Subsclass UserControl in SilverLight

By default Expression creates custom controls derived from UserControls

partial class MyControl : UserControl
{
}

In order to derive from a different class you will have to change code in several places

Define new base class with required virtual methods. Do not make class abstract

public class MyUserControl : UserControl
{
public virtual void Foo() {;}
}

Change MyControl to use MyUserControl

partial class MyControl : UserControl
{
public override void Foo() {;}
}

Change XAML for MyControl to use

<MyUserControl x:Class="MyControl">
</MyUserControl>

To make Blend happy, add following lines to AssemblyInfo.cs

// Make blend happy
using System.Windows.Markup;
[assembly: XmlnsDefinition("http://schemas.microsoft.com/client/2007", "YourNamespace")]