Offline emulation using virtual methods

Service-offline-emulation-virtual.cs

public class APIService {
  public virtual string UserId {
    get {
      return _userId;
    }
  }
    
  public virtual void Login()
  {
    // send request to server here
  }

  private string _userId;
}

public class OfflineAPIService : APIService {
  public override string UserId {
    get {
      return "offlineUser";
    }
  }
    
  public override void Login()
  {
    // do nothing
  }
}