WPF Not sending MouseMove events after CaptureMouse();

Question

I'm trying to have a WPF canvas with rounded rectangles on that I can drag round using the mouse. However once I try and capture the mouse on the canvas I don't get the move events any more.

This is a "mycanvas" user control and the rectangles are "foo" user controls. The XAML for these (minus the preamble) are:

mycanvas.xaml:

<Canvas MouseDown="CanvasMouseDown" MouseMove="CanvasMouseMove" MouseUp="CanvasMouseUp" Background="White">

    <my:Foo HorizontalAlignment="Left" Canvas.Left="97" Canvas.Top="30" x:Name="m_foo" VerticalAlignment="Top" Height="87" Width="128" />
</Canvas>

foo.xaml:

<Border BorderThickness="2" BorderBrush="Black" CornerRadius="15" Background="Plum">
    <Grid>
        <Label Content="Foo" Height="28" HorizontalAlignment="Left" Margin="6,6,0,0" Name="label1" VerticalAlignment="Top" />
    </Grid>
</Border>

And then the handlers are: mycanvas.xaml.cs:

private void CanvasMouseDown(object sender, MouseButtonEventArgs e)
{
    if (e.Source is Foo)
    {
        m_moving = e.Source as Foo;
        CaptureMouse();
        e.Handled = true;
    }
}

private void CanvasMouseMove(object sender, MouseEventArgs e)
{
    if (m_moving != null)
    {
        Canvas.SetLeft(m_moving, e.GetPosition(this).X);
        Canvas.SetTop(m_moving, e.GetPosition(this).Y);
    }
}

private void CanvasMouseUp(object sender, MouseButtonEventArgs e)
{
    ReleaseMouseCapture();
    m_moving = null;
}

The MouseDown fires and so the CaptureMouse gets called (and works because I can no longer close the app or click anything else in it!) but the MouseMove never gets called anymore - so where do the MouseMove events get sent now???

If I alt-tab to another application and then go back now suddendly the MouseMove is called and the Foo moves with the mouse.

Answer 1

Try either:

Mouse.Capture(this, CaptureMode.SubTree);

or

m_moving.CaptureMouse();
...
if (m_moving != null)
{
    m_moving.ReleaseMouseCapture();
    m_moving = null;
}

The mouse events were being raised by the Foo, not by the Canvas, so when you capture the mouse with the Canvas you prevent them from being raised.