Cannot render image to HttpContext.Response.OutputStream

Question

Basically I am trying to render a simple image in an ASP.NET handler:

public void ProcessRequest (HttpContext context)
{
    Bitmap image = new Bitmap(16, 16);
    Graphics graph = Graphics.FromImage(image);

    graph.FillEllipse(Brushes.Green, 0, 0, 16, 16);

    context.Response.ContentType = "image/png";
    image.Save(context.Response.OutputStream, ImageFormat.Png);
}

But I get the following exception:

System.Runtime.InteropServices.ExternalException: A generic error
occurred in GDI+.
    at System.Drawing.Image.Save(Stream stream, ImageCodecInfo encoder,
    EncoderParameters encoderParams)

The solution is to use this instead of having image write to OutputStream:

MemoryStream temp = new MemoryStream();
image.Save(temp, ImageFormat.Png);
byte[] buffer = temp.GetBuffer();
context.Response.OutputStream.Write(buffer, 0, buffer.Length);

So I'm just curious as to why the first variant is problematic?

Edit: The HRESULT is 80004005 which is just "generic".

Answer 1

The writer indeed needs to seek to write in the stream properly.

But in your last source code, make sure that you do use either MemoryStream.ToArray() to get the proper data or, if you do not want to copy the data, use MemoryStream.GetBuffer() with MemoryStream.Length and not the length of the returned array.

GetBuffer will return the internal buffer used by the MemoryStream, and its length generally greater than the length of the data that has been written to the stream.

This will avoid you to send garbage at the end of the stream, and not mess up some strict image decoder that would not tolerate trailing garbage. (And transfer less data...)