Whats wrong with this trigger?

I'm learning about WPF templates and I'm creating one for a button. I'm using a trigger to change the Fill property of the Ellipse on 'IsMouseOver'. When I set the trigger to the 'Fill' property directly, it works. But when I try to reference a specific SolidColorBrush, I get a compile error.

This works:

<ControlTemplate x:Key="GhostButtonTemplate" TargetType="Button">
       开发者_运维知识库 <Grid>
            <Ellipse Name="Border" Stroke="DarkGray" Fill="Gray">        
        </Ellipse>
            <ContentPresenter ... />
        </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsMouseOver" Value="True">
        <Setter TargetName="Border" Property="Fill" Value="Black"/>
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

This causes error:

<ControlTemplate x:Key="GhostButtonTemplate" TargetType="Button">
        <Grid>
            <Ellipse Name="Border" Stroke="DarkGray">
                <Ellipse.Fill>
                    <SolidColorBrush Color="Gray" x:Name="FillBrush"/>
            </Ellipse.Fill>         
        </Ellipse>
            <ContentPresenter ... />
        </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsMouseOver" Value="True">
        <Setter TargetName="FillBrush" Property="Color" Value="Black"/>
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

Error is:

Cannot find the Trigger target 'FillBrush'. (The target must appear before any Setters,Triggers, or Conditions that use it.)

Can anyone explain why the second case doesn't work? Thanks.

---

Rather than naming the brush you use the Ellipse Edit, yeah you know this :P

<ControlTemplate x:Key="GhostButtonTemplate" TargetType="Button">
    <Grid>
        <Ellipse Name="Border" Stroke="DarkGray">
            <Ellipse.Fill>
                <SolidColorBrush Color="Gray" x:Name="FillBrush"/>
        </Ellipse.Fill>         
    </Ellipse>
        <ContentPresenter ... />
    </Grid>
<ControlTemplate.Triggers>
    <Trigger Property="IsMouseOver" Value="True">
    <Setter TargetName="Border" Property="Fill" Value="Black"/>
    </Trigger>
</ControlTemplate.Triggers>