模拟视觉拖/放（此代码有什么问题？）

Question

我有一个 WPF 4/VB.net 2010 项目，我正在尝试进行可视化“拖放”（让一个对象在 MouseUp 上跟随鼠标并在 MouseDown 上“粘住”。）我有以下代码：

Private Sub Tile1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Input.MouseButtonEventArgs) Handles Tile1.MouseDown
    Tile1.CaptureMouse()
    IsDragging = True
End Sub

Private Sub Tile1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Input.MouseEventArgs) Handles Tile1.MouseMove
    If IsDragging = True Then
        Dim canvPosToWindow As Point = canv.TransformToAncestor(Me).Transform(New Point(0, 0))

        Dim r As Rectangle = TryCast(sender, Rectangle)
        Dim upperlimit As Double = canvPosToWindow.Y + (r.Height / 2)
        Dim lowerlimit As Double = canvPosToWindow.Y + canv.ActualHeight - (r.Height / 2)

        Dim leftlimit As Double = canvPosToWindow.X + (r.Width / 2)
        Dim rightlimit As Double = canvPosToWindow.X + canv.ActualWidth - (r.Width / 2)

        Dim absmouseXpos As Double = e.GetPosition(Me).X
        Dim absmouseYpos As Double = e.GetPosition(Me).Y

        If (absmouseXpos > leftlimit AndAlso absmouseXpos < rightlimit) AndAlso (absmouseYpos > upperlimit AndAlso absmouseYpos < lowerlimit) Then
            r.SetValue(Canvas.LeftProperty, e.GetPosition(canv).X - (r.Width / 2))
            r.SetValue(Canvas.TopProperty, e.GetPosition(canv).Y - (r.Height / 2))
        End If
    End If
End Sub

Private Sub Tile1_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Input.MouseButtonEventArgs) Handles Tile1.MouseUp
    Tile1.ReleaseMouseCapture()
    IsDragging = False
End Sub

我收到此错误：

NullReferenceException，未将对象引用设置为对象的实例。

在以下每一行中：

 Dim upperlimit As Double = canvPosToWindow.Y + (r.Height / 2)
 Dim lowerlimit As Double = canvPosToWindow.Y + canv.ActualHeight - (r.Height / 2)

 Dim leftlimit As Double = canvPosToWindow.X + (r.Width / 2)
 Dim rightlimit As Double = canvPosToWindow.X + canv.ActualWidth - (r.Width / 2)

我做错了什么？

Answer 1

我怀疑这个语句使 r “空”而不是一个正确的矩形。

Dim r As Rectangle = TryCast(sender, Rectangle)

发送者可能不是一个矩形，而可能是一个 WPF-Control（System.Windows.Controls.Control 的某个子类）。

您可以通过在该行上放置一个调试点来轻松地使用 Visual Studio 调试器进行检查，并查看该语句是否计算结果。

祝您的拖放实施顺利:)。