<Canvas x:Class="MoonTest.System.Windows.ManagedTestClass"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:custom="clr-namespace:MoonTest.System.Windows">
    <Canvas.Style>
        <Style TargetType="custom:ManagedTestClass">
            <Setter Property="A" Value="ok" /> <!-- This should work -->
        </Style>
    </Canvas.Style>
</Canvas>
