this._validator = validator;
    }

    public (bool flag, string msg) QueryHobbies(QueryStudentHobbiesDto dto)
    {
        var res = _validator.Validate(dto, ruleSet: "all");

        if(!res.IsValid)
        {
            return (false, res.Errors.FirstOrDefault().ErrorMessage);
        }
        else
        {
            //query ....

            return (true, string.Empty);
        }
    }
}

Go back to the controller.
private readonly IStudentService _service;
public ValuesController(IStudentService service)
{
    this._service = service;
}

// GET api/values/hobbies4
[HttpGet("hobbies4")]
public ActionResult GetHobbies4([FromQuery]QueryStudentHobbiesDto dto)
{
    var (flag, msg) = _service.QueryHobbies(dto);

    return !flag
        ? Ok(new { code = -1, data = new List<string>(), msg })
        : Ok(new { code = 0, data = new List<string> { "v1", "v2" }, msg = "" });
}

This also has an easy way which is similar to Model Binding.

Validator customization
Using the CustomizeValidatorAttribute to configure how the validator will be run.
// GET api/values/hobbies2
[HttpGet("hobbies2")]
public ActionResult GetHobbies2([FromQuery][CustomizeValidator(RuleSet = "all")]QueryStudentHobbiesDto dto)
{
    return Ok(new { code = 0, data = new List<string> { "v1", "v2" }, msg = "" });
}

Let's run it up.

It didn't seem to work!

We should add FluentValidation in the Startup class so that we can enable this feature!
public void ConfigureServices(IServiceCollection services)
{
    //others...

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1)
            //when using CustomizeValidator, should add the following code.
            .AddFluentValidation(fv =>
            {
                fv.RegisterValidatorsFromAssemblyContaining<Startup>();
                //fv.RunDefaultMvcValidationAfterFluentValidationExecutes = false;
                //fv.ImplicitlyValidateChildProperties = true;
            });
}

At this time, when we visit api/values/hobbies2, we can find out that we can not get the query result but the validated message.

However, we don't want to return this message to the users. We should follow the previous sample here!
How can we format the result?

We could use Filter to deal with the validated result here.
public class ValidateFilterAttribute : ResultFilterAttribute
{
    public override void OnResultExecuting(ResultExecutingContext context)
    {
        base.OnResultExecuting(context);

        //model valid not pass
        if(!context.ModelState.IsValid)
        {
            var entry = context.ModelState.Values.FirstOrDefault();

            var message = entry.Errors.FirstOrDefault().ErrorMessage;

            //modify the result
            context.Result = new OkObjectResult(new
            {
                code = -1,
                data = new JObject(),
                msg= message,
            });
        }
    }
}

And mark the attribute at the action method.
// GET api/values/hobbies3
[HttpGet("hobbies3")]
[ValidateFilter]
public ActionResult GetHobbies3([FromQuery][CustomizeValidator(RuleSet = "all")]QueryStudentHobbiesDto dto)
{
    //isn't valid will not visit the okobjectresult, but visit the filter
    return Ok(new { code = 0, data = new List<string> { "v1", "v2" }, msg = "" });
}

And we will get what we want.

This article introduced three ways to use FluentValidation in ASP.NET Core.

I hope this will help you!

Distributed cache plays a significant role in optimizing the efficacy of the application by caching data in memory and reducing database calls. Simply put, caching is the act of storing the result of an operation so that subsequent requests deliver results more quickly.

When do we perform caching?

• When the processing is sluggish.
• Multiple computations will be performed.
• When the output is identical for a given input, we know we do not need to recompute it because it will always be the same.
• When your host charges for database access. By caching the response and decreasing the number of superfluous server requests, you can save money. For instance, the Google APP engine provides a fixed number of reads and writes to the data repository per day; if you exceed this limit, you must pay, even if the website does not receive a great deal of traffic.  

With NCache, there are numerous methods to optimize performance in ASP.NET Core applications.

• Response Storage
• Dispersed Caching
• Object Caching Session Caching
• SignalR

What exactly is NCache?
NCache is an open-source distributed cache for.NET, Java, and Node.js that operates in memory. NCache is incredibly quick and scalable, and it caches application data to minimize database access. NCache is utilized to address the performance issues associated with data storage, databases, and the scalability of.NET, Java, and Node.js applications.

To get started with NCache, kindly follow the installation instructions.

Response Caching
Response caching will cache the server responses for a request so that the subsequent request can access the data from the cache instead of hitting a server. For example, assume we have a country and state cascading  dropdown list  in our web form, so based on the country selected, the request will pass to the server, and we use it to get a respective state list,

In this case, the country and state data will never change frequently, so it is unnecessary to make country and state requests to the server whenever the web form loads; initially, we can save the response in the cache and for the consecutive calls instead of hitting the server we can get a saved response from a cache. We can cache any response from the HTTP request; it can be XML, Text, JSON, or HTML. We will get complete control over cached response using the cache-control property; for example, if cache-control: max-age=45 implies that the server response is valid for 45 seconds.

There are different ways to cache the response.

In-memory
Response cache via HTTP Header
Distributed Caching

1. In-Memory
In this approach, the response content will be cached in-memory on the server side and served for all subsequent calls. We can use built-in <cache> tags to define the section of the views to be cached.
Since the content is cached in-memory, it will be cleared on the application restarts.

2. Response cache via HTTP Header

This approach caches the response content on the browser's end. It can reduce the server hits because the browser cache serves the subsequent calls.

3. Distributed Caching
In this approach, the response cache is distributed across several servers in a web farm that is external to an application. For subsequent calls, any server can be responded to. NCache provides a powerful distributed cache service to implement this approach, improving the application's performance by reducing server hits.
NCache as a response caching middleware

NCache is one of the best response caching middleware for .NET web applications, providing the following benefits as a distributed cache.

• 100% .NET- It is one of the distributed caches in the market, which is entirely built with .NET.
• Fast and Scalable- It is very fast since it uses an in-memory distributed cache. We can scale it linearly, so NCache will be a great choice when we experience performance challenges in our ASP.NET Core application during high traffic.
• High Availability- It supports peer clustering architecture, so there is no data loss when the cache server is down.

Distributed Caching
Distributed Caching is used to overcome the drawbacks of in-memory caching mechanisms regarding reliability and scalability. It will improve the application's performance when cloud services host the app. In the distributed cache, the cache is shared by multiple app servers and maintained as an external service.

It has several advantages,
    It doesn't use local memory
    Reliability -The cached data is available in the different data center
    It's consistency across requests to multiple servers

By default, ASP.NET Core has a distributed caching mechanism that persists the data in memory. The main drawback is the memory limit; the cached data will be cleared during the server restarts. Another approach is using the SQL Server, which has a drawback in performance; it will impact the application's performance when the load is high. So, the best approach is distributed caching with NCache. NCache is one of the distributed caching service providers. Using NCache, we can configure the ASP.NET Core application locally and in Azure as a distributed cache cluster.

The AddNCacheDistributedCache extension method configures NCache as a provider for Distributed Caching for the ASP.NET Core web application.

Download the NuGet package and add the below code to the program.cs file.
builder.Services.AddNCacheDistributedCache(configuration => {
    configuration.CacheName = "democache";
    configuration.EnableLogs = true;
    configuration.ExceptionsEnabled = true;
});

If there is any update in the record from the database, it will cache the new data.

Session Storage in ASP.NET Core using NCache
HTTP is a stateless protocol, which means the information will not persist across multiple requests; we need to use a session storage mechanism for the ASP.NET Core application to persist the information across multiple requests. By default, ASP.NET Core will use in-memory storage to store the session information, this is fine to some extend, but whenever there is high traffic, this session storage may fail due to some limitations.

Memory Limitation
Since the session information is directly stored in-memory, memory space might be a major concern here; if the memory space exceeds more than the limitation, the session storage will fail.

Session Loss
If the application server goes down, the session loss occurs.

ASP.NET Core provides two options to perform session caching,
    Session storage provider and  
    IDistributedCache interface

Both of them are fast in managing the session information; however, both are stand-alone caches with a single point of failure and no option for data replication when the server goes down.

So, the perfect solution to overcome this issue is storing the session information in distributed session storage. NCache is an in-memory distributed Cache provider developed natively in .NET and .NET Core; it's fast and scalable.

Configuring the NCache on the default Sessions Storage provider and IDistributed Cache interface is simple. Know more about session caching in NCache here.
Advantages of using NCache

1. Data replication capability
NCache provides data replication capability so that the session information will be available in the different centers for different regions; if any region went down, it would reroute the traffic from one region to another.

2. It also offers session sharing between separate cache clusters.
Configuring NCache as a Session Storage Provider

1. Install the following NuGet Package Manager
a.    For Open source
Install-Package AspNetCore.Session.NCache.OpenSource

b.    For Professional
Install-Package AspNetCore.Session.NCache.Professional

c.    For Enterprise
Install-Package AspNetCore.Session.NCache

2. Configure the session using the AddNCacheSession extension method, and add the below code to the program.cs file.
builder.Services.AddNCacheSession(configuration => {
    configuration.CacheName = "[your clustered cache name]";
    configuration.EnableLogs = true;
    configuration.SessionAppId = "demoAppSession";
    configuration.SessionOptions.IdleTimeout = 5;
    configuration.SessionOptions.CookieName = "AspNetCore.Session";
});

3. Configure the NCache settings in appsettings.cs file
{
  "NCacheSettings": {
    "SessionAppId": "demoAppSession",
    "SessionOptions": {
      "CookieName": "AspNetCore.Session",
      "CookieDomain": null,
      "CookiePath": "/",
      "CookieHttpOnly": "True",
      "IdleTimeout": "5",
      "CookieSecure": "None",
     "useJsonSerialization": true,
    },

    "CacheName": "demoClusteredCache",
    "EnableLogs": "True",
    "RequestTimeout": "90"
  },
}

Reference: ASP.NET Core Session Provider Configuration | NCache Docs (alachisoft.com)

Object Caching
Object Caching is one of the techniques to cache the object data to improve the ASP.NET Core application performance.
Assume we have an ASP.NET Core Web API service that returns a list of available countries in the world; in this case, whenever there is a request from the client, the service will fetch the country list from the database and return it to the client. In this case, we know the Country object data will not be frequently updated. So, caching this object in a distributed cache will avoid unnecessary database trips and improve the application's performance.

NCache provides two APIs to achieve object caching,
    NCache API
    IDistributedCache API

1. NCache API
NCache API has an extension method GetCache which is used to connect to an instance of the NCache within the application.

2. IDistributedCache API
If your application already uses the default distributed cache, the IDistributedCache API of NCache can be easily configured on top of the default one.

We already discussed IDistributedCache in the above DistributedCache section.

What is ASP.NET Core SignalR?
SignalR is used to fetch the content from the server side to clients instantly and manages the connection seamlessly. It is an open-source Microsoft API. It uses remove procedure call shortened to RPC to call the client from the server. Hubs establish a core connection between the server and the client, and it will have complete control over the connections.  

Use cases for SignalR,
    Real-time monitoring applications
    Chat application
    Scoreboard application
    IoT Device control and so on.  

SignalR Backplane
It is a shared bus or repository. We can configure a Webfarm as a backplane so that instead of sending a message from a web server to the clients, the backplane can be broadcast to all the web servers with all the servers connected to the backplane.

Main bottlenecks with SignalR Backplane,
    Database as SignalR Backplane is slow; it cannot scale out with increasing applied load.
    The database can slow down under high load, which may led to single-point failure sometime.

Integrating a scalable NCache In-memory Distributed Cache to our ASP.NET Core SignalR application can overcome all these bottlenecks and improve performance.

Configuring NCache as a backplane for ASP.NET Core SignalR application,

1. Download and install the NCache Nuget Package
dotnet add package AspNetCore.SignalR.NCache --version 5.3.1

2. Configure NCache settings in appsettings.json
"NCacheConfiguration": {
  "CacheName": "myLocalCache",
  "ApplicationID": "chatApplication"
},

3. Using the AddNCache extension method, we can easily configure our application and add the below code in the program.cs.
builder.Services.AddSignalR().AddNCache(ncacheOptions => {
    ncacheOptions.CacheName = Configuration["NCacheConfiguration:CacheName"];
    ncacheOptions.ApplicationID = Configuration["NCacheConfiguration:ApplicationID"];
});

We have seen a different technique for improving the performance of the ASP.NET Core applications using response cache, distributed cache, SignalR cache, and session and object cache with NCache. We also saw the overview of response, distributed, SignalR, session, and object cache on what, why, and the advantages of using this caching mechanism with NCache, which is super-fast and linearly scalable, and it is built on .NET to overcome the performance bottleneck in our ASP.NET Core web applications.

This article describes how to encrypt and decrypt text using the AES Encryption standard.

Using C# to implement encryption and decryption in an ASP.NET Windows console application
Open Visual Studio, Create a new console application.

Provide a project name, select the storage location, and then click next.

Select the.Net framework based on the needs of your project, then select Create.

Once the project has been created, then Right Click on the project name, choose to add, and click on the new item.
Add the class file to the existing project.

From the C# node, choose the class definition and provide the class file name like Encrypt and decrypt file.cs.
After we added the class file to our project solution.

Once the class file is created successfully, we can write the  Encryption and decryption methods in the class file.

Use the below method to encrypt the given text input.

public static string EncryptString(string plainText)
       {
           byte[] array;
           using (Aes aes = Aes.Create())
           {
               aes.KeySize = 256;
               aes.Key = new byte[32];
               aes.IV = new byte[16];
               aes.Padding = PaddingMode.PKCS7;
               ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);
               using (MemoryStream memoryStream = new MemoryStream())
               {
                   using (CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, encryptor, CryptoStreamMode.Write))
                   {
                       using (StreamWriter streamWriter = new StreamWriter((Stream)cryptoStream))
                       {
                           streamWriter.Write(plainText);
                       }
                       array = memoryStream.ToArray();
                   }
               }
           }
           return Convert.ToBase64String(array);
       }

Use the below Decrypt method to decrypt the encrypted text input.
public static string DecryptString(string cipherText)
       {
           byte[] buffer = Convert.FromBase64String(cipherText);
           using (Aes aes = Aes.Create())
           {
               aes.Padding = PaddingMode.PKCS7;
               aes.KeySize = 256;
               aes.Key = new byte[32];
               aes.IV = new byte[16];
               ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
               using (MemoryStream memoryStream = new MemoryStream(buffer))
               {
                   using (CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, decryptor, CryptoStreamMode.Read))
                   {
                       using (StreamReader streamReader = new StreamReader((Stream)cryptoStream))
                       {
                           return streamReader.ReadToEnd();
                       }
                   }
               }
           }
       }

The full code implementation is like in "Program.cs".
using System;
using Encrypt__Decrypt__AES_Operation__file;

namespace EncryptionDecryptionUsingSymmetricKey
{
    class Program
    {
        public static void Main(string[] args)
        {
            while (true)
            {
                ProcessEncryptDecrypt();
            }
        }
        public static void ProcessEncryptDecrypt()
        {
            int iChoice = 0;
            string strPwd = string.Empty;
            var encryptedString = string.Empty;
            Console.WriteLine("Enter your choice:");
            Console.WriteLine("1.Decryption   2.Encryption  3.Exit ");
            Console.WriteLine("******************************");
            iChoice = Convert.ToInt32(Console.ReadLine());
            if (iChoice == 1)
            {
                Console.WriteLine("Enter the Password:");
                strPwd = Convert.ToString(Console.ReadLine());
                encryptedString = AesOperation.EncryptString(strPwd);
                Console.WriteLine($"encrypted string : {encryptedString}");


            }
            else if (iChoice == 2)
            {
                Console.WriteLine("Enter the Password:");
                strPwd = Convert.ToString(Console.ReadLine());
                var decryptedString = AesOperation.DecryptString(strPwd);
                Console.WriteLine($"decrypted string : {decryptedString}");
            }
            else
            {
                Environment.Exit(0);
            }
        }

    }
}

And run the application. Happy Coding!

In ASP.NET Core, App Trim refers to a feature that automatically removes unused assemblies from the application's memory during runtime. This helps to reduce the memory footprint of the application and improve its overall performance.

What is Trimming?
When an ASP.NET Core application starts up, it loads all the required assemblies into memory. However, not all the assemblies may be needed throughout the lifetime of the application. App Trim identifies and removes the assemblies that are no longer required by the application. This process is called trimming.

The trimming process is done automatically by the .NET runtime, and it works by analyzing the code paths in the application to determine which assemblies are required and which ones are not. The unused assemblies are then removed from memory, freeing up space for other tasks.

App Trim is particularly useful in scenarios where memory usage is critical, such as in cloud-based environments where resources are limited and scaling can be expensive. It can also be used to improve the startup time of the application.

App Trim is available in ASP.NET Core 3.0 and later versions, and it is enabled by default in ASP.NET Core projects created with the .NET Core 3.0 SDK or later. However, it can be disabled or customized using the PublishTrimmed property in the project file or by using command-line options during the publishing process.

Trimming Options
PublishTrimmed, which enables trimming during the publishing process, was first introduced in .NET Core 3.0. However, the other options for trimming during publishing are only available in .NET 5 and newer versions.

To enable App Trim in the project file, open the .csproj file of your ASP.NET Core project and add the following property:
<PropertyGroup>
  <PublishTrimmed>true</PublishTrimmed>
</PropertyGroup>

Note:
    In .NET 6 and newer versions, trimming is fully supported. However, in .NET Core 3.1 and .NET 5, it was considered an experimental feature.
    It's worth noting that trimming is only available for applications that are published as self-contained deployments.

Hello, In.Net 7 today, I'll build the ASP.NET Web API, and I'll use the MSTest Unit Testing Project to test the module.

Microsoft's Visual Studio includes the native unit testing library MsTest.

I'll build a calculator service using the add/subtract/multiply/divide method in this tutorial. I'll make a Rest API that the Client uses to correlate to each Method.

Let's begin WebAPI Unit Testing with the.Net 7 MSTest Project.

1. From VS 2022, choose the ASP.Net Core Web API project.

2. Use .Net7 STS

3. Now WEB API project has been created. I must add CalculatorService and ICalculatorService interface to implement the Repository Design Pattern.

public interface ICalculatorService
{
    public int Add(int a, int b);

    public int Subtract(int a, int b);

    public int Multiply(int a, int b);

    public int Divide(int a, int b);
}
public class CalculatorService:ICalculatorService
{
    public int Add(int a, int b)
    {
        return a + b;
    }
    public int Subtract(int a, int b)
    {
        return a - b;
    }
    public int Multiply(int a, int b)
    {
        return a * b;
    }
    public int Divide(int a, int b)
    {
        return a / b;
    }
}

4. Now Add Service to the Dependency Injection container.
The below three methods define the lifetime of the services,

    AddTransient
    Transient lifetime services are created each time they are requested. This lifetime works best for lightweight, stateless services.
     
    AddScoped
    Scoped lifetime services are created once per request.
     
    AddSingleton
    Singleton lifetime services are created the first time they are requested (or when ConfigureServices is run if you specify an instance there), and then every subsequent request will use the same instance.

But here we use AddSingleton as per requirement.
builder.Services.AddSingleton<ICalculatorService, CalculatorService>();

5. Create the CalculatorController and Inject ICalculatorService via Constructor Injection. And created the Add/Subtract/Multiply/Divide API Method.
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

namespace WebApiCore7.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class CalculatorController : ControllerBase
    {
        private ICalculatorService _calculatorService;
        public CalculatorController(ICalculatorService calculatorService)
        {
            _calculatorService = calculatorService;
        }
        // GET: api/<CalculatorController>


        // GET api/<CalculatorController>/5
        [HttpGet("Add")]
        public int Get(int a, int b)
        {
            return _calculatorService.Add(a, b);
        }

        [HttpGet("Subtract")]
        public int Subtract(int a, int b)
        {
            return _calculatorService.Subtract(a, b);
        }

        [HttpGet("Multiply")]
        public int Multiply(int a, int b)
        {
            return _calculatorService.Multiply(a, b);
        }

        [HttpGet("Divide")]
        public int Divide(int a, int b)
        {
            return _calculatorService.Divide(a, b);
        }
    }
}

6. It's time to build and run the Web API project. We will test using the SWAGGER tool.
Swagger is an open-source framework for designing and describing APIs. Swagger's journey started in 2010 when it was developed by Reverb Technologies (formerly called Wordnik) to solve the need for keeping the API design and documentation in sync. Fast forward 6 years, and Swagger has become the de-facto standard for designing and describing RESTful APIs used by millions of developers and organizations for developing their APIs, be it internal or client facing.

7. Testing ADD API using Swagger

Till Now, we have developed the Web API, which implements the Calculator functionality.

It's time to add the MSUnit Test project to unit test the Calculator Service.

8. Right-click on Solution Explorer and Add Project and select MSTest Test Project.

9. Set the Project Name

10. Set the Framework as .Net 7 STS

11. Right-click on the UnitTest Project and add the WebAPIcore7 Project dependency As we have to Test the Calculator Service.

12. Add CalculatorUT Class and Decorate the class with TestClass and Method with TestMethod.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using WebApiCore7;
namespace UnitTestProject
{
    [TestClass]
    public class CalculatorUT
    {
        private ICalculatorService _calculatorService;
        public CalculatorUT()
        {
            _calculatorService = new CalculatorService();
        }

        [TestMethod]
        public void AddWithPass()
        {
            int actual = 5;
            int expected = _calculatorService.Add(3, 2);
            Assert.AreEqual(actual, expected);
        }
        [TestMethod]
        public void AddWithFail()
        {
            int actual = 5;
            int expected = _calculatorService.Add(3, 3);
            Assert.AreEqual(actual, expected);
        }

        [TestMethod]
        public void MultiplyWithPass()
        {
            int actual = 80;
            int expected = _calculatorService.Multiply(20, 4);
            Assert.AreEqual(actual, expected);
        }
    }
}

13. It's time to build and test our unit test cases.
Hit the test button on the top menu. Test showing in Green color are passed.

14. Those method that are tested are shown in the API project like this(Green/Red color).

Unit testing helps identify all kinds of issues with the software at a very early stage. Software developers can then work on those issues first before progressing any further. The main advantage of this is when the issues are resolved at an early stage, no other part of the software is impacted.

Tenant application in ASP.NET Core
First of all, we have to know what a tenant is. Simply tenant is a customer. Meaning each customer is called a tenant.

Multi-tenant application in ASP.NET Core
Multi-tenancy is an architecture in which a single software application instance serves multiple customers.

In this case, a single software will manage multiple customer databases. But you have needed a tenant database for multiple tenants. This process is also called SaaS (Software as a Service).

Moreover, you can also manage the software design style by multi-tenant architecture. In the case of a single tenant, every software-service instance has a separate database. Otherwise, in the case of multi-tenant only one software service instance, but there are multiple databases.

Tenant Database
I have to maintain a Tenant database to manage multiple customer Databases. I am using two tables, Like
CREATE DATABASE [TenantDB]

USE [TenantDB]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Tenants](
    [CustomerId] [int] NOT NULL,
    [Customer] [varchar](50) NOT NULL,
    [Host] [varchar](50) NULL,
    [SubDomain] [varchar](50) NOT NULL,
    [Logo] [varchar](50) NULL,
    [ThemeColor] [varchar](50) NULL,
    [ConnectionString] [varchar](max) NOT NULL,
 CONSTRAINT [PK_Customer] PRIMARY KEY CLUSTERED
(
    [CustomerId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO

USE [TenantDB]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[TenantUsers](
    [Id] [int] NOT NULL,
    [CustomerId] [int] NOT NULL,
    [Email] [varchar](50) NOT NULL,
 CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]
GO

USE [TenantDB]
GO
INSERT [dbo].[Tenants] ([CustomerId], [Customer], [Host], [SubDomain], [Logo], [ThemeColor], [ConnectionString]) VALUES (1, N'Red Customer', N'localhost:5057', N'rc', NULL, N'Red', N'Server=Rohol;Database=App-DB1; user id=sa; password=123456; MultipleActiveResultSets=true')
GO
INSERT [dbo].[Tenants] ([CustomerId], [Customer], [Host], [SubDomain], [Logo], [ThemeColor], [ConnectionString]) VALUES (2, N'Green Customer', N'localhost:5057', N'gc', NULL, N'Green', N'Server=Rohol;Database=App-DB2; user id=sa; password=123456; MultipleActiveResultSets=true')
GO
INSERT [dbo].[TenantUsers] ([Id], [CustomerId], [Email]) VALUES (1, 1, N'[email protected]')
GO
INSERT [dbo].[TenantUsers] ([Id], [CustomerId], [Email]) VALUES (2, 2, N'[email protected]')
GO

Application Database
Now I will add two application databases; these databases will access by a single web portal. Databases are like App-DB1 and App-DB2. Each database has one table, like Users.

CREATE Database [App-DB1]
GO
USE [App-DB1]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Users](
    [UserId] [int] NOT NULL,
    [UserName] [varchar](50) NULL,
    [UserEmail] [varchar](50) NULL,
    [Password] [varchar](50) NULL,
 CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED
(
    [UserId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]
GO

USE [App-DB1]
GO
INSERT [dbo].[Users] ([UserId], [UserName], [UserEmail], [Password]) VALUES (1, N'Red Customer', N'[email protected]', N'123456')
GO

CREATE Database [App-DB2]
GO
USE [App-DB2]
GO
/****** Object:  Table [dbo].[Users]    Script Date: 04/15/23 5:26:26 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Users](
    [UserId] [int] NOT NULL,
    [UserName] [varchar](50) NULL,
    [UserEmail] [varchar](50) NULL,
    [Password] [varchar](50) NULL,
 CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED
(
    [UserId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]
GO

USE [App-DB2]
GO
INSERT [dbo].[Users] ([UserId], [UserName], [UserEmail], [Password]) VALUES (1, N'Green Customer', N'[email protected]', N'123456')
GO

Create a Multi-tenant Application
Open visual studio editor 2022 and choose an ASP.Net Core Web App (MVC).

Install SaasKit.Multitenancy
Go to the NuGet Package Manager and install the SaasKit.Multitenancypackage

This SaasKit.Multitenancy package will manage a multi-tenancy strategy.

Now add two classes, Tenants and TenantUsers.
public class Tenants
    {
        [Key]
        public int CustomerId { get; set; }
        public string Customer { get; set; }
        public string Host { get; set; }
        public string SubDomain { get; set; }
        public string Logo { get; set; }
        public string ThemeColor { get; set; }
        public string ConnectionString { get; set; }
    }

C#

public class TenantUsers
   {
       [Key]
       public int Id { get; set; }
       public int CustomerId { get; set; }
       public string Email { get; set; }
   }

Now add another class like TenantResolver. Here I will use TenantContext, which is come from SaasKit.Multitenancy package.
public interface ITenantResolver
    {
        Task<TenantContext<Tenants>> ResolveAsync(HttpContext context);
    }
public class TenantResolver : ITenantResolver<Tenants>
    {
      public async Task<TenantContext<Tenants>> ResolveAsync(HttpContext context)
        {
         throw new NotImplementedException();
        }
    }

Service Registration
Go to the program file and register the Tenant class with TenantResolver class
// Multitenancy
builder.Services.AddMultitenancy<Tenants, TenantResolver>();

Middleware Setup
app.UseMuttitenancy<Tenant>();

This Tenant middleware will call with every HTTP request.

Install Entity Framework Core
I will useEFCore ORM to access SQL Database. So, we need to install the required packages. Like

After installing these three packages, I will add Two database contexts. One context is for Tenant-Database and another for App-Database. Like

TenantDBConnection
{
  "ConnectionStrings": {
    "TenantConnection": "Server=Rohol;Database=TenantDB; user id=sa; password=123456; MultipleActiveResultSets=true"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

// Sql Server TenantDb Connection
builder.Services.AddDbContextPool<TenantDbContext>(options => options.
        UseSqlServer(builder.Configuration.GetConnectionString("TenantConnection")));

Now add Signin and Users class in the Models folder. Like
public class Signin
    {
        [Required(ErrorMessage ="email address is required")]
        [EmailAddress]
        public string Email { get; set; }
        [DataType(DataType.Password)]
        public string? Password { get; set; }
    }

public class Users
    {
        [System.ComponentModel.DataAnnotations.Key]
        public int UserId { get; set; }
        [Required]
        public string UserName { get; set; }
        [Required]
        public string UserEmail { get; set; }
        [Required]
        public string Password { get; set; }

    }

DB context Implementation
Now time to implement Db contexts. Like
public class TenantDbContext : DbContext
    {
        public TenantDbContext(DbContextOptions<TenantDbContext> option) : base(option)
        {
        }

        public DbSet<Tenants> Tenants { get; set; }
        public DbSet<TenantUsers> TenantUsers { get; set; }

    }

public class AppDbContext : DbContext
    {
        private readonly Tenants tenant;

        public AppDbContext(DbContextOptions<AppDbContext> options, Tenants tenant):base(options)
        {
            this.tenant = tenant;
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(tenant.ConnectionString);
        }

        public DbSet<Users> Users { get; set; }

    }

Add Services
I have used two services for this application. One for tenant operation and another for application operation.

namespace MultiTenantApp.Services
{
    public interface ITenantService
    {
       Tenants GetTenantBySubDomain(string subDomain);
       Tenants GetTenantByEmail(string email);
    }

    public class TenantService : ITenantService
    {
        private readonly TenantDbContext tdbc;

        public TenantService(TenantDbContext tdbc)
        {
            this.tdbc = tdbc;
        }

        public Tenants GetTenantByEmail(string email)
        {
         throw new NotImplementedException();
        }

        public Tenants GetTenantBySubDomain(string subdomain)
        {
         throw new NotImplementedException();
        }
    }
}

    GetTenantBySubDomain()- This method will return a tenant by sub-domain.
    GetTenantByEmail ()- This method will return a tenant by email.

namespace MultiTenantApp.Services
{
    public interface IAppUserService
    {
        public string GetTenantByEmail(string email);
        public string Signin(Signin model);
    }
    public class AppUserService : IAppUserService
    {
        private readonly AppDbContext adbc;
        private readonly ITenantService tenantService;
        private readonly IHttpContextAccessor httpContextAccessor;

        public AppUserService(AppDbContext adbc, ITenantService tenantService, IHttpContextAccessor httpContextAccessor)
        {
            this.adbc = adbc;
            this.tenantService = tenantService;
            this.httpContextAccessor = httpContextAccessor;
        }

        public string GetTenantByEmail(string email)
        {
            throw new NotImplementedException();
        }

        public string Signin(Signin model)
        {
            throw new NotImplementedException();
        }
    }
}

    GetTenantByEmail()- Here, this method will return a valid URL with a sub-domain.
    Signin()- This method is used to sign in to this portal. It will return a URL as a string.

TenantResolver implementation
public async Task<TenantContext<Tenants>> ResolveAsync(HttpContext context)
        {   // get sub-domain form browser current url. if sub-domain is not exists then will set empty string
            string subDomainFromUrl = context.Request.Host.Value.ToLower().Split(".")[0] ?? string.Empty;
            // checking has any tenant by current sub-domain.
            var result = this.tenantService.GetTenantBySubDomain(subDomainFromUrl);
            Tenants tenant = new();
            // checking has any subdomain is exists in current url
            if (!string.IsNullOrEmpty(result.SubDomain))
            {
                // checking orginal sub-domain and current url sub-domain
                if (!result.SubDomain.Equals(subDomainFromUrl)) return null; // if sub-domain is different then return null
                else
                {
                    tenant.CustomerId = result.CustomerId;
                    tenant.Customer = result.Customer;
                    tenant.Host = result.Host;
                    tenant.SubDomain = result.SubDomain;
                    tenant.Logo = result.Logo;
                    tenant.ThemeColor = result.ThemeColor;
                    tenant.ConnectionString = result.ConnectionString;
                    return await Task.FromResult(new TenantContext<Tenants>(tenant));
                }
            }
            else return await Task.FromResult(new TenantContext<Tenants>(tenant));

        }

This resolver will resolve a multitenant strategy in each HTTP request. If the tenant is valid, then the HTTP request will execute else. The application will show an error. Here I am checking the sub-domain in each request. So, if the sub-domain exists and is valid, the app will work fine; otherwise shows an error. This is a demo and my logic and implementation so anyone can implement his logic.

Controller Implementation
Now add a Signin action in HomeController for view. Like,
public IActionResult Signin(string emailid="")
        {
            ViewBag.Email = emailid;
            return View();
        }

Also, add Signin Post Action for Signin. Like,
[HttpPost]
        public IActionResult Signin(Signin model)
        {
            // checking model state
            if (ModelState.IsValid)
            {
                // checking email at first time
                if (model.Password is null)
                {
                    // retrieve tenant information by user email
                    var result = this.appUserService.GetTenantByEmail(model.Email);
                    // if valid email then redirect for password
                    if (result is not null) return Redirect(result + "?emailid=" + model.Email);
                    else // if email is invalid then clear Email-ViewBag to stay same page and get again email
                    {
                        ViewBag.Email = string.Empty;
                        ViewBag.Error = "Provide valid email";
                    }
                }
                else // this block for password verification, when user provide password to signin
                {
                    var result = this.appUserService.Signin(model);
                    if (result is null) // if password is wrong then again provide valid password
                    {
                        ViewBag.Email = model.Email;
                        ViewBag.Error = "Provide valid password";
                    }
                    else return Redirect(result); // if password is valid then portal will open for user access
                }
            }
            else ViewBag.Email = ""; // if email is invalid then clear Email-ViewBag to stay same page and get again email
            return View();
        }

The detail I have implemented the into the source code. So don't worry; I have provided the source file.

Logout action
public IActionResult Logout()
        {
            return Redirect("http://localhost:5057");
        }

Go to the master _Layout.cshtml page and inject the Tenant class to access the required property. I will use the ThemeColor property to change the theme according to the user's colour. Like,
@inject Tenants tenants;

<body style="background-color:@tenants.ThemeColor">

Same as the index and privacy files. I will use the Customer name from the tenant class. Like
@inject Tenants tenants;
@{
    ViewData["Title"] = "Home Page";
}

<h1>@ViewData["Title"]</h1>

<h4>of @tenants.Customer</h4>

Run the project and sign in by different users. Like
First, for the red user

and second for green user

Razor expressions are used to generate HTML content in views dynamically. Razor is a syntax for combining HTML markup with C# code to produce dynamic web pages. Razor expressions are enclosed in the @ symbol, including C# code, HTML markup, and another Razor-specific syntax. Razor expressions can perform conditional logic, loop over collections, display data, and format content.

Examples of Razor expressions,
1. Display a value
<p>The value of x is @x</p>

This expression uses the @ symbol to output the value of the x variable.

Perform a conditional check,
// Controller
public IActionResult Index() {
    bool isAdmin = true;
    return View(isAdmin);
}
// View
@model bool

@if (Model)
{
    <h1>Welcome Admin</h1>
}
else
{
    <h1>Welcome Guest</h1>
}

If the Model variable is true, the view will display an <h1> element with the "Welcome Admin" text. Otherwise, it will display an <h1> element with the "Welcome Guest" text. The value is True to show Welcome Admin in H1 Font.

2. List Data Bind in Table
// Model Class
public class ProductM {
    public int Id {
        get;
        set;
    }
    public string Name {
        get;
        set;
    }
    public string Description {
        get;
        set;
    }
    public decimal Price {
        get;
        set;
    }
}
// Controller
public ActionResult Product() {
    List < ProductM > products = new List < ProductM > {
        new ProductM {
            Id = 1, Name = "Laptop", Description = "Hp 5262 I5 ", Price = 55000
        },
        new ProductM {
            Id = 2, Name = "Laptop", Description = "Dell 52 I5 ", Price = 58000
        },
        new ProductM {
            Id = 3, Name = "Laptop", Description = "Lenovo 3", Price = 45000
        },
        new ProductM {
            Id = 4, Name = "Mobile", Description = "Nokia ", Price = 5000
        },
        new ProductM {
            Id = 5, Name = "Mobile", Description = "RealMe", Price = 58000
        },
        new ProductM {
            Id = 6, Name = "Mobile", Description = "MI", Price = 45000
        }
    };
    return View(products);
}
// View page


@using CSharpCornerTest.Models
@model List<ProductM>
@{
        ViewBag.Title = "Product";
}

<h2>Product</h2>

<table class="table table-striped">
    <thead>
        <tr>
            <th>ID</th>
            <th>Name</th>
            <th>Description</th>
            <th>Price</th>
        </tr>
    </thead>
    <tbody>
        @foreach (var product in Model)
        {
            <tr>
                <td>@product.Id</td>
                <td>@product.Name</td>
                <td>@product.Description</td>
                <td>@product.Price.ToString("C")</td>
            </tr>
        }
    </tbody>
</table>

3 . Perform a Switch Case
// View Page
@using CSharpCornerTest.Models
@model StatusM
@{
    ViewBag.Title = "StatusCode";
}

<style>
    span.badge.badge-primary {
        font-size: 48px;
        margin: -73px 0 0 139px;
    }
</style>

@switch (Model.Status)
{
    case "Open":
        <h1>Status :  </h1>
        <span class="badge badge-primary">Open</span>
        break;
    case "Closed":
        <span class="badge badge-secondary">Closed</span>
        break;
    default:
        <span class="badge badge-danger">Invalid</span>
        break;
}
// Class
public class StatusM {
    public string Status {
        get;
        set;
    }
}
// Controller
public ActionResult StatusCode() {
    var model = new StatusM {
        Status = "Open"
    };
    return View(model);
}