图片框上的MouseDown错误: Handles子句需要WithEvents

Question

我是VB的新手--在下面的代码中，我得到了这个错误。

Handles子句需要在包含类型或其基类型之一中定义的WithEvents变量。(BC30506)

Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown

基本上，我正在尝试移动一个带有mousedown事件的图片框对象，就像这段代码一样

  Private Offset As Size 'used to hold the distance of the mouse`s X and Y position to the picturebox Left and Top postition

Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown
    Dim ptc As Point = Me.PointToClient(MousePosition) 'get the mouse position in client coordinates
    Offset.Width = ptc.X - PictureBox1.Left 'get the width distance of the mouse X position to the picturebox`s Left position
    Offset.Height = ptc.Y - PictureBox1.Top 'get the height distance of the mouse Y position to the picturebox`s Top position
End Sub

Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseMove
    If e.Button = MouseButtons.Left Then 'make sure the left mouse button is down first
        Dim ptc As Point = Me.PointToClient(MousePosition) 'get the mouse position in client coordinates
        PictureBox1.Left = ptc.X - Offset.Width 'set the Left position of picturebox to the mouse position - the offset width distance
        PictureBox1.Top = ptc.Y - Offset.Height 'set the Top position of picturebox to the mouse position - the offset height distance
    End If
End Sub

我已经读过其他问题了，似乎不能准确地理解为什么这段代码不能工作。

Answer 1

为了使用您的代码，您需要将PictureBox1声明为Friend WithEvents，就像设计器创建的每个控件一样：

Friend WithEvents PictureBox1

但是您只能在模块、接口或名称空间级别使用Friend。因此，Friend元素的声明上下文必须是源文件、名称空间、接口、模块、类或结构；不能是过程。

如果无法在运行时使用此声明创建控件，则必须使用AddHandler将事件与事件处理程序相关联。

例如：

AddHandler Me.PictureBox1.MouseDown, AddressOf PictureBox1_MouseDown
AddHandler Me.PictureBox1.MouseMove, AddressOf PictureBox1_MouseMove

您还必须从过程声明中删除Handles；即：

Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs)
    'your code here
End Sub

提示:如果需要，可以为事件处理程序使用更好的名称，而不是默认名称；例如：

AddHandler Me.PictureBox1.MouseMove, AddressOf movePictureBoxOnMouseLeftClick

Private Sub movePictureBoxOnMouseLeftClick(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs)
    'your code here
End Sub