用vb.net 写一个屏幕画框的程序

Public Class Form1 Dim m_IsMouseDown As Boolean = False Dim m_StartPoint As Point Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown m_IsMouseDown = True m_StartPoint = New Point(e.X, e.Y) End Sub Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseMove If (m_IsMouseDown) Then '如果鼠标左键按下，则绘制图形 Dim g As Graphics = Me.CreateGraphics() '创建Graphics对象 g.DrawRectangle(Pens.Blue, _ '绘制一个蓝色的方框 Math.Min(m_StartPoint .X, e .X), _ '左上角X坐标 Math .Min (m_StartPoint .Y ,e .Y), _ '左上角Y坐标 Math .Abs (e .X -m _StartPoint .X ) , _ '宽度：取两点之间的绝对值 Math .Abs (e .Y -m _StartPoint .Y )) '高度：取两点之间的绝对值 g = Nothing '释放Graphics对象所占用的内存区域 End If End Sub Private Sub PictureBox1_MouseUp(ByVal sender As Object, ByVal e As System