为什么我的矩形没有绘制在图片框中？ （拖动）

Question

因此，我尝试通过在表单中​​拖动鼠标来绘制矩形，并且成功了，但是当我尝试在图片框中以相同的方式绘制时，没有创建矩形。

Private Sub PictureBox1_MouseMove(sender As Object, e As MouseEventArgs) Handles PictureBox1.MouseMove

        If fGMouseIsDown And Not PictureBox1.Image Is Nothing Then

            rect.Width = e.X - rect.X
            rect.Height = e.Y - rect.Y
             Invalidate()


        End If
    End Sub

Private Sub PictureBox1_MouseDown(sender As Object, e As MouseEventArgs) Handles PictureBox1.MouseDown

        fGMouseIsDown = True
        rect.Location = e.Location
        rect.Width = 0
        rect.Height = 0
        Invalidate()
    End Sub 

Private Sub PictureBox1_Paint(sender As Object, e As PaintEventArgs) Handles PictureBox1.Paint

        e.Graphics.DrawRectangle(Pens.Blue, rect)

End Sub

Answer 1

Per @HansPassant：当您想要使图片框无效时，PictureBox1_MouseDown() 方法中的 Invalidate() 调用会使表单无效。

那个调用应该是：

PictureBox1.Invalidate()

此外，请确保以正确的方式拖动；这仅在您从左上角转到右下角时有效。