what problem with my code

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Controls;
using System.Windows;
using System.Windows.Media.Imaging;

namespace SellApp.Views.Control
{
    class ComboBoxValidator:Image
    {
        public ComboBoxValidator()
        {
            this.Visibility = Visibility.Hidden;
            VerifyControl.SelectionChanged+=new SelectionChangedEventHandler(VerifyControl_SelectionChanged);
        }
        private void VerifyControl_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            Check();
        }
        public bool Check()
        {
            if (VerifyControl.SelectedIndex == 0)
            {
                BitmapImage myBitmapImage = new BitmapImage();
                myBitmapImage.BeginInit();
                myBitmapImage.UriSource = new Uri("Resources/check_error.gif", UriKind.Relative);
                myBitmapImage.EndInit();
                this.Source = myBitmapImage;

                this.Visibility = Visibility.Visible;
                return false;
            }
            else
            {
                BitmapImage myBitmapImage = new BitmapImage();
                myBitmapImage.BeginInit();
                myBitmapImage.UriSource = new Uri("Resources/access_allowu.gif", UriKind.Relative);
                myBitmapImage.EndInit();
                this.Source = myBitmapImage;

                this.Visibility = Visibility.Visible;
                return true;
            }
           
        }
        #region  VerifyControl
        /// <summary>
        /// 检查是否允许为空
        /// </summary>
        public static DependencyProperty VerifyControlProperty =
                DependencyProperty.Register(
                        "VerifyControl",
                        typeof(ComboBox),
                        typeof(ComboBoxValidator),
                        new FrameworkPropertyMetadata(null));
        /// <summary>
        /// 是否允许为空
        /// </summary>

        public ComboBox VerifyControl
        {
            get { return (ComboBox)GetValue(VerifyControlProperty); }
            set
            {
                SetValue(VerifyControlProperty, value);
            }
        }
        #endregion
    }
}