Place ChildWindow Control in Center of Screen

Question

I use separated usercontrol to create my application toolbar in WPF.
I want to open WPF extended ChildWindow when user click on one of toolbar buttons. I should place ChildWindow in center of screen but when I set WindowStartupLocation="Center" ChildWindow placed at center of toolbar view. What should I do ?

Toolbar usercontrol :

   <Grid>
        <ToolBar>
            <Button ToolTip="tooltip" Margin="2" Click="Button_Click">
     </Button>
            <Separator/>
        </ToolBar>
    <extToolkit:ChildWindow     Name="childwindow"  WindowStartupLocation="Center"  Left="300" Top="400"  Height="300" Width="300"  Caption="caption"  IsModal="True">
        <Views:NewCompany />
    </extToolkit:ChildWindow>
   </Grid>

Answer 1

try this

   <Grid>
        <Grid.RowDefinition>
            <RowDefinition Height="50" />
            <RowDefinition Height="*" />
        </Grid.RowDefinition>
        <ToolBar Grid.Row="0">
            <Button ToolTip="tooltip" Margin="2" Click="Button_Click">
            </Button>
            <Separator/>
        </ToolBar>
        <Grid Grid.Row="1">
            <extToolkit:ChildWindow Name="childwindow"  WindowStartupLocation="Center"  Left="300" Top="400"  Height="300" Width="300"  Caption="caption"  IsModal="True">
                <Views:NewCompany />
            </extToolkit:ChildWindow>
        </Grid>
   </Grid>