Fiddler throws an out-of-memory exception

Sometimes, Fiddler may show a dialog containing the following text:

  Exception of type 'System.OutOfMemoryException' was thrown.
     at System.IO.MemoryStream.set_Capacity(Int32 value)
     at System.IO.MemoryStream.EnsureCapacity(Int32 value)
     at System.IO.MemoryStream.Write(Byte[] buffer, Int32 offset, Int32 count)
     at Fiddler.Session.Execute(Object objThreadstate)

Fiddler works by storing the entire request and response in memory. If you are performing a huge download (hundreds of megabytes) it's possible that Fiddler cannot find a free memory block large enough to hold the entire contiguous response, and hence you'll run into this "out of memory" problem. It's also possible that if you have thousands of sessions in the Fiddler session list, even a relatively small memory block will not be available to store a response a few megabytes in size. You can reduce the incidence of this problem by clearing the Web Sessions list (CTRL+X) or configuring it to automatically trim to the most recent two hundred sessions (Click the Filters tab, and click the "Keep only the most recent sessions" option at the bottom).

Developers can learn more about this here: http://blogs.msdn.com/ericlippert/archive/2009/06/08/out-of-memory-does-not-refer-to-physical-memory.aspx and here http://blogs.msdn.com/b/dotnet/archive/2011/10/04/large-object-heap-improvements-in-net-4-5.aspx.

Update: Fiddler2 now supports running on 64bit computers. If you're on a 64-bit machine, you'll never hit a problem.

If you're on a 32-bit machine, you can avoid out-of-memory errors when downloading huge files by adding the following code inside the OnPeekAtResponseHeaders function inside Rules > Customize Rules. The line in red will cause Fiddler not to keep a copy of the large file:

// This block enables streaming for files larger than 5mb
if (oSession.oResponse.headers.Exists("Content-Length"))
{
  var sLen = oSession.oResponse["Content-Length"];
  var iLen: Int32 = 0;
  if (!isNaN(sLen)){ 
    iLen = parseInt(sLen); 
    if (iLen > 5120000) {
      oSession.bBufferResponse = false; 
      oSession["ui-color"] = "yellow";
      oSession["log-drop-response-body"] = "save memory";
    }
  }
}

If you're using FiddlerCore or writing a Fiddler Extension, you can use code like this:

       Fiddler.FiddlerApplication.ResponseHeadersAvailable += delegate(Fiddler.Session oS)
       {
          // This block enables streaming for files larger than 5mb
            if (oS.oResponse.headers.Exists("Content-Length"))
            {
                int iLen = 0;
                if (int.TryParse(oS.oResponse["Content-Length"], out iLen))
                {
                    // File larger than 5mb? Don't save its content
                    if (iLen > 5000000)
                    {
                        oS.bBufferResponse = false;
                        oS["log-drop-response-body"] = "save memory";
                    }
                }
            }
        };