How to get the current logged in user ID in ASP.NET Core?

Update in ASP.NET Core Version >= 2.0

In the Controller:

public class YourControllerNameController : Controller
{
    private readonly UserManager<ApplicationUser> _userManager;
    
    public YourControllerNameController(UserManager<ApplicationUser> userManager)
    {
        _userManager = userManager;
    }

    public async Task<IActionResult> YourMethodName()
    {
        var userId =  User.FindFirstValue(ClaimTypes.NameIdentifier) // will give the user's userId
        var userName =  User.FindFirstValue(ClaimTypes.Name) // will give the user's userName
        
        // For ASP.NET Core <= 3.1
        ApplicationUser applicationUser = await _userManager.GetUserAsync(User);
        string userEmail = applicationUser?.Email; // will give the user's Email

       // For ASP.NET Core >= 5.0
       var userEmail =  User.FindFirstValue(ClaimTypes.Email) // will give the user's Email
    }
}

In some other class:

public class OtherClass
{
    private readonly IHttpContextAccessor _httpContextAccessor;
    public OtherClass(IHttpContextAccessor httpContextAccessor)
    {
       _httpContextAccessor = httpContextAccessor;
    }

   public void YourMethodName()
   {
      var userId = _httpContextAccessor.HttpContext.User.FindFirstValue(ClaimTypes.NameIdentifier);
   }
}

Then you should register IHttpContextAccessor in the Startup class as follows:

public void ConfigureServices(IServiceCollection services)
{
    services.TryAddSingleton<IHttpContextAccessor, HttpContextAccessor>();

    // Or you can also register as follows

    services.AddHttpContextAccessor();
}

For more readability write extension methods as follows:

public static class ClaimsPrincipalExtensions
{
    public static T GetLoggedInUserId<T>(this ClaimsPrincipal principal)
    {
        if (principal == null)
            throw new ArgumentNullException(nameof(principal));

        var loggedInUserId = principal.FindFirstValue(ClaimTypes.NameIdentifier);

        if (typeof(T) == typeof(string))
        {
            return (T)Convert.ChangeType(loggedInUserId, typeof(T));
        }
        else if (typeof(T) == typeof(int) || typeof(T) == typeof(long))
        {
            return loggedInUserId != null ? (T)Convert.ChangeType(loggedInUserId, typeof(T)) : (T)Convert.ChangeType(0, typeof(T));
        }
        else
        {
            throw new Exception("Invalid type provided");
        }
    }

    public static string GetLoggedInUserName(this ClaimsPrincipal principal)
    {
        if (principal == null)
            throw new ArgumentNullException(nameof(principal));

        return principal.FindFirstValue(ClaimTypes.Name);
    }

    public static string GetLoggedInUserEmail(this ClaimsPrincipal principal)
    {
        if (principal == null)
            throw new ArgumentNullException(nameof(principal));

        return principal.FindFirstValue(ClaimTypes.Email);
    }
}

Then use as follows:

public class YourControllerNameController : Controller
{
    public IActionResult YourMethodName()
    {
        var userId = User.GetLoggedInUserId<string>(); // Specify the type of your UserId;
        var userName = User.GetLoggedInUserName();
        var userEmail = User.GetLoggedInUserEmail();
    }
}

public class OtherClass
{
     private readonly IHttpContextAccessor _httpContextAccessor;
     public OtherClass(IHttpContextAccessor httpContextAccessor)
     {
         _httpContextAccessor = httpContextAccessor;
     }

     public void YourMethodName()
     {
         var userId = _httpContextAccessor.HttpContext.User.GetLoggedInUserId<string>(); // Specify the type of your UserId;
     }
}