update comboBox

  /// <summary>
    /// AutoCompleteComboBox
    /// </summary>
    public class AutoCompleteComboBox : ComboBox
    {
        #region DependencyProperty
        public string WaterMark
        {
            get { return (string)GetValue(WaterMarkProperty); }
            set { SetValue(WaterMarkProperty, value); }
        }

        // Using a DependencyProperty as the backing store for WaterMark.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty WaterMarkProperty =
            DependencyProperty.Register("WaterMark", typeof(string), typeof(AutoCompleteComboBox), new PropertyMetadata(null, new PropertyChangedCallback(OnWaterMarkChanged)));

        public bool? IsNull
        {
            get { return (bool?)GetValue(IsNullProperty); }
            set { SetValue(IsNullProperty, value); }
        }

        // Using a DependencyProperty as the backing store for IsNull.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty IsNullProperty =
            DependencyProperty.Register("IsNull", typeof(bool?), typeof(AutoCompleteComboBox), new PropertyMetadata(null));

        public bool? SetFocuse
        {
            get { return (bool?)GetValue(SetFocuseProperty); }
            set { SetValue(SetFocuseProperty, value); }
        }

        // Using a DependencyProperty as the backing store for SetFocuse.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SetFocuseProperty =
            DependencyProperty.Register("SetFocuse", typeof(bool?), typeof(AutoCompleteComboBox), new PropertyMetadata(null, new PropertyChangedCallback(OnSetFocuseChanged)));

        public bool IsAllowNull
        {
            get { return (bool)GetValue(IsAllowNullProperty); }
            set { SetValue(IsAllowNullProperty, value); }
        }

        // Using a DependencyProperty as the backing store for IsAllowNull.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty IsAllowNullProperty =
            DependencyProperty.Register("IsAllowNull", typeof(bool), typeof(AutoCompleteComboBox), new PropertyMetadata(true, new PropertyChangedCallback(OnIsAllowNullChanged)));

        private static void OnIsAllowNullChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {

        }

        #endregion

        #region Event
        private static void OnWaterMarkChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {

        }

        private static void OnSetFocuseChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            AutoCompleteComboBox wm = obj as AutoCompleteComboBox;

            if (e.NewValue != null && wm.EditableTextBox != null)
            {

                if ((bool)e.NewValue == true && wm.EditableTextBox.IsFocused != true)
                {
                    wm.EditableTextBox.Focus();
                }

            }
        }

        public IList DataSource
        {
            get { return (IList)GetValue(DataSourceProperty); }
            set { SetValue(DataSourceProperty, value); }
        }

        // Using a DependencyProperty as the backing store for DataSource.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty DataSourceProperty =
            DependencyProperty.Register("DataSource", typeof(IList), typeof(AutoCompleteComboBox), new UIPropertyMetadata(null, new PropertyChangedCallback(OnDataSourceChanged)));

        private static void OnDataSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            AutoCompleteComboBox a = d as AutoCompleteComboBox;
            //if (a.IsAllowNull)
            //{
                a.Items.Clear();
                if (a.DataSource != null)
                {

                    Type b = null;

                    foreach (var item in a.DataSource)
                    {
                        if (b == null)
                        {
                            b = item.GetType();
                            object o = Activator.CreateInstance(b);
                            //o.GetType().GetProperty(a.DisplayMemberPath).SetValue(o, "----Select----", null);
                            a.Items.Add(o);
                        }
                        a.Items.Add(item);
                    }

                }
                else
                {
                    a.ItemsSource = null;
                }

            //}

        }

        protected override void OnDropDownOpened(EventArgs e)
        {
            base.OnDropDownOpened(e);
            if (this.HasItems)
            {
                this.Items[].GetType().GetProperty(this.DisplayMemberPath).SetValue(this.Items[], "----Select----", null);
            }

        }

        protected override void OnDropDownClosed(EventArgs e)
        {
            base.OnDropDownClosed(e);
            if (this.SelectedIndex == )
            {
                //this.SelectedItem = null;
                //this.SelectedItem = null;
                //this.Text = null;
                //this.SelectedValue = null;
                this.SelectedIndex = -;
            }
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            //load the text box control
            if (this.EditableTextBox != null && this.MyWaterMarkTextBlock != null)
            {
                if (SetFocuse == true)
                {
                    if (!this.EditableTextBox.IsFocused)
                    {
                        this.EditableTextBox.Focus();
                    }

                }
                this.EditableTextBox.TextChanged += new TextChangedEventHandler(EditableTextBox_TextChanged);
                this.EditableTextBox.GotFocus += EditableTextBox_GotFocus;
                this.Loaded += AutoCompleteComboBox_Loaded;
            }

        }

        void AutoCompleteComboBox_Loaded(object sender, RoutedEventArgs e)
        {

            if (string.IsNullOrWhiteSpace(this.Text))
            {

                this.MyWaterMarkTextBlock.Visibility = Visibility.Visible;
                //this.MyWaterMarkTextBlock.Text = WaterMark;
                this.IsNull = false;
            }
            else
            {

                this.MyWaterMarkTextBlock.Visibility = Visibility.Collapsed;
                //this.MyWaterMarkTextBlock.Text = WaterMark;
                this.IsNull = true;
            }
            try
            {
                //if (this.HasItems)
                //{
                //    this.Items[0].GetType().GetProperty(this.DisplayMemberPath).SetValue(this.Items[0], "----Select----", null);
                //}
                //if (!this.IsAllowNull)
                //{
                //    this.EditableTextBox.IsEnabled = false;
                //}
            }
            catch (Exception)
            {

            }

        }

        void EditableTextBox_GotFocus(object sender, RoutedEventArgs e)
        {
            if (this.MyWaterMarkTextBlock.Visibility == Visibility.Visible)
            {
                // this.Text = null;
                this.MyWaterMarkTextBlock.Visibility = Visibility.Collapsed;
                //this.MyWaterMarkTextBlock.Text = WaterMark;
            }
            this.IsDropDownOpen = true;
        }

        protected override void OnSelectionChanged(SelectionChangedEventArgs e)
        {
           base.OnSelectionChanged(e);

                if (this.SelectedIndex == )
                {
                    this.SelectedIndex = -;
                } 

        }

        void EditableTextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            if (this.EditableTextBox.IsFocused && this.IsDropDownOpen == false)
            {

                this.IsDropDownOpen = true;
            }
            if (string.IsNullOrWhiteSpace(this.Text))
            {
                if (!this.EditableTextBox.IsFocused)
                {
                    this.MyWaterMarkTextBlock.Visibility = Visibility.Visible;
                    // this.MyWaterMarkTextBlock.Text = WaterMark;

                }
                this.IsNull = false;

            }
            else
            {
                this.IsNull = true;
                this.MyWaterMarkTextBlock.Visibility = Visibility.Collapsed;
                // this.MyWaterMarkTextBlock.Text = WaterMark;

            }

        }

        void SimpleAutoCompleteComboBox_LostFocus(object sender, RoutedEventArgs e)
        {
            this.IsDropDownOpen = false;
            // to prevent misunderstanding that user has entered some information
            if (this.SelectedIndex == - || this.SelectedIndex == )
            {
                this.Text = null;
                this.SelectedItem = null;
                this.SelectedValue = null;
                this.MyWaterMarkTextBlock.Visibility = Visibility.Visible;
                this.IsNull = false;
                // this.MyWaterMarkTextBlock.Text = WaterMark;

            }
            // syncronize text
            else
            {
                this.Text = this.SelectedText;
                this.IsNull = true;
            }
            // release timer resources

            try
            {
                this.EditableTextBox.CaretIndex = ;
            }
            catch { }
        }

        #endregion

        #region Construtor
        static AutoCompleteComboBox()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(AutoCompleteComboBox), new FrameworkPropertyMetadata(typeof(AutoCompleteComboBox)));

        }

        public AutoCompleteComboBox()
        {
            this.StaysOpenOnEdit = true;
            this.IsEditable = true;
            this.IsTextSearchEnabled = true;
            this.LostFocus += SimpleAutoCompleteComboBox_LostFocus;
            this.IsAllowNull = true;

        }
        #endregion

        protected override void OnItemsSourceChanged(System.Collections.IEnumerable oldValue, System.Collections.IEnumerable newValue)
        {
            base.OnItemsSourceChanged(oldValue, newValue);
        }

        #region property
        /// <summary>
        /// Gets the waterMark  .
        /// </summary>
        protected TextBlock MyWaterMarkTextBlock
        {
            get
            {
                return base.GetTemplateChild("PART_MyWaterMarkTextBlock") as TextBlock;
            }
        }

        /// <summary>
        /// Gets the text box in charge of the editable portion of the combo box.
        /// </summary>
        protected TextBox EditableTextBox
        {
            get
            {
                return base.GetTemplateChild("PART_EditableTextBox") as TextBox;
            }
        }

        private string SelectedText
        {
            get
            {
                try
                {
                    if (this.SelectedIndex == -) return null;

                    if (this.SelectedItem != null)
                    {
                        return this.SelectedItem.GetType().GetProperty(this.DisplayMemberPath).GetValue(this.SelectedItem, null).ToString();
                    }
                    else
                    {
                        return null;
                    }
                }
                catch (System.Exception)
                {

                    return null;
                }

            }
        }
        #endregion

    }