how to search in combo box when I type first letter

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VS2008

C#

Visual-Studio

WPF

I want to know that when I type a first character in combo box it should display all item which are start that character and the character remains in combo box text edit.

xmlns:myControl="clr-namespace:Enrollment.Controls"
<myControl:AutoCompleteComboBox Grid.Row="7" Grid.Column="1" SelectedValue="{Binding Path=SelectedCurrentCity}" ItemsSource="{Binding Path=CurrentCityList}">

I have added a control

<UserControl x:Class="Enrollment.Controls.AutoCompleteComboBox"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  >
    <Canvas>
        <TextBox x:Name="autoTextBox"  Height="25" MinWidth="240" TextWrapping="NoWrap"></TextBox>
        <ComboBox MinWidth="240" x:Name="suggestionListBox" Margin="0,25,0,-25" removed="LightYellow" 
                 Visibility="Collapsed" SelectionChanged="suggestionListBox_SelectionChanged" />
    </Canvas>
</UserControl>

public Windows2()
{
fillCurrentCityList();
}
public fillCurrentCityList ()
{
CurrentCityList = new List<string>();
            string tt1 = CurrStateIDcmb.SelectedValue.ToString();
            string str1 = "Select CityName from mstCity where StateId = (Select StateId from mstState where StateName =@tt1)";
            SqlConnection cn2 = new SqlConnection(@"Data Source=SUMIT\SQLEXPRESS;AttachDbFilename='C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data\Enrollment.mdf';Integrated Security=True");

            SqlCommand cmd2 = new SqlCommand(str1, cn2);
            cn2.Open();

       
            cmd2.Parameters.AddWithValue("@tt1", CurrStateIDcmb.SelectedValue.ToString());


            SqlDataReader dm;
            try
            {

                dm = cmd2.ExecuteReader();
                while (dm.Read())
                {
                
                
                    CurrentCityList.Add(dm["CityName"].ToString());


                }

            }

            catch (Exception ex)
            {
               
            }

            finally
            {
                cn2.Close();
            }
}



 public partial class AutoCompleteComboBox : UserControl
    {
        #region Constructor
        public AutoCompleteComboBox()
        {
            InitializeComponent();
            // Attach events to the controls
            autoTextBox.TextChanged +=
                new TextChangedEventHandler(autoTextBox_TextChanged);
            autoTextBox.PreviewKeyDown +=
                new KeyEventHandler(autoTextBox_PreviewKeyDown);
            suggestionListBox.SelectionChanged +=
                new SelectionChangedEventHandler(suggestionListBox_SelectionChanged);
        }
        #endregion
        #region Properties

        /// <summary>
        /// Gets or sets the items source.
        /// </summary>
        /// <value>The items source.</value>
        public IEnumerable<string> ItemsSource
        {
            get 
            { 
                return (IEnumerable<string>)GetValue(ItemsSourceProperty);
            }
            set { SetValue(ItemsSourceProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ItemsSource.  
        // This enables animation, styling, binding, etc...
        public static readonly DependencyProperty ItemsSourceProperty =DependencyProperty.Register("ItemsSource"
                                , typeof(IEnumerable<string>)
                                , typeof(AutoCompleteComboBox)
                                , new UIPropertyMetadata(null));

        /// <summary>
        /// Gets or sets the selected value.
        /// </summary>
        /// <value>The selected value.</value>
        public string SelectedValue
        {
            get { return (string)GetValue(SelectedValueProperty); }
            set { SetValue(SelectedValueProperty, value); }
        }

        // Using a DependencyProperty as the backing store for SelectedValue.  
        // This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SelectedValueProperty =
            DependencyProperty.Register("SelectedValue"
                            , typeof(string)
                            , typeof(AutoCompleteComboBox)
                            , new UIPropertyMetadata(string.Empty));

        #endregion


        #region Methods
        /// <summary>
        /// Handles the TextChanged event of the autoTextBox control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The instance containing the event data.</param>
        void autoTextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            // Only autocomplete when there is text
            if (autoTextBox.Text.Length > 0)
            {
                // Use Linq to Query ItemsSource for resultdata
                string condition = string.Format("{0}%", autoTextBox.Text);
                IEnumerable<string> results = ItemsSource.Where(delegate(string s){return s.ToLower().StartsWith(autoTextBox.Text.ToLower());
                }
                );

                if (results.Count() > 0)
                {
                    suggestionListBox.ItemsSource = results;
                    suggestionListBox.Visibility = Visibility.Visible;
                }
                else
                {
                    suggestionListBox.Visibility = Visibility.Collapsed;
                    suggestionListBox.ItemsSource = null;
                }
            }
            else
            {
                suggestionListBox.Visibility = Visibility.Collapsed;
                suggestionListBox.ItemsSource = null;
            }
        }

        /// <summary>
        /// Handles the PreviewKeyDown event of the autoTextBox control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The instance containing the event data.</param>
        void autoTextBox_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Down)
            {
                if (suggestionListBox.SelectedIndex < suggestionListBox.Items.Count)
                {
                    suggestionListBox.SelectedIndex = suggestionListBox.SelectedIndex + 1;
                }
            }
            if (e.Key == Key.Up)
            {
                if (suggestionListBox.SelectedIndex > -1)
                {
                    suggestionListBox.SelectedIndex = suggestionListBox.SelectedIndex - 1;
                }
            }
            if (e.Key == Key.Enter || e.Key == Key.Tab)
            {
                // Commit the selection
                suggestionListBox.Visibility = Visibility.Collapsed;
                e.Handled = (e.Key == Key.Enter);
            }

            if (e.Key == Key.Escape)
            {
                // Cancel the selection
                suggestionListBox.ItemsSource = null;
                suggestionListBox.Visibility = Visibility.Collapsed;
            }
        }

        /// <summary>
        /// Handles the SelectionChanged event of the suggestionListBox control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="System.Windows.Controls.SelectionChangedEventArgs"/> instance containing the event data.</param>
        private void suggestionListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (suggestionListBox.ItemsSource != null)
            {
                autoTextBox.TextChanged -= new TextChangedEventHandler(autoTextBox_TextChanged);
                if (suggestionListBox.SelectedIndex != -1)
                {
                    autoTextBox.Text = suggestionListBox.SelectedItem.ToString();
                }
                autoTextBox.TextChanged += new TextChangedEventHandler(autoTextBox_TextChanged);
            }
        }

        #endregion
    }

Exception in autoTextBox_TextChanged
ItemsSource shows null
http://screencast.com/t/ObdoUxAmW[^]