I would like to know that how [Authorize] attribute recognize that this user is authenticate?
If user is valid then we call FormsAuthentication.SetAuthCookie() method and as per MSDN this method:
Creates an authentication ticket for the supplied user name and adds it to the cookies collection of the response, or to the URL if you are using cookieless authentication.
Is [Authorize] attribute checks authentication ticket or cookies collection?
[Authorize] does not deal with any authentication mechanism itself. It merely looks in the users IIdentity for the IsAuthenticated flag. It will also look in the users IsMemberOf method, for authorization based on roles.
All the work to decode the authentication ticket is done in the early stages of the app pipeline, which sets those flags. By the time the Authorization Attribute methods are called, all that work has already been done and is stored in the users runtime data.
You can easily check the source code for the Authorize attribute, and you will see that it's quite simple in nature. It just returns true or false based on some simple lookups.
It's become more complicated in .net core, where it's based on policies and what not, but the original MVC implementation was quite simple.
My answer relates to ASP.NET Core I'm not sure if you asked about classic ASP.NET but this should be similar.
There's a middleware that you have to add for [Authorize] to work. ASP.NET Core provides this middleware out of the box and you can add your custom authentication handlers too.
You can check how it's implemented by reading: https://github.com/aspnet/Security/tree/dev/src
For example you want to use JWT bearer authentication, you have to add JWT bearer middleware, this is simply extension of AuthenticationBuilder: https://github.com/aspnet/Security/blob/dev/src/Microsoft.AspNetCore.Authentication.JwtBearer/JwtBearerExtensions.cs which calls AddScheme under the hood.
You want to use cookie based authentication you just call AddCookie which is also extension that calls AddScheme under the hood: https://github.com/aspnet/Security/blob/dev/src/Microsoft.AspNetCore.Authentication.Cookies/CookieExtensions.cs
Usage of it is documented here: https://learn.microsoft.com/en-us/aspnet/core/migration/1x-to-2x/identity-2x?view=aspnetcore-2.1
See also Using the [Authorize] Attribute
Web API provides a built-in authorization filter, AuthorizeAttribute. This filter checks whether the user is authenticated. If not, it returns HTTP status code 401 (Unauthorized), without invoking the action.
If you are interested how this filter works under the hood you can check it here.
You must be authenticated before you can be authorized, this is the logic responsible for it: https://github.com/aspnet/Security/blob/644f34e90d35b369efdce9c11ab1db42e0a7f4a7/src/Microsoft.AspNetCore.Authorization.Policy/PolicyEvaluator.cs#L91
In summary
how [Authorize] attribute knows that this user is authenticated.
Authorize attribute alone doesn't know if this user is authenticated. This is handled by authentication middleware and depends stricly on the scheme it tries to authenticate with. It simply tries to authenticate with schemes you have added(cookie,jwt etc.) by calling HttpContext.AuthenticateAsync which is simply calling AuthenticationService.AuthenticateAsync under the hood and sets HttpContext.User from the result ClaimsPrincipal, which is simply result from schema handler like jwt handler for instance. I think this should give you more in-depth idea how this works.
Generally if you're starting new project I don't recommend using classic ASP.NET and prepare for the future with .NET Core as everything is now going in this direction. ASP.NET 5 ( I also refer to it as "classic") is pretty much dead now.
