Expander control in WPF

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.NET4

Hi,
Please tell how to make expander control to expand Horizontally from right to left.
I have tried with following Xaml code but it is not working

<canvas height="338" name="canvas1" width="280">
        <expander canvas.left="10" canvas.top="10" header="expander1" height="100" name="expander1" expanddirection="Right">
            <stackpanel height="100" name="stackPanel1" width="200">
                &lt;Button Content="Button" Height="23" Name="button1" Width="75" /&gt;
            </stackpanel>
        </expander>
    </canvas>

please help me out....