public class LookupCombo : ComboBox
    {
        private LookupListModel<String> model;

        public LookupCombo()
            : base()
        {
            this.SelectedValuePath = "Key";
            this.DisplayMemberPath = "Name";
            this.IsEditable = false;
        }

        public void SetModel(LookupListModel<String> model)
        {
            this.model = model;
            this.ItemsSource = this.model.Items;
        }

        public void SetValue(String keyvalue)
        {
            if (keyvalue.Length > 0)
            {
                this.model.AssureItemExists(keyvalue);
                this.Items.Refresh();
                this.SelectedValue = keyvalue;
            }
            else
            {
                if (Items.Count > 0)
                    this.SelectedIndex = 0;
            }
        }

    }

<cr:CustomDocument x:Class="XDNG3.DOC14"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:cr="clr-namespace:XDNG3.Core">
    <Grid Height="213" Width="432">
        <cr:LookupCombo x:Name="KONTR" VerticalAlignment="Top" 
                        HorizontalAlignment="Left" Width="250" Margin="85,6,0,0" />
        <ComboBox Height="23" Margin="84,40,97,0" Name="comboBox1" VerticalAlignment="Top" />
    </Grid>
</cr:CustomDocument>

<cr:CustomDocument x:Class="XDNG3.DOC14"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:cr="clr-namespace:XDNG3.Core">
    <Grid Height="213" Width="432">
        <cr:LookupCombo x:Name="KONTR" VerticalAlignment="Top" 
                        Style="{StaticResource {x:Type ComboBox}}"
                        HorizontalAlignment="Left" Width="250" Margin="85,6,0,0" />
        <ComboBox Height="23" Margin="84,40,97,0" Name="comboBox1" VerticalAlignment="Top" />
    </Grid>
</cr:CustomDocument>