Offline_implementaiton_using_interface.cs

using System;

public static class Services
{
  public static IAPIService api { get { return _api.Value; } }
  public static bool isOffline { get; set; }
  
  private static Lazy<IAPIService> _api = new Lazy<IAPIService>(() => isOffline
                                        ? (IAPIService)new OfflineAPIService() 
                                        : new APIService());
}

public interface IAPIService
{
  string UserId { get; }
  void Login();
}

public class APIService : IAPIService
{
  public string UserId
  {
    get
    {

      return _userId;
    }
  }

  public void Login()
  {
    // send request to server
  }

  private string _userId;
}

public class OfflineAPIService : IAPIService
{
  public string UserId
  {
    get
    {

      return "offlineUser";
    }
  }

  public void Login()
  {
    // do nothing
  }
}