Custom WebView Renderes is called, but Delegate is not working

I defined a delegate for a Custom WebView Renderer and assigned it – but the override methods are never called. Below my code. Any idea?
Thanks for any help.

using Xamarin.Forms.Platform.iOS;
using Xamarin.Forms;
using MonoTouch.UIKit;
using MonoTouch.Foundation;
using System.Diagnostics;
using System;

[assembly: ExportRenderer (typeof (tg.CustomWebView), typeof (tg.iOS.CustomWebViewRenderer))]

namespace tg.iOS
{

public class CustomWebViewRenderer: WebViewRenderer
{
    // ok - Get's called !
    public CustomWebViewRenderer ()
    {
        Debug.WriteLine ("CustomWebViewRenderer ()");
        this.Delegate = new MyCustomWebViewDelegate();
        this.LoadUrl ("http://www.google.ch");

        // does not work:
        //this.ShouldStartLoad = delegate (UIWebView webViewParam, NSUrlRequest request, UIWebViewNavigationType navigationType) {
        //  Debug.WriteLine("ShouldStartLoad");
        //  return false;
        //} ;

    }


    public class MyCustomWebViewDelegate : UIWebViewDelegate
    {
        // get's never called:
        public override bool ShouldStartLoad (UIWebView webView, NSUrlRequest request, UIWebViewNavigationType navigationType)
        {
            Debug.WriteLine("ShouldStartLoad");
            return false;
        }

        public override  void LoadFailed (UIWebView webView, NSError error)
        {
            Debug.WriteLine("Entered");
        }


        public override  void LoadingFinished (UIWebView webView)
        {
            Debug.WriteLine("Entered");
        }


        public override  void LoadStarted (UIWebView webView)
        {
            Debug.WriteLine("Entered");
        }

    }
}

}