Null-sichere Eingabevalidierung

using System;
 
public class Program {
    public static void RegisterUser(string username) {
        if (string.IsNullOrWhiteSpace(username)) {
            throw new ArgumentException("Username cannot be empty or whitespace.");
        }
        Console.WriteLine($"User {username} registered.");
    }
 
    public static void Main() {
        RegisterUser("Alex");
    }
}