In this article, we will see the difference between AddScoped vs AddTransient vs AddSingleton in .net core.
Why we require

• It defines the lifetime of object creation or a registration in the .net core with the help of Dependency Injection.
• The DI Container has to decide whether to return a new object of the service or consume an existing instance.
• The lifetime of the Service depends on how we instantiate the dependency.
• We define the lifetime when we register the service.

Three types of lifetime and registration options

• Scoped
• Transient
• Singleton

Scoped

• In this service, with every HTTP request, we get a new instance.
• The same instance is provided for the entire scope of that request. eg., if we have a couple of parameter in the controller, both object contains the same instance across the request
• This is a better option when you want to maintain a state within a request.

services.AddScoped<IAuthService,AuthService>();

Transient
    A new service instance is created for each object in the HTTP request.
    This is a good approach for the multithreading approach because both objects are independent of one another.
    The instance is created every time they will use more memory and resources and can have a negative impact on performance
    Utilize for the lightweight service with little or no state.

services.AddTransient<ICronJobService,CronJobService>();

Singleton
    Only one service instance was created throughout the lifetime.
    Reused the same instance in future, wherever the service is required
    Since it's a single lifetime service creation, memory leaks in these services will build up over time.
    Also, it has memory efficient as they are created once reused everywhere.

services.AddSingleton<ILoggingService, LoggingService>();

When to use which Service
Singleton approach => We can use this for logging service, feature flag(to on and off module while deployment), and email service
Scoped approach => This is a better option when you want to maintain a state within a request.
Transient approach =>  Use this approach for the lightweight service with little or no state.

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Combining Lists comparing AddRange, Arrays and Concat
I had to combine two large lists, and was curious what the performance was of various methods. The 3 methods that came to mind was:
    Using AddRange
    Using an array to copy both lists into, then convert the array back to the final list.
    Using Linq's Concat extension method.

I was looking for the straight union of the two lists. No sorting, merging or filtering.

To compare the methods I had in mind, I decided to combine 2 huge lists and measure the time it took. That's not perfect, since it's a rare use-case, but performance tests are necessarily contrived in one dimension or other. Milliseconds and up are easier to compare than ticks, so I use a dataset that gets me in that range.

When it comes to measuring performance, I am a bit paranoid about compiler and run-time optimizations, and a collection with all zeroes strikes me as a good target to optimize for. So I add some guff to the collections before combining them.

I use the StopWatch class as time-taker. I don't like repeated start/stop calls, as it's easy to inadvertently get some code in between start and stop that was not part of the code to be measured. I isolated the StopWatch calls using an interface like this:
interface ITimedWorker
{
    string Label { get; }
    bool Check();
    void Work();
}

Then I can make a single measure with a method like this:
static void RunTimedOperation(ITimedWorker op)
{
    Stopwatch sw = new Stopwatch();

    Log("Starting {0}", op.Label);
    GC.Collect();
    sw.Start();
    op.Work();
    sw.Stop();
    if (!op.Check())
        Log("{0} failed its check", op.Label);
    Log("{0} finished in:\n\t{1:D2}:{2:D2}:{3:D2}:{4:D4}", op.Label, sw.Elapsed.Hours, sw.Elapsed.Minutes, sw.Elapsed.Seconds, sw.Elapsed.Milliseconds);
}

To get around garbage collection interference, I tried various ways to disable or discourage it during the actual combine operation I wanted to measure, but it did not work for the data sizes I was using. I found that inducing the collect just before the operation was enough to not trigger any collections where I didn't want them.

The test program is written to run either a single test or all three. When running all three tests, to alleviate the issues with garbage collection, the code picks pseudo-randomly which method to run first. It is an attempt to be fair to each of the three methods, since the garbage collector will be more likely to run in subsequent operations compared to the first one completed.

For the combining logic, I created a common base class for the three methods:
abstract class ListJoinerBase
{
    protected List<object> m_l1, m_l2, m_lCombined;
    private long m_goalCount;

    protected ListJoinerBase(List<object> l1, List<object> l2)
    {
        m_l1 = l1; m_l2 = l2;
        m_goalCount = m_l1.Count + m_l2.Count;
    }

    public abstract void Work();
    public bool Check()
    {
        Debug.Assert(m_lCombined.Count == m_goalCount);
        return m_lCombined.Count == m_goalCount;
    }
}

I checked correctness with the debugger. To minimize the chance of new errors getting introduced as code is added, I use a simple check method, which just verifies that the combined list is of the expected size.

Then remaining work was to add labels and implement the Work() method for the three specializations:
class ListJoinerArray : ListJoinerBase, ITimedWorker
{
    [...]
    public override void Work()
    {
        object[] combined = new object[m_l1.Count + m_l2.Count];

        m_l1.CopyTo(combined);
        m_l2.CopyTo(combined, m_l1.Count);
        m_lCombined = new List<object>(combined);
    }
}
class ListJoinerAddRange : ListJoinerBase, ITimedWorker
{
    [...]
    public override void Work()
    {
        m_lCombined = new List<object>();
        m_lCombined.AddRange(m_l1);
        m_lCombined.AddRange(m_l2);
    }
}
class ListJoinerConcat : ListJoinerBase, ITimedWorker
{
    [...]
    public override void Work()
    {
        m_lCombined = m_l1.Concat(m_l2).ToList();
    }
}

Results
The Array and AddRange methods were close, with an edge to Array copying. The Concat method was somewhat behind. The first two methods came in at 18-24 milliseconds for the data size I was using. The Concat method took 144-146 milliseconds. Based on this, I decided to use the array copying method. Before deciding yourself, I encourage you to download the program, play with the methods and add any others you want to compare with, and come to your own conclusions.

Appendix: Sample output from program
[usrdir]\source\repos\ListCombineDemo\results>..\bin\Release\ListCombineDemo.exe /all
Starting Join lists Using Concatenation
Join lists Using Concatenation finished in:
        00:00:00:0147
Starting Join lists Using Arrays
Join lists Using Arrays finished in:
        00:00:00:0018
Starting Join lists Using AddRange
Join lists Using AddRange finished in:
        00:00:00:0024

[usrdir]\source\repos\ListCombineDemo\results>..\bin\Release\ListCombineDemo.exe /all
Starting Join lists Using AddRange
Join lists Using AddRange finished in:
        00:00:00:0024
Starting Join lists Using Concatenation
Join lists Using Concatenation finished in:
        00:00:00:0140
Starting Join lists Using Arrays
Join lists Using Arrays finished in:
        00:00:00:0019

[usrdir]\source\repos\ListCombineDemo\results>..\bin\Release\ListCombineDemo.exe /random
Starting Join lists Using AddRange
Join lists Using AddRange finished in:
        00:00:00:0023

[usrdir]\source\repos\ListCombineDemo\results>..\bin\Release\ListCombineDemo.exe /concat
Starting Join lists Using Concatenation
Join lists Using Concatenation finished in:
        00:00:00:0145

[usrdir]\source\repos\ListCombineDemo\results>..\bin\Release\ListCombineDemo.exe /arrays
Starting Join lists Using Arrays
Join lists Using Arrays finished in:
        00:00:00:0020

Appendix: Some raw results in seconds

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API key authentication will keep a secure line between the API and clients, however, if you wish to have user authentication, go with token-based authentication, aka OAuth2.0. In this article, you will learn how to implement the API Key Authentication to secure the ASP.NET Core Web API by creating a middleware.
API Key Authentication

Step 1
Open Visual Studio Create or open a ASP.NET Core Web API Project, in my case I’m creating a new project with .NET 6.

Creating a new project

Select a template as shown in the below figure 

Step 2
Run the application and you will get swagger UI to access WeatherForecast API.   

public class ApiKeyMiddleware {
    private readonly RequestDelegate _next;
    private
    const string APIKEY = "XApiKey";
    public ApiKeyMiddleware(RequestDelegate next) {
        _next = next;
    }
    public async Task InvokeAsync(HttpContext context) {
        if (!context.Request.Headers.TryGetValue(APIKEY, out
                var extractedApiKey)) {
            context.Response.StatusCode = 401;
            await context.Response.WriteAsync("Api Key was not provided ");
            return;
        }
        var appSettings = context.RequestServices.GetRequiredService < IConfiguration > ();
        var apiKey = appSettings.GetValue < string > (APIKEY);
        if (!apiKey.Equals(extractedApiKey)) {
            context.Response.StatusCode = 401;
            await context.Response.WriteAsync("Unauthorized client");
            return;
        }
        await _next(context);
    }
}

The middleware will check the API key in the header and validate the key by extracting it from the header and compare with the key defined in code.

InvokeAsync method is defined in this middleware so that it will contain the main process, in our case, the main process will be to search and validate the ApiKey header name and value within the httpcontext request headers collection
if (!context.Request.Headers.TryGetValue(APIKEY, out
        var extractedApiKey)) {
    context.Response.StatusCode = 401;
    await context.Response.WriteAsync("Api Key was not provided ");
    return;
}

If there is no header with APIKEY it will return “Api Key was not provided”

Step 4
Open Program.cs file to register the middleware
app.UseMiddleware<ApiKeyMiddleware>();

Step 5
Open appsettings.json file and add an API Key
"XApiKey": "pgH7QzFHJx4w46fI~5Uzi4RvtTwlEXp"

Step 6
Run the application, and test the API using POSTMAN without passing the ApiKey in header, you will get “Api Key was not provided” message in payload, as shown in the below figure.

Passing wrong API Key

Providing correct API Key

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ASP.NET Core is a new open-source and cross-platform framework for building modern cloud-based and internet-connected applications using the C# programming language. Google Cloud Shell is a browser-based command-line tool to access the resources provided by Google Cloud. Cloud Shell makes it really easy to manage your Cloud Platform Console projects and resources without having to install the Cloud SDK and other tools on your system.

In this article, I will demonstrate how to build and launch an ASP.NET Core App from the Google Cloud Shell.

Prerequisites
Basic Linux commands and text editors like vim, nano, etc..,
Google Cloud Account(You can get a free account from this Link)

Get Started !
Kindly login to the google cloud account which you have created.

After the successful login. You will see the welcome page like this.

Cloud Shell is a virtual machine that is loaded with development tools. It offers a constant 5GB memory to store the data and runs on the Google Cloud. Cloud Shell provides command-line access to our Google Cloud resources.

On the Cloud Console, in the top right toolbar, click the Activate Cloud Shell button.

In prompts the tab and press the Continue button.

It takes some time to provision and connects to the environment. When it's connected, those already authenticated, and the project is set to our PROJECT_ID like this

gcloud is the command-line tool for Google Cloud. It's pre-installed on Cloud Shell and supports tab completion.
We can see the active account name with this command: gcloud auth list.

We can see the project ID using this command: gcloud config list project

Creating an ASP.NET Core App in Cloud Shell
Create a global.json file to provide the .NET core version. Type nano global.json
It will automatically create a JSON file and open it to edits it. Paste the following line to define the version
{
    "sdk": {
        "version": "3.1.401"
    }
}

JavaScript
Press Ctrl+X to exit, Y to save the file, then Enter to confirm the filename.
The dotnet the command-line tool is already installed in Cloud Shell.
Verify by checking the version: dotnet --version

Use the following command to disable Telemetry coming from our new app: export DOTNET_CLI_TELEMETRY_OPTOUT=1
Create a structure of an ASP.NET Core web app using the following dotnet command: dotnet new razor -o HelloWorldAspNetCore

Building and Launching an ASP.NET Core App from Google Cloud Shell

The above command creates a project and then restores all its dependencies.

Build the ASP.NET Core App
Find the default project name created using the ls command: ls

The default project name is "HelloWorldAspNetCore". Navigate to our project folder: cd HelloWorldAspNetCore

We can see all our project files inside the folder:

Enter the following command to run the app: dotnet run --urls=http://localhost:8080

To check that the app is running, click on the web preview button on the top right in Cloud Shell and select Preview on port 8080.

It will open the tab with the URL and then loads the site successfully,

In this article, I have shown the practical use of Google Cloud Shell and ASP.NET basics, then created a simple ASP.NET core App using Cloud Shell and launched it on the Google Cloud without once leaving the browser. You can also use the different versions of DOTNET on the platform by tweaking the version in the global.json file.

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As we know .NET 6 is the latest version of .NET and is generally available with lots of improvement in performance, security, code quality, developer experience and provides long-term support. It is recommended for beginners or newbies who want to start their .NET learning journey using the latest framework and latest Visual Studio IDE. Additionally, sooner or later we need to upgrade our existing solutions to the latest framework.

This article describes how to get started with .NET 6 and what is new in .NET 6 with comparison to .NET5. We will create a console application in .NET6 and .NET 5 and compare the differences between them. Additionally, the article will show how to add more classes in .NET 6 console application and call it from the Program.cs.

Let’s move on.
Create Console App in .NET 6

Step 1
Open Visual Studio 2022 and click Create a new project.

Step 2
Select Console App and click Next.

Step 3
Give the project name and location of the project.

Step 4
Select framework: .NET 6.0 (Long-term support).

This creates the console app which looks like below.

Default Program.cs file is listed below.

// See https://aka.ms/new-console-template for more information
Console.WriteLine("Hello, World!");

The project.csproj file is given below.

Now, let’s compile and run the program. Click on Strat without debugging option or press Ctrl+F5 as illustrated below.

You can see the FirstConsoleApp.exe is created in the folder location of the project under the directory: “E:\SampleProject\FirstConsoleApp\bin\Debug\net6.0” as shown below.

We can run this .exe file which displays the “Hello, World!” message.
Create Project in .NET 5

Let’s create a console application in Visual Studio using .NET 5. Follow all steps as per previous, only choose framework .NET 5 in Step 4.

Below is Program.cs of console app using .NET 5

Comparing .NET6 and .NET 5

We can see here, Program.cs of .NET 5 contains:

    Using system
    Namespace
    class keyword
    Main method

If we compare both the Program.cs file from the above two console apps, these are not available in .NET 6. so, this is the difference between .NET5 and .NET6.
Extending .NET 6 Console Application

Add New Class
Right-click on the project—Go to Add-->Class and click add.

We have added a class with the name: Class1.

Now, will create a public void method named Sum: in this Class1.

public void Sum() {
    int a = 5;
    int b = 6;
    int Sum = a + b;
    Console.WriteLine("Sum : {0}", Sum);
}

Complete code of Class1.cs

namespace FirstConsoleApp {

    internal class Class1 {
        public void Sum() {
            int a = 5;
            int b = 6;
            int Sum = a + b;
            Console.WriteLine("Sum : {0}", Sum);
        }
    }
}

You might have noticed that this class contains:

• the namespace
• internal Class
• using statements (by default)

Whereas Program.cs doesn’t contain those and you can write logic directly.

Call Sum() Method

In Program.cs class we will call the sum method of the Class1.cs. Furthermore, to call a method from class, here we need to add namespace: using FirstConsoleApp;

using FirstConsoleApp; //need to call method from Class1
Class1 class1 = new Class1();
class1.Sum();

Now, if we run the app then we will get Sum:11 in the console.

Complete code of Program.cs

// See https://aka.ms/new-console-template for more information
using FirstConsoleApp;
Console.WriteLine("Hello, World!");
Class1 class1 = new Class1();
class1.Sum();

When we run the console application, the output looks like below.

Hence, in this article, we created our first Console application in .NET 6 and also created a console app in .NET 5 and compared the differences between them. Additionally, we added the new class in .NET 6 console application and learned to call the method from class in Program.cs file. I hope it helps you to understand the differences between .NET 5 and .NET 6 and get start your journey in the latest .NET framework.

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Web scraping is a programmed strategy to get enormous amounts of information from websites. Most of this information is unstructured information in an HTML format which is at that point changed over into organized information in a spreadsheet or a database so that it can be used in different applications. There are many distinctive ways to perform web scraping to get information from websites. These incorporate using online administrations, specific API’s or indeed making your code for web scraping from scratch. Many websites allow you to get to their information in an organized format. This is often the most excellent choice, but there are other sites that do not allow users to get massive amounts of information in an organized format or they are not that innovatively progressed. In that circumstance, it is best to use web scraping to scrape the site for information.

Python is the most popular language in the current days used for web scraping. Python has various libraries available for web scraping. At the same time, we can use .NET also for web scraping. Some third-party libraries allow us to scrape data from various sites.  

HtmlAgilityPack is a common library used in .NET for web scraping. They have recently added the .NET Core version also for web scraping. 

We will use our C# Corner site itself for web scraping. C# Corner gives RSS feeds for each author. We can get information like articles / blogs link, published date, title, feed type, author name from these RSS feeds. We will use HtmlAgilityPack library to crawl the data for each article / blog post and get required information. We will add this information to an SQL database. So that we can use this data for future usage like article statistics. We will use Entity Framework and code first approach to connect SQL server database. 

Create ASP.NET Core Web API using Visual Studio 2022
We can use Visual Studio 2022 to create an ASP.NET Core Web API with .NET 6.0.

We have chosen the ASP.NET Core Web API template from Visual Studio and given a valid name to the project.

We can choose the .NET 6.0 framework. We have also chosen the default Open API support. This will create a swagger documentation for our project.  

We must install the libraries below using NuGet package manger.
HtmlAgilityPack
Microsoft.EntityFrameworkCore.SqlServer
Microsoft.EntityFrameworkCore.Tools

We can add database connection string and parallel task counts inside the appsettings.  

appsettings.json
{
"Logging": {
"LogLevel": {
  "Default": "Information",
  "Microsoft.AspNetCore": "Warning"
}
},
"AllowedHosts": "*",
"ConnectionStrings": {
"ConnStr": "Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=AnalyticsDB;Integrated Security=True;ApplicationIntent=ReadWrite;MultiSubnetFailover=False"
},
"ParallelTasksCount": 20
}

Database connection string will be used by entity framework to connect SQL database and parallel task counts will be used by web scraping parallel foreach code.  

We can create a Feed class inside a Models folder. This class will be used to get required information from C# Corner RSS feeds.

Feed.cs
namespace Analyitcs.NET6._0.Models
{
public class Feed
{
    public string Link { get; set; }
    public string Title { get; set; }
    public string FeedType { get; set; }
    public string Author { get; set; }
    public string Content { get; set; }
    public DateTime PubDate { get; set; }

    public Feed()
    {
        Link = "";
        Title = "";
        FeedType = "";
        Author = "";
        Content = "";
        PubDate = DateTime.Today;
    }
}
}

We can create an ArticleMatrix class inside the Models folder. This class will be used to get information for each article / blog once we get after web scraping.  

ArticleMatrix.cs
using System.ComponentModel.DataAnnotations.Schema;

namespace Analyitcs.NET6._0.Models
{
public class ArticleMatrix
{
    public int Id { get; set; }
    public string? AuthorId { get; set; }
    public string? Author { get; set; }
    public string? Link { get; set; }
    public string? Title { get; set; }
    public string? Type { get; set; }
    public string? Category { get; set; }
    public string? Views { get; set; }
    [Column(TypeName = "decimal(18,4)")]
    public decimal ViewsCount { get; set; }
    public int Likes { get; set; }
    public DateTime PubDate { get; set; }
}
}

We can create our DB context class for Entity framework.  

MyDbContext.cs
using Microsoft.EntityFrameworkCore;

namespace Analyitcs.NET6._0.Models
{
public class MyDbContext : DbContext
{
    public MyDbContext(DbContextOptions<MyDbContext> options)
        : base(options)
    {
    }
    public DbSet<ArticleMatrix>? ArticleMatrices { get; set; }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
    }
}
}

We will use this MyDbContext class later for saving data to the database.  

We can create our API controller AnalyticsController and add web scraping code inside it.

AnalyticsController.cs
using Analyitcs.NET6._0.Models;
using HtmlAgilityPack;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using System.Globalization;
using System.Net;
using System.Xml.Linq;

namespace Analyitcs.NET6._0.Controllers
{
[Route("api/[controller]")]
[ApiController]
public class AnalyticsController : ControllerBase
{
    readonly CultureInfo culture = new("en-US");
    private readonly MyDbContext _dbContext;
    private readonly IConfiguration _configuration;
    public AnalyticsController(MyDbContext context, IConfiguration configuration)
    {
        _dbContext = context;
        _configuration = configuration;
    }

    [HttpPost]
    [Route("CreatePosts/{authorId}")]
    public async Task<ActionResult> CreatePosts(string authorId)
    {
        try
        {
            XDocument doc = XDocument.Load("https://www.c-sharpcorner.com/members/" + authorId + "/rss");
            if (doc == null)
            {
                return BadRequest("Invalid Author Id");
            }
            var entries = from item in doc.Root.Descendants().First(i => i.Name.LocalName == "channel").Elements().Where(i => i.Name.LocalName == "item")
                          select new Feed
                          {
                              Content = item.Elements().First(i => i.Name.LocalName == "description").Value,
                              Link = (item.Elements().First(i => i.Name.LocalName == "link").Value).StartsWith("/") ? "https://www.c-sharpcorner.com" + item.Elements().First(i => i.Name.LocalName == "link").Value : item.Elements().First(i => i.Name.LocalName == "link").Value,
                              PubDate = Convert.ToDateTime(item.Elements().First(i => i.Name.LocalName == "pubDate").Value, culture),
                              Title = item.Elements().First(i => i.Name.LocalName == "title").Value,
                              FeedType = (item.Elements().First(i => i.Name.LocalName == "link").Value).ToLowerInvariant().Contains("blog") ? "Blog" : (item.Elements().First(i => i.Name.LocalName == "link").Value).ToLowerInvariant().Contains("news") ? "News" : "Article",
                              Author = item.Elements().First(i => i.Name.LocalName == "author").Value
                          };

            List<Feed> feeds = entries.OrderByDescending(o => o.PubDate).ToList();
            string urlAddress = string.Empty;
            List<ArticleMatrix> articleMatrices = new();
            _ = int.TryParse(_configuration["ParallelTasksCount"], out int parallelTasksCount);

            Parallel.ForEach(feeds, new ParallelOptions { MaxDegreeOfParallelism = parallelTasksCount }, feed =>
            {
                urlAddress = feed.Link;

                var httpClient = new HttpClient
                {
                    BaseAddress = new Uri(urlAddress)
                };
                var result = httpClient.GetAsync("").Result;

                string strData = "";

                if (result.StatusCode == HttpStatusCode.OK)
                {
                    strData = result.Content.ReadAsStringAsync().Result;

                    HtmlDocument htmlDocument = new();
                    htmlDocument.LoadHtml(strData);

                    ArticleMatrix articleMatrix = new()
                    {
                        AuthorId = authorId,
                        Author = feed.Author,
                        Type = feed.FeedType,
                        Link = feed.Link,
                        Title = feed.Title,
                        PubDate = feed.PubDate
                    };

                    string category = "Uncategorized";
                    if (htmlDocument.GetElementbyId("ImgCategory") != null)
                    {
                        category = htmlDocument.GetElementbyId("ImgCategory").GetAttributeValue("title", "");
                    }

                    articleMatrix.Category = category;

                    var view = htmlDocument.DocumentNode.SelectSingleNode("//span[@id='ViewCounts']");
                    if (view != null)
                    {
                        articleMatrix.Views = view.InnerText;

                        if (articleMatrix.Views.Contains('m'))
                        {
                            articleMatrix.ViewsCount = decimal.Parse(articleMatrix.Views[0..^1]) * 1000000;
                        }
                        else if (articleMatrix.Views.Contains('k'))
                        {
                            articleMatrix.ViewsCount = decimal.Parse(articleMatrix.Views[0..^1]) * 1000;
                        }
                        else
                        {
                            _ = decimal.TryParse(articleMatrix.Views, out decimal viewCount);
                            articleMatrix.ViewsCount = viewCount;
                        }
                    }
                    else
                    {
                        articleMatrix.ViewsCount = 0;
                    }
                    var like = htmlDocument.DocumentNode.SelectSingleNode("//span[@id='LabelLikeCount']");
                    if (like != null)
                    {
                        _ = int.TryParse(like.InnerText, out int likes);
                        articleMatrix.Likes = likes;
                    }

                    articleMatrices.Add(articleMatrix);
                }

            });

            _dbContext.ArticleMatrices.RemoveRange(_dbContext.ArticleMatrices.Where(x => x.AuthorId == authorId));

            foreach (ArticleMatrix articleMatrix in articleMatrices)
            {
                await _dbContext.ArticleMatrices.AddAsync(articleMatrix);
            }

            await _dbContext.SaveChangesAsync();
            return Ok(articleMatrices);
        }
        catch
        {
            return BadRequest("Invalid Author Id / Unhandled error. Please try again.");
        }
    }

}
}

We have created a “CreatePosts” method inside the API controller. We are passing C# Corner author id to this method and get all the author post details from RSS feeds.
XDocument doc = XDocument.Load("https://www.c-sharpcorner.com/members/" + authorId + "/rss");
            if (doc == null)
            {
                return BadRequest("Invalid Author Id");
            }
            var entries = from item in doc.Root.Descendants().First(i => i.Name.LocalName == "channel").Elements().Where(i => i.Name.LocalName == "item")
                          select new Feed
                          {
                              Content = item.Elements().First(i => i.Name.LocalName == "description").Value,
                              Link = (item.Elements().First(i => i.Name.LocalName == "link").Value).StartsWith("/") ? "https://www.c-sharpcorner.com" + item.Elements().First(i => i.Name.LocalName == "link").Value : item.Elements().First(i => i.Name.LocalName == "link").Value,
                              PubDate = Convert.ToDateTime(item.Elements().First(i => i.Name.LocalName == "pubDate").Value, culture),
                              Title = item.Elements().First(i => i.Name.LocalName == "title").Value,
                              FeedType = (item.Elements().First(i => i.Name.LocalName == "link").Value).ToLowerInvariant().Contains("blog") ? "Blog" : (item.Elements().First(i => i.Name.LocalName == "link").Value).ToLowerInvariant().Contains("news") ? "News" : "Article",
                              Author = item.Elements().First(i => i.Name.LocalName == "author").Value
                          };

            List<Feed> feeds = entries.OrderByDescending(o => o.PubDate).ToList();

After that we use a parallel foreach statement to loop entire article / blog detail and scrape the data from each post.  

We will get article / blog category from below code.
string category = "Uncategorized";
                    if (htmlDocument.GetElementbyId("ImgCategory") != null)
                    {
                        category = htmlDocument.GetElementbyId("ImgCategory").GetAttributeValue("title", "");
                    }

                    articleMatrix.Category = category;

We will get article / blog views from the code below.
var view = htmlDocument.DocumentNode.SelectSingleNode("//span[@id='ViewCounts']");
                    if (view != null)
                    {
                        articleMatrix.Views = view.InnerText;

                        if (articleMatrix.Views.Contains('m'))
                        {
                            articleMatrix.ViewsCount = decimal.Parse(articleMatrix.Views[0..^1]) * 1000000;
                        }
                        else if (articleMatrix.Views.Contains('k'))
                        {
                            articleMatrix.ViewsCount = decimal.Parse(articleMatrix.Views[0..^1]) * 1000;
                        }
                        else
                        {
                            _ = decimal.TryParse(articleMatrix.Views, out decimal viewCount);
                            articleMatrix.ViewsCount = viewCount;
                        }
                    }
                    else
                    {
                        articleMatrix.ViewsCount = 0;
                    }

We will get article / blog user likes from below code.
var like = htmlDocument.DocumentNode.SelectSingleNode("//span[@id='LabelLikeCount']");
                    if (like != null)
                    {
                        _ = int.TryParse(like.InnerText, out int likes);
                        articleMatrix.Likes = likes;
                    }

After getting all this information from each article / blog using parallel foreach statement, we have saved entire information to database using below code.
_dbContext.ArticleMatrices.RemoveRange(_dbContext.ArticleMatrices.Where(x => x.AuthorId == authorId));

           foreach (ArticleMatrix articleMatrix in articleMatrices)
           {
               await _dbContext.ArticleMatrices.AddAsync(articleMatrix);
           }

           await _dbContext.SaveChangesAsync();

We must change the Program.cs class with the code change below. So that, the Entity framework connection must be established.

We can create the SQL database and table using migration commands.

We can use the command below to create migration scripts in Package Manager Console.  
PM > add-migration InititalScript

Above command will create a new migration script. We can use this script to create our database and table.

We can use the command below to update the database.
PM> update-database

If you look at the SQL server explorer, and you can see that our new database and table is created now.

We can run our application and use swagger to execute the CreatePosts method.

We must use our correct C# corner author id. You can easily get your author id from the profile link
Above is my profile link. sarath-lal7 is my author id.  

We can use the above user id in the swagger and get all the article / blog details.

You can see that authors’ post details received in the swagger. Please note that, currently C# Corner returns maximum 100 posts details only. 

If you look at the database, you can see that 100 records are available in the database as well.

If you enter the same author id again, the earlier data in the database will be removed and new data will be inserted. Hence, for a particular author a maximum of 100 records will be available on the database at any time. We can use this data to analyze the post details using any of the client-side applications like Angular or React.  

I have already created a client application using Angular 8 and hosted it on Azure. You may try this Live App. I will create an Angular 13 version of this application soon.  

In this post, we have seen how to scrape data from websites in .NET 6.0 application using HtmlAgilityPack library. We have used C# Corner site to scrape data from and we have scraped all the post information for a particular author using his author id. C# Corner allows us RSS feeds for each author, and we will get maximum of 100 posts for a user.

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In this article, you will learn how to send a One Time Password[OTP] on registered mobile number using C# and asp.net.

Step 1
First create one web page in your Visual Studio and design it. Design is given below,
<asp:Panel ID="pnl1" runat="server">
  <table>
    <tr>
      <td>Enter Your Mobile Number:</td>
      <td>
        <asp:TextBox ID="txtmobileNo" runat="server"></asp:TextBox>
      </td>
    </tr>
    <tr>
      <td></td>
      <td>
        <asp:Button ID="btnsendOtp" runat="server" Text="Send OTP" OnClick="btnsendOtp_Click" />
      </td>
    </tr>
  </table>
</asp:Panel>
<asp:Panel ID="pnl2" runat="server" Visible="false">
  <table>
    <tr>
      <td>Enter Your OTP:</td>
      <td>
        <asp:TextBox ID="txtverifyMobileNO" runat="server"></asp:TextBox>
      </td>
    </tr>
    <tr>
      <td></td>
      <td>
        <asp:Button ID="btnverify" runat="server" Text="Verify" OnClick="btnverify_Click" />
      </td>
    </tr>
  </table>
</asp:Panel>

Step 2
Add below namespace in .cs page,
using System.Data.SqlClient;
using System.Data;
using System.Net;
using System.Web.ClientServices;
using System.Collections.Specialized;
using System.Configuration;

Step 3
For sending OTP you need API Key. Register your details for API KEY and get 10 SMS.

Step 4
After registering successfully go to setting option and click on API Key for API key generatiton

Step 5
Click on create API Key and no need to enter IP address and notes just save it.

Step 6
Write the below code on btnOtp_click
protected void btnsendOtp_Click(object sender, EventArgs e) {
    pnl1.Visible = false;
    pnl2.Visible = true;
    Random random = new Random();
    int value = random.Next(1001, 9999);
    string destinationaddress = "+91" + txtmobileNo.Text;
    string message = "Your OTP is " + value + "(Send by R.R.Research and development founder is Ramesh Chandra)";
    string message1 = HttpUtility.UrlEncode(message);
    using(var wb = new WebClient()) {
        byte[] response = wb.UploadValues("https://api.textlocal.in/send/", new NameValueCollection() {
            {
                "apikey",
                "here is enter your API Key"
            }, {
                "numbers",
                destinationaddress
            }, {
                "message",
                message1
            }, {
                "sender",
                "TXTLCL"
            }
        });
        string result = System.Text.Encoding.UTF8.GetString(response);
        Session["OTP"] = value;
    }
}

Step 6
Verify your OTP. Write the below code on verify button.

protected void btnverify_Click(object sender, EventArgs e) {
    if (txtverifyMobileNO.Text == Session["OTP"].ToString()) {
        pnl2.Visible = false;
        ScriptManager.RegisterStartupScript(this, typeof(string), "Message", "confirm('Your Mobilenumber has been verify sccessfully.');", true);
    } else {
        ScriptManager.RegisterStartupScript(this, typeof(string), "Message", "confirm('Your OTP is not correct please enter correct OTP');", true);
        pnl2.Visible = true;
    }
}

Step 7
Now run the project.

Step 8
After entering the mobile number and click on send OTP first panel will be false and the second panel will be true.

Step 9
Enter OTP and click on verify button, you will get the below message.
"Your Mobile number has been verified sccessfully."

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Have you ever wanted the weather info for your app without all of the HttpClient stuff?

You are in the right place.

I've got an amazing yet beautiful plugin for you!

Let's get started!

Open up a new project or use the project which you are currently working on.

Today I am going to use a console project.

As always right click on the solution and click on manage nuget packages.

Search for plugin.weather and install the first package.

Add the following namespaces to your file.
using Plugin.Weather;

Let's first get the weather of current time using the city.

For that use the following code.
Weather weather = Api.GetWeatherByCity("City Name");

Now we can access all the weather information using the "weather" object.

The below image shows the information which we can access using the weather object.

Now I am going to write all the information to the console. You can also get the weather image URL and save the image as Png/Jpg/Gif Format.

Code

Weather weather = Api.GetWeatherByCity("City Name");
Console.WriteLine($"Temperature: {weather.Temperature}, " +
$"Humidity: {weather.Humidity}, Looks like Day? {weather.IsDay}," +
$" Latitude: {weather.Latitude}, Longitude: {weather.Longitude}, " +
$"State: {weather.State}, Country: {weather.Country}," +
$" Weather: {weather.Weather_Current}, Cloud Cover {weather.Cloud_Cover}," +
$" Time Zone: {weather.TimeZone}, Wind Speed {weather.Wind_Speed}," +
$" Wind Degree {weather.Wind_Degree}, Wind Direction: {weather.Wind_Direction}, " +
$"Pressure: {weather.Pressure}, Icon Url: {weather.Weather_Icon_url}");
weather.Save_Weather_Icon_As_Jpg("C:\\weather");
Console.WriteLine("Press Enter to continue...");
Console.Read();

Let's take a city. Example: Wellington.

Output

We have got the following information for Wellington, New Zealand.
    Temperature: 17,
    Humidity: 94,
    Looks like Day? no,
    Latitude: -41.300,
    Longitude: 174.783, State: ,
    Country: New Zealand,
    Weather: Partly cloudy,
    Cloud Cover 50,
    Time Zone: Pacific/Auckland,
    Wind Speed 4,
    Wind Degree 320,
    Wind Direction: NW, Pressure: 1018,
    Icon Url: https://assets.weatherstack.com/images/wsymbols01_png_64/wsymbol_0004_black_low_cloud.png

Let's take a look at a city in the northern hemisphere. Example: Seoul

Output

We have got the following information for Seoul, South Korea.
    Temperature: -2,
    Humidity: 63,
    Looks like Day? no,
    Latitude: 37.566,
    Longitude: 127.000,
    State: null,
    Country: South Korea,
    Weather: Clear,
    Cloud Cover 0,
    Time Zone: Asia/Seoul,
    Wind Speed 2,
    Wind Degree 210,
    Wind Direction: SSW,
    Pressure: 1029,
    Icon Url: https://assets.weatherstack.com/images/wsymbols01_png_64/wsymbol_0008_clear_sky_night.png

Now let's take a look at the other functions of this package.

We can get the current season in the north/south hemisphere.

Code
Console.WriteLine(Api.Get_Curent_Season_Northern_Hemisphere_As_String());
Console.WriteLine(Api.Get_Curent_Season_Southern_Hemisphere_As_String());

Output

At the time of writing the article, it's Winter in the north and summer in the south.

We can also get images of various seasons using the following code.

You will have to pass a random value between 1 to 5.

Image image = Api.GetSummerImage(2);
Image winter = Api.GetWinterImage(3);
Image autumn = Api.GetAutumnImage(5);
Image spring = Api.GetSpringImage(4);
image.Save("C:\\test\\Summer.png");
winter.Save("C:\\test\\Winter.png");
autumn.Save("C:\\test\\autumn.png");
spring.Save("C:\\test\\Spring.png");
Console.WriteLine("Files Saved..");
Console.Read();

Let's check the folder.
There are a few files created!

And they show pics of the different seasons of nature.

You can also get an image based on the season in the northern and the southern hemisphere.

Use this code:
Again you will need to specify a random number between 5 to get the image.
Image image_north = Api.Get_Curent_Season_Image_Northern_Hemisphere(4);
Image image_south = Api.Get_Curent_Season_Image_Southern_Hemisphere(2);
image_north.Save("C:\\North.png");
image_south.Save("C:\\South.png");

Output

There are two files created.

At the time of writing the season in the north it is Winter and it's summer in the south.
By using just two lines of code we can get a random image based on the current season and the value given.

It's easy right.

In this article, we have learned how to get the weather info and how to get not only the current season but also how to get the image of a season or get the image using the current season using C#.

This works on all the .net core projects such as ASP.NET, Xamarin.Forms, WinForms, WPF, .NET MAUI.

Please download the nuget package used in this article.

Link:  https://www.nuget.org/packages/Plugin.Weather/

(OR)

Execute the following command in the package console.
PM> Install-Package Plugin.Weather -Version 2.0.0

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What are Guards?
Guard, is a check of integrity preconditions used to avoid errors during execution. The main purpose and use of this is to avoid unnecessary nested branching conditions and give meaningful errors and hence to simplify the code.

In simple language we can say that the code that validates your method's input is called a Guard clause. It makes your code more understandable and it protects you from bugs and unexpected behaviors.

The if block act as a guard clause by protecting the GetStudent method against any null _student arguments. But the main drawback is even though we have null check over here but to reuse this across the code we need to have multiple null checks in order to escape the crash or unexpected behavior.

public Students GetStudents(StudentModel _students) {
    if (_strudents != null) {
        // Write the code.
    } else {
        Console.WriteLine("Student model do not contain any data");
    }
}

How to handle guard clauses exceptions?
"Guard clauses exceptions should never be caught."

This means that most of the time, we should let the caller hit those exceptions as most of the time, guard clauses will guard against scenarios that should never happen like null arguments. We all must have encountered the time when the bug is caused because of the null reference exception. So should we catch a bug and take the chance of never discover it? The answer is No! Instead, we should let the application fail immediately so that we can discover the bug before deploying it to production during the development process.

But what if we have preconditions that don't rely on bugs? What if we have preconditions that could occur sometimes like business logic preconditions? The solution is to expose your guard clauses!

How to use Guard clause?
We can write our own defensive code to use the guards or one library which is simple to understand and solves this problem is Dawn.Guard library.

The below example is one step solution of getting rid of nested if conditions. So let's say if the condition is not fulfilled the code will throw exception so that the developer can get to know the error and can fix it. This library is not limited to null check. There are guards like Equality guards, Boolean guards, Comparison guards, etc. which makes our task easy when it comes to validation conditions.
public Students GetStudentsData(StudentsModel _students) {
    _students = Guard.Argument(_students, nameof(_students)).NotNull().Value;
    //This will let the user know if the _student is null before navigating to next step.
}

Let's take an example and explore more. Suppose we want to check if the Student's name is not null and length should not exceed more than 25 characters.

Code without using Dawn.Guards Library

public Student(string studentName) {
    if (studentName == null) throw new ArgumentNullException(nameof(studentName), "studentName cannot be null.");
    if (studentName.Length > 25) throw new ArgumentException("studentName cannot exceed more than 25 characters.", nameof(studentName));
}

That if expression is a Guard Clause. However, we can do better than that, since repeating this code over and over in all of our methods is a bad practice.

Code using Guards(Dawn.Guards library)

Guard needs to know the argument's value to test it against preconditions and its name to include in a potential exception. There are three ways to initialize a guarded argument,
// First, by specifying the argument value and name separately.
Guard.Argument(arg, nameof(arg));
// Second, omitting the optional argument name.
Guard.Argument(arg);
// Third, creating a MemberExpression via a lambda expression.
Guard.Argument(() => arg);

    The first sample initializes a guarded argument by specifying both the argument's value and name.
    The second sample does not specify the argument name. This is allowed but not recommended since the argument name proves a valuable piece of information when you try to identify the cause of an error from logs or crash dumps.
    The third sample initializes a MemberExpression that provides both the argument's value and name. Although compiling an expression tree is an expensive operation, it is a convenient alternative that can be used in applications that are not performance-critical.

With Guard, if you want to guard an argument against null or max length, you just write NotNull and MaxLength and that's it
public Student(string StudentName) {
    Guard.Argument(StudentName, nameof(StudentName)).NotNull().MaxLength(25);
}

If the argument is passed null, you'll get an ArgumentNullException thrown with the correct parameter name and a clear error message out of the box. Similarly, it will throw an exception if the criteria for max length is not met. By pulling out all of the validation checks from throughout the method we can remove a number of nested conditionals. This definitely improves the readability of the code and makes the code easier to maintain in the future.

The above example illustrates the use of the Dawn.Guard library. We can extend the guard class to handle our own custom exceptions as well.

Exception Messages
Guard creates a meaningful exception message that contains the argument name and a description specific to the validation when a precondition can't be satisfied. Additionally, every validation in Guard accepts an optional parameter letting the user specify a custom error message.

// Throws an ArgumentException if the arg is not null.
Guard.Argument(() => arg).Null(a => "The argument must be null but it is: " + a);
// Throws an ArgumentNullException if the arg is null.
Guard.Argument(() => arg).NotNull("The argument cannot be null.");

In this article, we have seen how guards clause can help us to discover the bugs before deploying in production and to make our code more readable. We have also learned about Dawn. Guard library and how to use it.

This article talks about the Singleton Design Pattern, why it is important and then demonstrates how to build a singleton DBManager (a class that wraps all calls to the database) using dotConnect for PostgreSQL and C#. It also discusses the pros and cons of this approach.
Pre-requisites

You’ll need the following tools to deal with code examples:
    Visual Studio 2019 Community Edition (download)
    dotConnect for PostgreSQL (download)

Introduction to Design Patterns
Design patterns can be used to solve recurring problems or complexities in software development. They are divided into three categories: structural, behavioral, and creational. Creational patterns can be used to create instances of classes and to manage them. However, structural patterns define the types and their relationships and help to understand the relationships between the entities. Behavioral design patterns emphasize on how objects collaborate and how responsibilities are delegated between them.
What is a Singleton Design Pattern?

The Singleton Design Pattern, as the name suggests, restricts the instantiation of a class to one object only. In other words, a class that follows the Singleton Design Pattern will not allow more than one instance of it to be created.

Use Cases
The typical use cases of the Singleton Design Pattern are:
    LogManager
    DbManager
    ServiceRegistry

Create a new ASP.NET Core Web API Project
Earlier, we mentioned the necessary tools to proceed to the practical scenarios. The time has come to use those tools.
First, we need to create a new ASP.NET Core Web API project:
    Open Visual Studio 2019.
    Click Create a new project.
    Select ASP.NET Core Web Application and click Next.
    Specify the project name and location to store that project in your system. Optionally, checkmark the Place solution and project in the same directory checkbox.
    Click Create.
    In the Create a new ASP.NET Core Web Application window, select API as the project template.
    Select ASP.NET Core 5 or later as the version.
    Disable the Configure for HTTPS and Enable Docker Support options (uncheck them).
    Since we won’t use authentication in this example, specify authentication as No Authentication.
    Click Create to finish the process.

We’ll use this project in this article.

Implement a Singleton DBManager in ASP.NET Core and C#
In this section we’ll implement a simple DBManager class that follows the Singleton Design Pattern.

Create a Database Table
Create a new database table using the following script:
CREATE TABLE books(
   book_id serial PRIMARY KEY,
   book_title VARCHAR (255) UNIQUE NOT NULL,
   book_pages INT NOT NULL
);

We’ll use this database table throughout this article to store and retrieve data.

Install NuGet Packages
To get started you should install the dotConnect for PostgreSql package in your project. You can install it either from the NuGet Package Manager tool inside Visual Studio or, from the NuGet Package Manager console using the following command:

    PM> Install-Package Devart.Data.PostgreSql

If the installation is successful, you're all set to get started using dotConnect for PostgreSQL in your application.

Configuring the Application
You should specify the database connection string in a config file, i.e., appsettings.json and then read the connection string in your application. Replace the default generated code of the appsettings.json with the following code:
{
    "PostgreSqlConnectionString": {
      "DefaultConnection": "UserId = postgres; Password =
       mypass;host=localhost;database=Test;"
  },
  "AllowedHosts": "*"
}

You should also add the IConfiguration instance to the services container so that you can access it from anywhere in the application.
public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton(Configuration);
    services.AddControllers();
}

Create the DBManager Class
To keep things simple, we'll build a DBManager with minimal features. Now, create a class called DBmanager inside a fie having the same primary name with a .cs extension and write the following code in there:
public class DBManager
{
   public int ExecuteNonQuery(string commandText)
   {
      return 1;
   }
   public int ExecuteReader(string commandText)
   {
       return 1;
   }
}

Read Data from the Database
To read data from the database using dotConnect for PostgreSQL, we can create a PgSqlDataAdapter instance and then use it to fill a data table and return it. The ExecuteReader method shown in the following code snippet illustrates how this can be achieved:

public DataTable ExecuteReader(string commandText)
{
    DataTable dataTable = new DataTable();
    using (PgSqlConnection pgSqlConnection = new
    PgSqlConnection(ConnectionString))
    {
        using (PgSqlCommand pgSqlCommand = new PgSqlCommand())
        {
            pgSqlCommand.CommandText = commandText;
            pgSqlCommand.Connection = pgSqlConnection;
            if (pgSqlConnection.State != ConnectionState.Open)
                pgSqlConnection.Open();
            PgSqlDataAdapter pgSqlDataAdapter = new
                PgSqlDataAdapter(pgSqlCommand);
            pgSqlDataAdapter.Fill(dataTable);
            if(dataTable.Rows.Count > 0)
                return dataTable;
            return null;
        }
    }
}

The ExecuteReader method of the DBManager class returns an instance of a DataTable. Now, add the following property in the DBManager class to store the database connection string:
public string ConnectionString
{
    get;set;
}

Return a List from the ExecuteReader Method
You can also return a List from the ExecuteReader method. Create a class named Book with the following content in there:
public class Book
{
    public int book_id { get; set; }
    public string book_title { get; set; }
    public int book_pages { get; set; }
}

Replace the source code of the ExecuteReader method of the DBManager class with the following code:
public List<Book> ExecuteReader(string commandText)
{
    DataTable dataTable = new DataTable();
    using (PgSqlConnection pgSqlConnection = new
    PgSqlConnection(ConnectionString))
    {
        using (PgSqlCommand pgSqlCommand = new PgSqlCommand())
        {
            pgSqlCommand.CommandText = commandText;
            pgSqlCommand.Connection = pgSqlConnection;
            if (pgSqlConnection.State != ConnectionState.Open)
                pgSqlConnection.Open();
            PgSqlDataAdapter pgSqlDataAdapter = new
            PgSqlDataAdapter(pgSqlCommand);
            pgSqlDataAdapter.Fill(dataTable);
            return dataTable.ToList<Book>();
        }
    }
}

Note that the return type has been changed from DataTable to List<Book> and how an instance of List<Book> is created using an extension method called ToList<T>().

The following code listing illustrates a class named Extensions that contains the ToList extension method.
public static class Extensions
{
    public static List<T> ToList<T>(this DataTable dataTable) where T : new()
    {
        List<T> data = new List<T>();
        foreach (DataRow row in dataTable.Rows)
        {
            T item = GetItemFromDataRow<T>(row);
            data.Add(item);
        }
        return data;
    }
    public static T GetItemFromDataRow<T>(DataRow dataRow)
    {
        Type temp = typeof(T);
        T obj = Activator.CreateInstance<T>();
        foreach (DataColumn column in dataRow.Table.Columns)
        {
            foreach (PropertyInfo propertyInfo in temp.GetProperties())
            {
                if (propertyInfo.Name == column.ColumnName)
                    propertyInfo.SetValue(obj, dataRow[column.ColumnName], null);
            }
        }
        return obj;
    }
}

Note how an instance of a DataTable is converted to an instance of List<T>. The GetItemFromDataRow method returns an instance of the Book class.

Insert Data to the Database
To insert data to the PostgreSQL database using dotConnect for PostgreSQL, you can use the following method:
public int ExecuteNonQuery(string commandText)
{
    using (PgSqlConnection pgSqlConnection = new
    PgSqlConnection(ConnectionString))
    {
        using (PgSqlCommand pgSqlCommand = new PgSqlCommand())
        {
            pgSqlCommand.CommandText = commandText;
            pgSqlCommand.Connection = pgSqlConnection;
            if (pgSqlConnection.State != ConnectionState.Open)
                pgSqlConnection.Open();
            return pgSqlCommand.ExecuteNonQuery();
        }
    }
}

The Complete Source Code
The complete source code of the DBManager class is given below:
public class DBManager
{
    public string ConnectionString
    {
        get;set;
    }
    public List<Book> ExecuteReader(string commandText)
    {
        DataTable dataTable = new DataTable();
        using (PgSqlConnection pgSqlConnection = new
        PgSqlConnection(ConnectionString))
        {
            using (PgSqlCommand pgSqlCommand = new PgSqlCommand())
            {
                pgSqlCommand.CommandText = commandText;
                pgSqlCommand.Connection = pgSqlConnection;
                if (pgSqlConnection.State != ConnectionState.Open)
                    pgSqlConnection.Open();
                PgSqlDataAdapter pgSqlDataAdapter = new
                   PgSqlDataAdapter(pgSqlCommand);
                pgSqlDataAdapter.Fill(dataTable);
                return dataTable.ToList<Book>();
            }
        }
    }
    public int ExecuteNonQuery(string commandText)
    {
        using (PgSqlConnection pgSqlConnection = new
        PgSqlConnection(ConnectionString))
        {
            using (PgSqlCommand pgSqlCommand = new PgSqlCommand())
            {
                pgSqlCommand.CommandText = commandText;
                pgSqlCommand.Connection = pgSqlConnection;
                if (pgSqlConnection.State != ConnectionState.Open)
                    pgSqlConnection.Open();
                return pgSqlCommand.ExecuteNonQuery();
            }
        }
    }
}

The Singleton DBManager Class
To make the DBManager class a singleton class, you should have a private constructor. A private constructor would prevent the class from being extended or instantiated. Since we don't want more than one instance of this class, a private constructor is needed.

You should then create an instance of the DBManager inside a static property as shown in the code snippet given below:
public class DBManager
{
    private static DBManager instance;
    private DBManager() { }
    public static DBManager Instance
    {
        get
        {
           if (instance == null)
           {
               instance = new DBManager();
           }
           return instance;
        }
   }
   //Other methods removed for brevity
}

Since the DBManager instance is static, you would have only one instance throughout the lifetime of the application. You can access the DBManager class, and its methods as shown in the following code snippet:

DBManager.Instance.ConnectionString = connectionString;
return DBManager.Instance.ExecuteReader("select * from public.books");

Make the DBManager Class ThreadSafe
But what if you would like to make it thread safe so that no two threads can have access to the critical section? To make this class thread safe, you can take advantage of the lock keyword as illustrated in the following code snippet:
public class DBManager
{
   private static object lockObj = new object();
   private static DBManager instance;
   private DBManager() { }
   public static DBManager Instance
   {
       get
       {
           lock(lockObj)
           {
               if (instance == null)
               {
                   instance = new DBManager();
               }
           }
           return instance;
       }
   }
   //Other methods removed for brevity
}

Use the DBManager Class in Controller Classes
You can now use the DBManager instance in your controller class like this:
[Route("api/[controller]")]
[ApiController]
public class BooksController : ControllerBase
{
    readonly IConfiguration _configuration;
    readonly string connectionString;
    public BooksController(IConfiguration configuration)
    {
        _configuration = configuration;
        connectionString =
        _configuration["PostgreSqlConnectionString:DefaultConnection"];
    }
    [HttpGet]
    public List<Book> Get()
    {
        DBManager.Instance.ConnectionString = connectionString;
        return DBManager.Instance.ExecuteReader("select * from public.books");
    }
}

A DBManager class serves as a helper to connect to databases and perform required CRUD operations. Ideally it should be a singleton since you don’t need multiple instances of it in your application. This article has demonstrated how we can build a thread-safe DBManager class in C# and use it in an ASP.NET Core application.