WPF画布中的形状不显示

我遇到了一个非常奇怪的问题,在WPF的画布上绘制形状。

<DockPanel Grid.Row="3">
            <Canvas Name="BottomCanvas" Margin="15" Background="Yellow">
                <Canvas Name="TransparentCanvas" Background="Transparent"
                        MouseDown="TransparentCanvas_MouseDown"
                        MouseUp="TransparentCanvas_MouseUp"
                        MouseMove="TransparentCanvas_MouseMove"
                       Width="{Binding RelativeSource={RelativeSource FindAncestor, 
                        AncestorType={x:Type Canvas}},Path=ActualWidth}"
                        Height="{Binding RelativeSource={RelativeSource FindAncestor, 
                        AncestorType={x:Type Canvas}},Path=ActualHeight}">
                </Canvas>
            </Canvas>

        </DockPanel>

上面是我定义的画布。

                Ellipse myEllipse = new Ellipse();
                SolidColorBrush mySolidColorBrush = new SolidColorBrush();
                mySolidColorBrush.Color = Color.FromArgb(0, 0, 255, 0);
                myEllipse.Fill = mySolidColorBrush;
                myEllipse.Width = myEllipse.Height = 100;
                Canvas.SetTop(myEllipse, 15);
                Canvas.SetLeft(myEllipse, 15); ;
                bottomCanvas.Children.Add(myEllipse);

有人知道怎么了吗?

bottomCanvas 这是添加到 MainWindow 我不知道为什么。这就是交易。