我的拖放代码出了什么问题......（Silverlight 4）

Question

我在Silverlight 4中对我的RichTextBox有拖放行为，但我遇到了一个小的捕捉问题。

我希望用户能够“抓住”RichTextBox的任何边框并拖放它。 RichTextBox应该相对于用户“抓住”RichTextBox的位置进行拖动。但相反，只要用户开始拖动，RichTextBox就会“捕捉”到鼠标位置的中间位置而不是相对于鼠标位置的位置。

所以，如果我抓住右下角，如下所示......

https://skydrive.live.com/redir?resid=DCC93DD825EF3F43!658

它会在拖动开始时“捕捉”到鼠标位置的中间位置，如下所示......

https://skydrive.live.com/redir?resid=DCC93DD825EF3F43!659

这是我的拖动代码。我假设我的数学错误（在MouseMove事件中）???

public class CustomRichTextBox : RichTextBox
{
    private bool isDragging = false;

    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
    {
        base.OnMouseLeftButtonDown(e);

        bool isDragAllowed = false;
        Point pt = e.GetPosition(this);

        if (pt.Y >= 0 && pt.Y <= this.BorderThickness.Top)
            // Allow dragging if the mouse is down on the top border
            isDragAllowed = true;
        else if (pt.Y >= (this.RenderSize.Height - this.BorderThickness.Bottom) && pt.Y <= this.RenderSize.Height)
            // Allow dragging if the mouse is down on the bottom border
            isDragAllowed = true;
        else if (pt.X >= 0 && pt.X <= this.BorderThickness.Left)
            // Allow dragging if the mouse is down on the left border
            isDragAllowed = true;
        else if (pt.X >= (this.RenderSize.Width - this.BorderThickness.Right) && pt.X <= this.RenderSize.Width)
            // Allow dragging if the mouse is down on the right border
            isDragAllowed = true;

        if (!isDragAllowed)
            return;

        this.CaptureMouse();
        isDragging = true;
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);

        if (isDragging)
        {
            CustomRichTextBox elem = this;
            CompositeTransform ct = (CompositeTransform)elem.RenderTransform;
            UIElement parent = (UIElement)elem.Parent;

            ct.TranslateX = e.GetPosition(parent).X - (parent.RenderSize.Width / 2);
            ct.TranslateY = e.GetPosition(parent).Y - (parent.RenderSize.Height / 2);
        }
    }

    protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
    {
        base.OnMouseLeftButtonUp(e);

        isDragging = false;
        this.ReleaseMouseCapture();
    }
}