vb.net用点画直线的问题

[dzweather]

找了半天,vb.net没的直接画点的东西。只有图像有一个setpixel

下面用画像素是1的矩形，来代替点，然后循环画出一条直线。

但是，画后，立即就闪电消失，什么原因？

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim bp As New Bitmap(PictureBox1.Width, PictureBox1.Height, Imaging.PixelFormat.Format32bppPArgb)
        Dim gr As Graphics = PictureBox1.CreateGraphics
        For i As Int32 = 0 To 200
            gr.DrawRectangle(Pens.Red, i, 40, 1, 1)
        Next
        PictureBox1.Image = bp
    End Sub