Public Class Form1
    ' True while we are drawing the new line.
    Private m_Drawing As Boolean

    ' Buffer for erasing rubberband lines.
    Private m_BufferBitmap As Bitmap
    Private m_BufferGraphics As Graphics

    ' The mouse position.
    Private m_X1 As Integer
    Private m_Y1 As Integer
    Private m_X2 As Integer
    Private m_Y2 As Integer

    ' Start drawing a rubberband line.
    Private Sub Form1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseDown
        ' Do nothing if this isn'tthe left mouse button.
        If e.Button <> MouseButtons.Left Then Exit Sub
        m_Drawing = True

        ' Save a snapshot of the form.
        SaveSnapshot()

        ' Save the current mouse position.
        m_X1 = e.X
        m_X2 = e.X
        m_Y1 = e.Y
        m_Y2 = e.Y
    End Sub

    Private Sub SaveSnapshot()
        Dim new_bitmap As Bitmap

        ' Make a new bitmap that fits the form.
        new_bitmap = New Bitmap(Me.Size.Width, Me.Size.Height, Me.CreateGraphics())
        m_BufferGraphics = Graphics.FromImage(new_bitmap)

        ' Clear the new bitmap.
        m_BufferGraphics.Clear(Me.BackColor)

        ' Copy the existing bitmap's contents into
        ' the new bitmap.
        If Not (m_BufferBitmap Is Nothing) Then
            m_BufferGraphics.DrawImage(m_BufferBitmap, 0, 0)
        End If

        ' Save the new bitmap and graphics objects.
        m_BufferBitmap = new_bitmap
    End Sub

    ' Continue drawing the rubberband line.
    Private Sub Form1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseMove
        ' Do nothing if we're not drawing.
        If Not m_Drawing Then Exit Sub

        ' Save the new point.
        m_X2 = e.X
        m_Y2 = e.Y

        ' Erase the previous line.
        DrawForm(Me.CreateGraphics())

        ' Draw the new line directly on the form.
        Me.CreateGraphics().DrawLine( _
        Pens.Gray, m_X1, m_Y1, m_X2, m_Y2)
    End Sub

    ' Redraw the saved buffer.
    Private Sub DrawForm(ByVal gr As Graphics)
        If Not (m_BufferBitmap Is Nothing) Then gr.DrawImage(m_BufferBitmap, 0, 0)
    End Sub

    ' Finish drawing the new line.
    Private Sub Form1_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseUp
        ' Do nothing if we're not drawing.
        If Not m_Drawing Then Exit Sub
        m_Drawing = False

        ' Save the new point.
        m_X2 = e.X
        m_Y2 = e.Y

        ' Draw the new line permanently on the buffer.
        m_BufferGraphics.DrawLine( _
        Pens.Blue, m_X1, m_Y1, m_X2, m_Y2)

        ' Redraw to show the new line.
        DrawForm(Me.CreateGraphics())
    End Sub

    ' Redraw the form.
    Private Sub Form1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles MyBase.Paint
        DrawForm(e.Graphics)
    End Sub
End Class