Check out example codes for ".net core identity get user id". It will help you in understanding the concepts better.

Code Example 1

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);
    }
}

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