The C1InputMaskExtender control allows you to create a mask for a drop-down list box. Use the ShowTrigger property to show the drop-down arrow and use the ComboItems property to populate the drop-down list.

1. Create an ASP.NET Web application.
2. Add a standard TextBox control to the main content of  your page.
3. Click the TextBox smart tag and select Add Extender from the Tasks menu.
4. In the Extender Wizard, select C1InputMaskExtender and click OK. A C1InputMaskExtender control is added to the page and the TargetControlID is set to TextBox1.
5. Select View | Properties Window in the Visual Studio menu.
6. Click the drop-down list at the top of the Properties window and select TextBox1_C1InputMaskExtender.
7. Set the C1InputMaskExtender.Mask property to 000-0000. 
8. Set the C1InputExtenderBase.ShowTrigger property to True.
9. Click the ellipsis button next to the C1InputExtenderBase.ComboItems property. Click the Add button three times and enter three values, for example, 100-1000, 200-2000, and 123-1234.

The markup should now look similar to the following:

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

<cc1:C1InputMaskExtender ID="TextBox1_C1InputMaskExtender" runat="server" Mask="000-0000" ShowTrigger="True" TargetControlID="TextBox1">

            <ComboItems> 

 <cc1:C1ComboBoxItem Value="100-1000" />

                <cc1:C1ComboBoxItem Value="200-2000" />

                <cc1:C1ComboBoxItem Value="123-1234" />

            </ComboItems>

        </cc1:C1InputMaskExtender>

10. Press F5 to run the application. The ShowTrigger property displays the drop-down arrow. The Mask requires digits in the format specified, and the drop-down list specified in the ComboItems property contains three preset values.