Auto sizing columns of grid

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C#

WPF

Gridview

I have a grid and two columns in it.. I place an expander in the second column... based on expanding or collapsing it, the width of the first column should vary. To be more specific, the contents in the first grid should occupy the whole window when the expander is in collapsed state and when expanded, the first column width should get adjusted to give space for the contents in the expander

<Window x:Class="GridSplitterExample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Width="1200">
    <Grid Style="{DynamicResource BaseGridStyle}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="900"/>
            <ColumnDefinition Width="300"/>
        </Grid.ColumnDefinitions>
        <Grid Width="1100">
            <GroupBox Header="Variable section" Width="1100"/>
        </Grid>
        <Grid Grid.Column="2">
            <Expander  ExpandDirection="Left" ToolTip="Show/Hide TaskPanel">
                <GroupBox Header="Expander section" Width="300"/>
            </Expander>
        </Grid>
    </Grid>
</Window>

The size of the contents in the groupbox also should occupy the whole window if expander is collapsed and shrink if expander is expanded