WPF，这段代码为什么报错，却能运行？
<Window.Resources>
<ControlTemplate x:Key="ResizeDecoratorTemplate" TargetType="Control">
<Grid>
<local:ResizeThumb Height="3" Cursor="SizeNS" Margin="0 -4 0 0"
VerticalAlignment="Top" HorizontalAlignment="Stretch"/>
<local:ResizeThumb Width="3" Cursor="SizeWE" Margin="-4 0 0 0"
VerticalAlignment="Stretch" HorizontalAlignment="Left"/>
<local:ResizeThumb Width="3" Cursor="SizeWE" Margin="0 0 -4 0"
VerticalAlignment="Stretch" HorizontalAlignment="Right"/>
<local:ResizeThumb Height="3" Cursor="SizeNS" Margin="0 0 0 -4"
VerticalAlignment="Bottom" HorizontalAlignment="Stretch"/>
<local:ResizeThumb Width="7" Height="7" Cursor="SizeNWSE" Margin="-6 -6 0 0"
VerticalAlignment="Top" HorizontalAlignment="Left"/>
<local:ResizeThumb Width="7" Height="7" Cursor="SizeNESW" Margin="0 -6 -6 0"
VerticalAlignment="Top" HorizontalAlignment="Right"/>
<local:ResizeThumb Width="7" Height="7" Cursor="SizeNESW" Margin="-6 0 0 -6"
VerticalAlignment="Bottom" HorizontalAlignment="Left"/>
<local:ResizeThumb Width="7" Height="7" Cursor="SizeNWSE" Margin="0 0 -6 -6"
VerticalAlignment="Bottom" HorizontalAlignment="Right"/>
</Grid>
</ControlTemplate>
<Style TargetType="Button">
<Setter Property="Template" >
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}}">
<Control Template="{StaticResource ResizeDecoratorTemplate}" />
<ContentPresenter Content="{TemplateBinding ContentControl.Content}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid HorizontalAlignment="Left" Height="485" VerticalAlignment="Top" Width="766">
<Button Content="Button" HorizontalAlignment="Left" Margin="158,184,0,0" VerticalAlignment="Top" Width="75"/>
</Grid>
class ResizeThumb : Thumb
{
...
...
}
