Silverlight:如何在generic.xaml中为子类设置默认样式?

我有以下层次:

public class A
{
  protected class B
  {
  }
}

我尝试用以下方法定义默认样式(在generic.xaml中):

<Style TargetType="local:A+B">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="local:A+B">
                <Grid/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style TargetType="local:A.B">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="local:A.B">
                <Grid/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style TargetType="local:B">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="local:B">
                <Grid/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

有人知道正确的语法吗?