Hello everybody , i use ASMX WebService with Xamarin.Forms , and i have two Methods in Web service , one Method returns String Value and the Other one returns Double , when i try these two methods with firefox (or any other internet Browser) , both methods work fine , but when i call those two method by Xamarin.forms , the one which returns double works fine ,but the other one with string value raises exception Error

Unhandled Exception:

System.Reflection.TargetInvocationException: An exception occurred during the operation, making the result invalid. Check InnerException for exception details.

where are my Web service Methods

{
[WebMethod]
public string ReturnString()
{
return "String Value Returned";
}

    [WebMethod]
    public double ReturnDouble()
    {
        return 5.500;
    }

}

And here is my Xamarin.forms Code

{
public MainPage()
{
InitializeComponent();

        BtReturnString.Clicked += BtReturnString_Clicked;
        BtReturnDouble.Clicked += BtReturnDouble_Clicked;
    }

    private void BtReturnDouble_Clicked(object sender, EventArgs e)
    {
        ServiceReference1.WebService1SoapClient WS = new ServiceReference1.WebService1SoapClient();
        WS.ReturnDoubleAsync();
        WS.ReturnDoubleCompleted += WS_ReturnDoubleCompleted;
    }

    private void WS_ReturnDoubleCompleted(object sender, ServiceReference1.ReturnDoubleCompletedEventArgs e)
    {
        Device.BeginInvokeOnMainThread(() =>
        {
            Result1.Text = e.Result.ToString();
        });
    }

    private void BtReturnString_Clicked(object sender, EventArgs e)
    {
        ServiceReference1.WebService1SoapClient WS = new ServiceReference1.WebService1SoapClient();
        WS.ReturnStringAsync();
        WS.ReturnStringCompleted += WS_ReturnStringCompleted;
    }

    private void WS_ReturnStringCompleted(object sender, ServiceReference1.ReturnStringCompletedEventArgs e)
    {
        Device.BeginInvokeOnMainThread(() =>
        {
            Result1.Text = e.Result.ToString();
        });
    }

}

any help please ?????