How to correctly inherit from a usercontrol defined in XAML in Silverlight

public class MyBase : UserControl
{
}

public class FirstUserControl : MyBase
{
...
}

<local:MyBase
    x:Class="FirstUserControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:local="YourAssembly" ...>

    <!-- Sticking with UserControl instead of local:MyBase makes this clearer -->
    <UserControl.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                ..
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </UserControl.Resources>

..Your XAML
</local:MyBase>