Resize problem due to GridSplitter.

Hi

I have a grid splitter between two grids. I am collapsing the visibilty of grid below the splitter on basis of boolean. But after collapsing the visibilty of my grid, my upper grid(above thegrid splitter0 is not resizing to window form.

Code:-

<Grid >
        <Grid.RowDefinitions>
            <RowDefinition  Height="*"></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition  Height="*"></RowDefinition>
            </Grid.RowDefinitions>

 <Expander  Grid.Row="0" ">
<grid  Name="grdResults>

' Have columns and rows  inside
</grid>
</Expander>
 <GridSplitter  Grid.Row="1"
                VerticalAlignment="Center" HorizontalAlignment="Stretch" SnapsToDevicePixels="True"
                     />
 <Expander  Grid.Row="2" ">
<grid Name="grdExams">

' Have columns and rows  inside
</grid>
</Expander>

Now i am collapsing the visibilty of second expander on basis of boolean and still my first grid is not exapanding to full window sapce. Do i need to set some property for this