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