Offline emulation mode per Service's method

Service-offline-emulation.cs

public static class Services
{
    public static bool isOffline
    {
        get; set;
    }
}

public class APIService
{
    public string UserId
    {
        get
        {
            if (Services.isOffline)
            {
                return "offlineUser";
            }

            return _userId;
        }
    }

    public void Login()
    {
        if (Services.isOffline)
        {
            return;
        }
    // send request to server here
    }

    private string _userId;
}