Q:

Xamarin alert failing

Getting error when i call the UIAlertView:
/Users/sun/Projects/csharp/csharp/ViewController.cs(13,13): Warning CS0618: 'UIAlertView.UIAlertView(string, string, UIAlertViewDelegate, string, params string[])' is obsolete: 'Use overload with a IUIAlertViewDelegate parameter' (CS0618) (csharp)

Code:
    void HandleTouchUpInside(object sender, EventArgs ea)
    {
        new UIAlertView("Touch2", "TouchUpInside handled", null, "OK", null).Show();
    }

A:

The warning and error are two different things.
Error:
The invalid selector on the TouchUpInside can be caused be a few different things, since you did not post code on how you are setting it up, but I would look at if you have any additional actions for the button assigned in the Storyboard, if the button (or its ViewController) is going out of scope, etc.. 
Warning:

Warning CS0618: 'UIAlertView.UIAlertView(string, string, UIAlertViewDelegate, string, params string[])' is obsolete: 

UIAlertView is deprecated in iOS 8, so if you are targeting iOS 9+, you should use UIAlertController
var uiAlert = UIAlertController.Create("Touch2", "TouchUpInside handled", UIAlertControllerStyle.Alert);
uiAlert.AddAction(UIAlertAction.Create("OK", UIAlertActionStyle.Default, (alertAction) => { 
    alertAction.Dispose(); 
    uiAlert.Dispose(); 
}));
await PresentViewControllerAsync(uiAlert, true);

