怎样做到messagebox不覆盖picturebox的内容？

[yinduasan111]

Dim oldpt As New Point(100, 100)
Dim newpt As New Point(500, 100)
Public Cache As New Bitmap(500, 500)
Public CacheG As Graphics

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
DrawImage()
End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
MessageBox.Show("The calculations are complete", "msg", MessageBoxButtons.OK, MessageBoxIcon.Asterisk)
End Sub

Private Function DrawImage()
PictureBox1.CreateGraphics.DrawLine(New Pen(Color.Black, 1), oldpt, newpt)
End Function

Protected Overrides Sub OnPaint(ByVal e As System.Windows.Forms.PaintEventArgs)
DrawImage()
End Sub

以上是我按照这个论坛里一篇帖子里抄来的，Button1_Click在PictureBox1里画一条直线，Button2_Click弹出 messagebox。虽然我发现在移动messagebox时候没有问题，但是如果最后点击OK的时候如果messagebox还在直线上，那么messagebox消失以后，直线被遮盖的部份也随着消失了。

[yinduasan111]

那怎么把直线画在image里呢，如果原来之间是在picturebox里画的，程序是否需要很大改动呢？

[Brunhild]

改动不大，在DrawImage中改就可以了：
With (Me.PictureBox1)
If Me.PictureBox1.Image Is Nothing Then
Dim bm As New System.Drawing.Bitmap(.Width, .Height)
.Image = bm
End If

Dim graphics As System.Drawing.Graphics = System.Drawing.Graphics.FromImage(.Image)
graphics.DrawLine(System.Drawing.Pens.Black, 0, 0, 100, 100)
End With

[citylamp]

那就再添加MessageBox（）按下OK以后的重画嘛。
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
MessageBox.Show("The calculations are complete", "msg", MessageBoxButtons.OK, MessageBoxIcon.Asterisk)
Me.Refresh() ‘先清空
DrawImage() ‘再重画
End Sub

[Brunhild]

因为DrawImage绘画的直线并没有重画的机制，一但刷新所在区域就没有了。解决方法有：
1. 在picturebox的paint事件中重画直线
2. 将直线画在image中，用picturebox显示此image