在Canvas中使控件居中

经常会遇到要在Canvas里将各类控件居中的问题，下面以TextBlock为例说明。

//在将TextBlock添加到Canvas前获取它的实际尺寸
TextBlock label = new TextBlock;
label.Text = "It is a test";
label.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
Size labelSize = label.DesiredSize;

Canvas canvas = new Canvas();
canvas.Width = 100;
canvas.Height = 40;
canvas.Children.Add(label);
Canvas.SetLeft(label, (canvas.Width - labelSize.Width)/2);
Canvas.SetTop(label, (canvas.Height - labelSize.Height)/2);