asp.net – 如何使用System.Drawing绘制透明图像？

有谁知道我做错了什么？

Protected Sub Page_Load(ByVal sender As Object,ByVal e As System.EventArgs) _
  Handles Me.Load
    '' Change the response headers to output a GIF image.
    Response.Clear()
    Response.ContentType = "image/gif"

    Dim width = 110
    Dim height = width

    '' Create a new 32-bit bitmap image
    Dim b = New Bitmap(width,height)

    '' Create Grahpics object for drawing
    Dim g = Graphics.FromImage(b)

    Dim rect = New Rectangle(0,width - 1,height - 1)

    '' Fill in with Transparent
    Dim tbrush = New System.Drawing.SolidBrush(Color.Transparent)
    g.FillRectangle(tbrush,rect)

    '' Draw Circle Border
    Dim bPen = Pens.Red
    g.DrawPie(bPen,rect,365)

    '' Fill in Circle
    Dim cbrush = New SolidBrush(Color.LightBlue)
    g.FillPie(cbrush,365)


    '' Clean up
    g.Flush()
    g.Dispose()

    '' Make Transparent
    b.MakeTransparent()

    b.Save(Response.OutputStream,Imaging.ImageFormat.Gif)
    Response.Flush()
    Response.End()
End Sub