How to add filtering to a combobox

Hi, I'm having a real problem trying to add filtering to a combobox in WPF. The idea is that I should be bale to typ some characters which filter the items in the dropdown list to make finding what I want easier. Here is the class I wrote (with help from the Internet) to do the job..

    /// <summary>
    /// A Combobox which adds filtering for text at any point in the list of items
    /// </summary>
    public class IpoComboBox : ComboBox
    {
        public static readonly DependencyProperty MinimumSearchLengthProperty = DependencyProperty.Register("MinimumSearchLength", typeof(int), typeof(IpoComboBox), new UIPropertyMetadata(1));

        private string oldFilter = string.Empty;
        private string currentFilter = string.Empty;

        public IpoComboBox()
        {
            this.Loaded += IpoComboBox_Loaded;
        }

        void IpoComboBox_Loaded(object sender, RoutedEventArgs e)
        {
        }

        [Description("Length of the search string that triggers filtering.")]
        [Category("IpoComboBox")]
        [DefaultValue(1)]
        public int MinimumSearchLength
        {
            get
            {
                return (int)this.GetValue(MinimumSearchLengthProperty);
            }
            set
            {
                this.SetValue(MinimumSearchLengthProperty, value);
            }
        }

        protected TextBox EditableTextBox
        {
            get
            {
                return this.GetTemplateChild("PART_EditableTextBox") as TextBox;
            }
        }

        protected override void OnItemsSourceChanged(IEnumerable oldValue, IEnumerable newValue)
        {
            if (newValue != null)
            {
                ICollectionView view = CollectionViewSource.GetDefaultView(newValue);
                view.Filter += this.FilterPredicate;
            }
            if (oldValue != null)
            {
                ICollectionView view = CollectionViewSource.GetDefaultView(oldValue);
                view.Filter -= this.FilterPredicate;
            }
            base.OnItemsSourceChanged(oldValue, newValue);
        }

        protected override void OnPreviewKeyDown(KeyEventArgs e)
        {
            if (e.Key == Key.Tab || e.Key == Key.Enter)
            {
                // Explicit Selection -> Close ItemsPanel                
                this.IsDropDownOpen = false;            
            }            
            else if (e.Key == Key.Escape)            
            {                
                // Escape -> Close DropDown and redisplay Filter                
                this.IsDropDownOpen = false;                
                this.SelectedIndex = -1;                
                this.Text = this.currentFilter;
            }            
            else            
            {                
                if (e.Key == Key.Down)                
                {                    
                    // Arrow Down -> Open DropDown                    
                    this.IsDropDownOpen = true;                
                }                 
                base.OnPreviewKeyDown(e);            
            }             
            // Cache text            
            this.oldFilter = this.Text;        
        }

        protected override void OnKeyUp(KeyEventArgs e)        
        {            
            if (e.Key == Key.Up || e.Key == Key.Down)            
            {                
                // Navigation keys are ignored            
            }            
            else if (e.Key == Key.Tab || e.Key == Key.Enter)            
            {                
                // Explicit Select -> Clear Filter                
                this.ClearFilter();
            }            
            else            
            {                
                // The text was changed                
                if (this.Text != this.oldFilter)                
                {    
                    // Clear the filter if the text is empty,                    
                    // apply the filter if the text is long enough                    
                    if (this.Text.Length == 0 || this.Text.Length >= this.MinimumSearchLength)
                    {
                        this.RefreshFilter();                        
                        this.IsDropDownOpen = true;                         
                        // Unselect                        
                        this.EditableTextBox.SelectionStart = int.MaxValue;
                    }                
                }                 
                base.OnKeyUp(e);                 
                // Update Filter Value                
                this.currentFilter = this.Text;            
            }        
        }

        protected override void OnPreviewLostKeyboardFocus(KeyboardFocusChangedEventArgs e)
        {
            this.ClearFilter();
            int temp = this.SelectedIndex;
            this.SelectedIndex = -1;
            this.Text = string.Empty;
            this.SelectedIndex = temp;
            base.OnPreviewLostKeyboardFocus(e);
        }

        private void RefreshFilter()
        {
            if (this.ItemsSource != null)
            {
                ICollectionView view = CollectionViewSource.GetDefaultView(this.ItemsSource);
                view.Refresh();
            }
        }

        private void ClearFilter()
        {
            this.currentFilter = string.Empty;
            this.RefreshFilter();
        }

        /// <summary>
        /// Filters the predicate but the object could be anything. We need to find the property specified in the xaml
        /// </summary>
        /// <param name="value">The value.</param>
        /// <returns></returns>
        private bool FilterPredicate(object value)
        {
            bool isViewable = true;

            string comparisonString = GetItemDisplayString(value);

            // No filter, no text            
            if (value == null)
            {
                isViewable = false;
            }
            else if (string.IsNullOrEmpty(this.Text)) // No text, no filter            
            {
                isViewable = true;
            }
            else
            {
                if (!this.IsTextSearchCaseSensitive)
                {
                    isViewable = comparisonString.ToLower().Contains(this.Text.ToLower());
                }
                else
                {
                    isViewable = comparisonString.Contains(this.Text);
                }
            }
            return isViewable;
        }

        /// <summary>
        /// Gets the item display string.
        /// </summary>
        /// <param name="value">The value.</param>
        /// <returns></returns>
        private string GetItemDisplayString(object value)
        {
            string retStr = null;

            if (!(value is string))
            {
                // Because of the dynamic binding we may be given more than normal strings
                Type type = value.GetType();
                PropertyInfo displayProp = type.GetProperty(this.DisplayMemberPath);
                retStr = (string)displayProp.GetValue(value, null);
            }
            else
            {
                retStr = (string)value;
            }

            return retStr;
        }
    }

This works fine at the start, when the control is first viewed, but everything goes wrong when you select an item in the dropdown. If I select an item in the list then hit delete or backspace on the resulting selected text in the editbox part of the combobox then the text reappears and the dropdown shows. I expect the drop down but can't figure out why the text is reappearing. I think it's something to do with the view refresh but don't know how to fix it.

Does anyone have a better solution out there for this?

I also have the added complexity that the current implementation has a viewmodel bound to it which uses the first item in the list as a special case an repopulates the list from the model which could have changed. I'm wondering whether I should move this out to a button they can press but it would be nice to keep this and have it working with the filtering if possible.

Thanks

Celestrium