How to set autosuggest of DataGridComboBoxColumn WPF?

Question

<DataGridComboBoxColumn x:Name="list_itemname" Width="*" Header="Item Name" >
 <DataGridComboBoxColumn.ElementStyle>
 <Style TargetType="ComboBox">
<Setter Property="IsEditable" Value="True"/>
 <Setter Property="ItemsSource" Value="{Binding Path=itemlist }" />
</Style>
DataGridComboBoxColumn.ElementStyle>
 <DataGridComboBoxColumn.EditingElementStyle>
 <Style TargetType="ComboBox">
<Setter Property="IsEditable" Value="True"/>
 <Setter Property="ItemsSource" Value="{Binding Path=itemlist }" />
 </Style>
</DataGridComboBoxColumn.EditingElementStyle>
</DataGridComboBoxColumn>

C# Code:

public partial class page_addsale : Page
 {
 List itemlist = new List();
 ObservableCollection<Proxy> collection = new ObservableCollection<Proxy>();
 DataTable item_dt = new DataTable();
public page_addsale()
{
             InitializeComponent();
             con.Open(); OleDbDataAdapter ad = new OleDbDataAdapter("select id, item_name from item_list", con);
             ad.Fill(item_dt);
             con.Close();
 }
public class Proxy
{
   public string itemname { get; set; }
   public string itemunit { get; set; }
}
private void add_sale_Loaded(object sender, RoutedEventArgs e)
 {
             item_list.CanUserAddRows = true;
             item_list.AutoGenerateColumns = false;
                      string[] arr_item = item_dt.AsEnumerable().Select<System.Data.DataRow, String>(x => x.Field<String>("item_name")).ToArray();
             itemlist.AddRange(arr_item);
             list_itemname.ItemsSource = itemlist;
             item_list.ItemsSource = collection;
 }
}

 

 

after tab in unit column then DataGridComboBoxColumn selected item empty