How to get a User ID in Asp.Net Core 3.1 (ClaimPrincipal) in a Service Repository

Question: How do you get user id in .net core MVC from a service repository?

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Answer: The User's Id can be obtained from .net core application in several ways, one of them is by getting User from an HttpContext:

1. Create an Interface with a name "IUserUtility" that has a function defined as "GetUserID"
   - This function can return a Task of a "String" if you like.
   
   
2. Create a Mock Repository that implements the "IUserUtility"
   - In the Mock Repository, create a constructor that will utilize "Dependency Injection".
   
   - Dependency Injection will help you get access to Asp.Net Core inbuilt API's and other Services (User-defined Interfaces registered as Services in Startup.cs File).
   
   - In the Mock Repository, Inject HttpContext in the Constructor as well as define a private property on top of the Constructor
      
        - Defining a private read-only helps your application not to create a memory consuming variable that sometimes hangs even when the Service is not being utilized. Creating private read-only will allocate small portion of memory to that private variable and return the memory to the application when the variable is not in use.
   
   - wire the private HttpContext _httpContext with the HttpContext httpContext injected in the Constructor, this will input the httpContext Service coming from the Application Startup.cs Services into a private readonly _httpContext.
   
   - After implementing the UserUtility Interface go ahead and write your code in the GetUserID() function. This function will contain code that gets User ID and returns a string or an object containing the Current Logged in User ID.
   
   
3. See the Code below for reference:
   

   //This an Interface
   namespace YourProjectName.FirstLevelFolder.FinalFolderYourInterfaceClassIsLocated
   {
       public interface IUserUtility
       {
           Task<string> GetUserID();
       }
   }
   ?

   //This is a Service Mock Repository Class that implements IUserUtility Interface
   namespace ProjectName.FolderName
   {
       public class UserUtilityRepository : IUserUtility
       {
           private readonly SignInManager<AnotherUserMockRepositoryClass> _signInManager;
           private readonly HttpContext _contextRequest;
           public UserUtilityRepository(SignInManager<AnotherUserMockRepositoryClass> signInManager, HttpContext contextRequest)
           {
               _signInManager = signInManager;
               _contextRequest = contextRequest;
   
           }
           public async Task<string> GetUserID()
           {
               ClaimsPrincipal principal = _contextRequest.User as ClaimsPrincipal;
              string UserID = _signInManager.UserManager.GetUserId(principal);
               return UserID;
           }
       }
   }
   ?

   
   [NB]: Errors: You might an error that says:

   An unhandled exception occurred while processing the request.

   InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Http.HttpContext' while attempting to activate 'YouProject.Controllers.YourController'.
   
   Try: Dont't forget to add services.AddHttpContextAccessor in Startup.cs class.