How to scale a shape around the mouse cursor location in a wpf application?

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C#

WPF

C#4.0

I was developing a CAD-like application recently with WPF. The panning and dragging are implemented in a scrollviewer which contains a canvas. All the shapes are drew in the canvas. Now I want to scale the shape in canvas but the point under the cursor has a offset after scaled.I just want the shape can scale around the mouse cursor location and can be written in C# code-only without xaml. Here is my code:

<local:sc x:Name="sc1" Height="263" HorizontalAlignment="Left"  Margin="51,34,0,0" VerticalAlignment="Top" Width="477" HorizontalScrollBarVisibility="Visible" VerticalScrollBarVisibility="Visible" Grid.ColumnSpan="2">
            <Canvas Name="canvas2" Height="337" Width="632" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                <Canvas.Background>
                    <LinearGradientBrush EndPoint="1,0.5" StartPoint="0,0.5">
                        <GradientStop Color="Black" Offset="0" />
                        <GradientStop Color="White" Offset="1" />
                    </LinearGradientBrush>
                </Canvas.Background>
                <Canvas.RenderTransform>
                    <TransformGroup>
                        <ScaleTransform ScaleY="-1"/>
                        <TranslateTransform Y="{Binding ElementName=canvas2,Path=Height}"/>
                    </TransformGroup>
                </Canvas.RenderTransform>
                <Label Content="duancong" Foreground="White" Canvas.Top="200">
                    <Label.RenderTransform>
                        <ScaleTransform ScaleY="-1"/>
                    </Label.RenderTransform>
                </Label>
            </Canvas>
        </local:sc>

And here is the behind code:

void canvas2_MouseWheel(object sender, MouseWheelEventArgs e)
        {

            if (e.Delta > 0)
                this.scale *= 1.1;
            if (e.Delta < 0)
                this.scale *= 0.9;

            label2.Content = scale;
            scaler();
        }

    void scaler()
        {
            canvas2.Width = sc1.Width * scale;
            canvas2.Height = sc1.Height * scale;
            l1.Scale = scale;
        }