Could you help me to embed C#WPF to VB.NET?

Question

using System;using System.Windows;
using System.Windows.Input;
using System.Windows.Media;

"SampleUserControl.xaml.cs"
namespace SampleWpfUserControlLibrary
{
    /// <summary>
    /// Interaction logic for SampleUserControl.xaml
    /// </summary>
    public partial class SampleUserControl
    {
        private IApplicationHostWindow _hostWindow;
     
        public SampleUserControl()
        {
            InitializeComponent();
        }
        [Obsolete("This constructor should not be used",false)]
        public SampleUserControl(IApplicationHostWindow applicationHostWindow)
        {
            _hostWindow = applicationHostWindow;
           
            IsHitTestVisible = true;


            InitializeComponent();
        }


        public double GetViewboxWidth()
        {
            return viewHost.ActualWidth;
        }


        public double GetViewboxHeight()
        {
            return viewHost.ActualHeight;
        }


        public void SetViewboxChild(UIElement child)
        {
            viewHost.Content = child;
        }


        public void ProcessLeftButtonDown(Point point)
        {
            var result = VisualTreeHelper.HitTest(this, point);
            return;
        }


        protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
        {
            return base.HitTestCore(hitTestParameters) ??
                   new PointHitTestResult(viewHost.Content as Visual ?? viewHost, hitTestParameters.HitPoint);
        }


        private void exitMenuItem_Click(object sender, RoutedEventArgs e)
        {
            _hostWindow.Exit();
        }


        private void newProjectMenuItem_Click(object sender, RoutedEventArgs e)
        {
            _hostWindow.OpenDocument();
           
           
        }


        private void UserControl_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            return;
        }


        private void UserControl_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            return;
        }
    }
}

"IApplicationHostWindow.cs"

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace SampleWpfUserControlLibrary

{

    public interface IApplicationHostWindow

    {

        void OpenDocument();

        void Exit();

    }

}

What I did as off now is I added the WPF to VB.Net. As I run the program, an error occurs being null of _hostWindow.

http://i46.tinypic.com/mrpxi.png

on the line
_hostwindow.OpenDocument();

says "Object reference not set to an instance of an object"

It seems to work fine when I run the WPF only
http://i45.tinypic.com/oppylf.png