How can set property in trigger, if it's null?

How can set property in trigger, if it'开发者_如何学运维s null?

<Style.Triggers>
    <Trigger Property="ContextMenu" Value="{x:Null}">
        <Setter Property="ContextMenu" Value="{DynamicResource ContextMenu}"/>
     </Trigger>
</Style.Triggers>

You can simply set the ContextMenu directly in your Style like so:

<Style ...>
    <Setter Property="ContextMenu" Value="{DynamicResource ContextMenu}"/>
</Style>

If the user sets the ContextMenu explicitly, then it will take precedence over the Style setter. See this MSDN article for the order or precedence. Your Style setter falls under #8, while a user setting is at #3.

Have you tried?

<Style.Triggers>
    <DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=Self}, Path=ContextMenu}" Value="{x:Null}">
        <Setter Property="ContextMenu" Value="{DynamicResource ContextMenu}"/>
     </DataTrigger>
</Style.Triggers>

But in reality you should be setting the context menu in the control's style and then any user of the control can override it in a derived style or in the control attributes.