While developing a real-time project, we should offer different authorizations to different users. This means, for example,  users who are "Admin" ,can access a timesheet, but users who are "HR" can rollout pay changes. Here we are trying to define different roles. A group of users belonging to a role will have same access or authorization. But users from different roles, will have different authorizations.

 
So, before hosting our application, we should define the basic roles in place. We don't expect consumers/end-users to define roles in general. We can achieve that using .Net core identity and entity framework.
 
Prerequisite
A .Net core project with Identity, Entity framework and language as C#.
 
In Entity framework , we can define default database table values before creating database model. This concept is called seeding. We are going to use seeding in order to define default users and roles here.
 
I have a class AppDbContext which inherits from IdentityDbContext. We will need to override OnModelCreating() method to meet seeding. Here is the sample code.
 
Startup.cs
    public void ConfigureServices(IServiceCollection services)  
           {  
               #region database configuration              
               string connectionString = config.GetConnectionString("AppDb");  
      
               services.AddDbContextPool<AppDbContext>(options => options.UseSqlServer(connectionString));  
      
               services.AddIdentity<User, IdentityRole>()  
                   .AddEntityFrameworkStores<AppDbContext>();  
               #endregion                         
           }  

AppDbContext.cs
    public class AppDbContext : IdentityDbContext<User>  
        {  
            public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)  
            {  
      
            }  
      
            protected override void OnModelCreating(ModelBuilder builder)  
            {  
                base.OnModelCreating(builder);  
                this.SeedUsers(builder);  
                this.SeedUserRoles(builder);  
            }  
      
            private void SeedUsers(ModelBuilder builder)  
            {  
                User user = new User()  
                {  
                    Id = "b74ddd14-6340-4840-95c2-db12554843e5",  
                    UserName = "Admin",  
                    Email = "[email protected]",  
                    LockoutEnabled = false,  
                    PhoneNumber = "1234567890"  
                };  
      
                PasswordHasher<User> passwordHasher = new PasswordHasher<User>();  
                passwordHasher.HashPassword(user, "Admin*123");  
      
                builder.Entity<User>().HasData(user);  
            }  
      
            private void SeedRoles(ModelBuilder builder)  
            {  
                builder.Entity<IdentityRole>().HasData(  
                    new IdentityRole() { Id = "fab4fac1-c546-41de-aebc-a14da6895711", Name = "Admin", ConcurrencyStamp = "1", NormalizedName = "Admin" },  
                    new IdentityRole() { Id = "c7b013f0-5201-4317-abd8-c211f91b7330", Name = "HR", ConcurrencyStamp = "2", NormalizedName = "Human Resource" }  
                    );  
            }  
      
            private void SeedUserRoles(ModelBuilder builder)  
            {  
                builder.Entity<IdentityUserRole<string>>().HasData(  
                    new IdentityUserRole<string>() { RoleId = "fab4fac1-c546-41de-aebc-a14da6895711", UserId = "b74ddd14-6340-4840-95c2-db12554843e5" }  
                    );  
            }  
        }  

After adding this piece of code, we need to build the application. Now, let's open Package Manager Console . We need to run "add-migration user_role" and "update-database" commands in order to reflect seeding changes in database.

Now lets open SQL query window just to make sure EF has updated User, Role and UserRole table. Here you go.

As you can see, we have default users and roles before we deploy our application.
 
Note

You can see few GUIDs that I have used as ID, you can customize it based on your project design. 

 
We have succeeded in seeding user and role data. Thanks for coming this far in the article. Please feel free to give your valuable comments/suggestions/feedback.

A special method of the class that is automatically invoked when an instance of the class is created is called a constructor. A constructor is a special method that is used to initialize objects. The advantage of a constructor, is that it is called when an object of a class is created.
 

When we create instance of a class a default constructor  is invoked automatically having the same name as Class . Some key points,
    Constructor is used to initialize an object (instance) of a class.
    Constructor is a like a method without any return type.
    Constructor has the same name as class name.
    Constructor follows the access scope (Can be private, protected, public, Internal and external).
    Constructor can be overloaded.

Following is the list of constructors in C#.
    Default constructor
    Parameterized constructor
    Private Constructor
    Static Constructor
    Copy Constructor

Default Constructor
A constructor without any parameters is called a default constructor. If we do not create constructor the class will automatically call default constructor when an object is created.
 
Example of Default Constructor,
    using System;  
    namespace DefaultConstractor {  
        class addition {  
            int a, b;  
            public addition() //default contructor  
            {  
                a = 100;  
                b = 175;  
            }  
            public static void Main() {  
                addition obj = new addition(); //an object is created , constructor is called  
                Console.WriteLine(obj.a);  
                Console.WriteLine(obj.b);  
                Console.Read();  
            }  
        }  
    }  

Parameter Constructor
A constructor with at least one parameter is called a parameterized constructor.
 
Private Constructor
Private constructors are used to restrict the instantiation of object using 'new' operator. A private constructor is a special instance constructor. It is commonly used in classes that contain static members only.
 
Static Constructor
A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced
 
Copy Constructor
The constructor which creates an object by copying variables from another object is called a copy constructor.
 
Is it possible to have a class without Constructor?
If a class contains no instance constructor declarations, a default instance constructor is automatically provided. The only classes in C# which don't have any instance constructors are static classes, and they can't have constructors.
 
Example
    class Person {  
        //This class has no constructor  
        public void getPersonProfile() {  
            Console.WriteLine("Base class:getPersonProfile() have no constructor");  
        }  
    }  
    class Program {  
        static void Main(string[] args) {  
            //Create object of person  
            Person p = new Person();  
            p.getPersonProfile();  
        }  
    }  

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Program.cs is where the application starts. Program.cs file will work the same as Program.cs file in traditional console application of .NET Framework. Program.cs fille is the entry point of application and will be responsible to register Startup.cs fill, IISIntegration and Create a host using an instance of IWebHostBuilder, the Main method.

Global.asax is no longer  in ASP.NET Core application. Startup.cs file is a replacement of Global.asax file in ASP.NET Core.

Startup.cs file is entry point, and it will be called after Program.cs file is executed at application level.  It handles the request pipeline. Startup class triggers the second the application launches.

Description
 
What is Program.cs ?

Program.cs is where the application starts. Program.cs class file is entry point of our application and creates an instance of IWebHost which hosts a web application.
    public class Program {  
        public static void Main(string[] args) {  
            BuildWebHost(args).Run();  
        }  
        public static IWebHost BuildWebHost(string[] args) => WebHost.CreateDefaultBuilder(args).UseStartup < startup > ().Build();  
    }   

WebHost is used to create instance of IWebHost and IWebHostBuilder and IWebHostBuilder which are pre-configured defaults. The CreateDefaultBuilder() method creates a new instance of WebHostBuilder. UseStartup<startup>() method specifies the Startup class to be used by the web host. We can also specify our custom class in place of startup.

Build() method returns an instance of IWebHost and Run() starts web application until it stops.

Program.cs in ASP.NET Core makes it easy for us to setup a web host.
    public static IWebHostBuilder CreateDefaultBuilder(string[] args) {  
        var builder = new WebHostBuilder().UseKestrel().UseContentRoot(Directory.GetCurrentDirectory()).ConfigureAppConfiguration((hostingContext, config) => {  
            /* setup config */ }).ConfigureLogging((hostingContext, logging) => {  
            /* setup logging */ }).UseIISIntegration()  
        return builder;  
    }   

the UseKestrel() specify method is an extension, which specifies Kestrel as an internal web server. The Kestrel is an open-source, cross-platform web server for ASP.NET Core. Application is running with Asp.Net Core Module and it is necessary to enable IIS Integration (UseIISIntegration()), which configures the application base address and port.
 
It also configures UseIISIntegration(), UseContentRoot(), UseEnvironment("Development"), UseStartup<startup>() and other configurations are also available, like Appsetting.json and Environment Variable. UseContentRoot is used to denote current directory path.
 
We can also register logging and set minimium loglevel as shown below. It also overrides loglevel configured in appsetting.json file.
    .ConfigureLogging(logging => { logging.SetMinimumLevel(LogLevel.Warning); })   

In the same way we can also control body size of our request and response by enabling in program.cs file as below.
    .ConfigureKestrel((context, options) => { options.Limits.MaxRequestBodySize = 20000000; });   

ASP.net Core is cross-platform and open-source and also it is compatible to host into any server rather than IIS, external web server such as IIS, Apache, Nginx etc.
 
What is Startup file?
Now the question is, is startup.cs file is mandatory or not? Yes, startup.cs is mandatory, it can be decorated with any access modifier like public, private, internal. Multiple Startup classes are allowed in a single application. ASP.NET Core will select the appropriate class based on its enviroment.

If a class Startup{EnvironmentName} exists, that class will be called for that EnvironmentName or Environment Specific startup file will be executed to avoid frequent changes of code/setting/configuration based on environment.
 
ASP.NET Core supports multiple environment variables like Development, Production and Staging. It reads the environment variable ASPNETCORE_ENVIRONMENT at application startup and store value into Hosting Environment interface.

Should we need to define class name with startup.cs? No, it is not necessary, that class name should be Startup.

We can define two methods in startup file like ConfigureServices and Configure along with constructor.

Startup file example
    public class Startup {  
        // Use this method to add services to the container.  
        public void ConfigureServices(IServiceCollection services) {  
            ...  
        }  
        // Use this method to configure the HTTP request pipeline.  
        public void Configure(IApplicationBuilder app) {  
            ...  
        }  
    }   

ConfigureServices Method
This is an optional method in Startup class which is used to configure services for application. When any request comes to the application, he ConfigureService method will be called first.

ConfigureServices method includes IServiceCollection parameter to register services. This method must be declared with a public access modifier, so that environment will be able to read the content from metadata.
    public void ConfigureServices(IServiceCollection services)  
    {  
       services.AddMvc();  
    }  

Configure Method
The Configure method is used to specify how the application will respond in each HTTP request. This method is mostly used for registering middleware in HTTP pipeline. This method method accept IApplicationBuilder parameter along with some other services like IHostingEnvironment and ILoggerFactory. Once we add some service in ConfigureService method, it will be available to Configure method to be used.
    public void Configure(IApplicationBuilder app)  
    {  
       app.UseMvc();  
    }   

Above example shows how to enable MVC feature in our framework. We need to register UseMvc() into Configure and also need to register service AddMvc() into ConfigureServices. UseMvc is know as middlware. Middleware is new concept introduce with Asp.net Core, thereaew  many inbuilt middleware available, and some are as below.
    app.UseHttpsRedirection();  
    app.UseStaticFiles();  
    app.UseRouting();  
    app.UseAuthorization();  
    UseCookiePolicy();  
    UseSession();   

Middleware can be configure in http pipeline using Use, Run, and Map.
 
Run Extension
The nature of Run extension is to short circuit the HTTP pipeline immediately. It is a shorthand way of adding middleware to the pipeline that does not call any other middleware which is next to it and immediately returns HTTP response.
 
Use Extension
It will transfer next invoker, so that HTTP request will be transferred to the next middleware after execution of current use if the next invoker is present.
 
Map Extension
Map simply accepts a path and a function that configures a separate middleware pipeline.
 
app.Map("/MyDelegate", MyDelegate);
 
To get more detailed information about middleware Click here
 
ASP.net core has built-in support for Dependency Injection. We can configure services to DI container using this method. Following ways are to configure method in startup class.
 
AddTransient
Transient objects are always different; a new instance is provided to every controller and every service.
 
Scoped
Scoped objects are the same within a request, but different across different requests.
 
Singleton
Singleton objects are the same for every object and every request.
 
Can we remove startup.cs and merge entire class with Program.cs ?
 
Answer is : Yes we can merge entire class  in to single file.

Here we have gone through basic understanding of how program.cs and startup.cs file is important for our Asp.net Core application and how both can be configured, we have also had a  little walk though  Environment Variable and middleware and how to configure middleware and dependency injection. Additionally we have seen how UseIIsintegration() and UseKestrel() can be configured.

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In this article, we will create an empty ASP.NET Core project that doesn’t include default features, i.e., an empty shell.

Step to be followed

First, create a new empty project using Visual Studio 2017.
    File > New > Project
    Under “.NET Core”, select “ASP.NET Core Web Application”. Enter Name and Location. Click OK.
    Select “Empty”. Click OK.

Next, remove the code from Program.cs and Startup.cs so that they look like below (keeping the necessary "using" statements).
    public class Program {  
        public static void Main(string[] args) {  
            BuildWebHost(args).Run();  
        }  
        public static IWebHost BuildWebHost(string[] args) => WebHost.CreateDefaultBuilder(args).UseStartup().Build();  
    }  
    public class Startup {  
        public Startup(IHostingEnvironment env, ILoggerFactory loggerFactory, IConfiguration config) {}  
        public void ConfigureServices(IServiceCollection services) {  
            // setup dependency injection in service container  
        }  
        public void Configure(IApplicationBuilder app, IHostingEnvironment env) {  
            // setup request pipeline using middleware  
        }  
    }  

Empty project template in Visual Studio 2017 creates a project with Program.cs and Startup.cs classes.

Program.cs
Just like a Console application, "public static void Main()" is the starting point of ASP.NET Core applications. We’re setting up a host (WebHost) that references the server handling requests (Kestrel). This is achieved using CreateDefaultBuilder() method that configures -
    Kestrel- cross-platform web server
    Root- content root will use the web project’s root folder
    IIS- as the reverse proxy server
    Startup- points to a class that sets up configuration, services, and pipeline (see next section).
    Configuration- adds JSON and environment variables to IConfiguration available via Dependency Injection (see the next section).
    Logging- adds a Console and Debugs the logging providers.

Once configured, we Build() and Run() the host, at which point, the main thread is blocked and the host starts listening to the request from the Server.

When setting up WebHostBuilder, you could set values for various settings via UseSetting()method, which takes in a key/value pair for a property. These properties include applicationName (string), contentRoot (string), detailedErrors (bool), environment (string), URLs (semicolon separated list) and Webroot (string). Some of these properties can also be set via extension methods on WebHostBuilder.

Startup.cs
This class sets up the application dependency injection services and requests pipeline for our application, using the following methods.
    ConfigureServices(): add services to the service container
    Configure(): setup request pipeline using Middleware.

The parameters for these methods can be seen in the code attachment above.

These methods are called in the same sequence as listed above. The important point to remember about the sequence in which they run is that the service container is available after ConfigureServices, i.e., in Configure() method, and not before that.