Medium 2

User.cs

public class User {

  // Terrible property - this can be set to anything without reason
  public string Username { get; set; }
  
  
  // slightly better, but requires additional field
  // and we still wouldn't know why "username2" changed
  private string username2;
  public string Username2 {
    get => username2;
    set {
      if (string.IsNullOrEmpty(value)) throw new ArgumentException("Must have a value");
      if (value.Length > 50) throw new ArgumentException("Too long");
      username2 = value;
    }
  }

  public string Username3 { get; private set; }
  
  public void ChangeUsername(string newUsername) {
    ValidateUsername(newUsername);
    // (maybe) add some additional logic
    Username3 = newUsername;
  }

  private void ValidateUsername(string username) {
    if (string.IsNullOrEmpty(username)) throw new ArgumentException("Must have a value");
    if (username.Length > 50) throw new ArgumentException("Too long");
  }
}