User.cs

 public sealed class User
  {
      public User(string firstName)
      {
          if (firstName == null)
              throw new ArgumentNullException(nameof(firstName), "First name cannot be null.");

          if (firstName.Length == 0)
              throw new ArgumentException("First name cannot be empty.", nameof(firstName));

          if (firstName.Length > 20)
              throw new ArgumentException("First name Length cannot has more than 20 characters.", nameof(firstName));
        }
  }