httpWebRequest (The underlying connection was closed: The connection was closed unexpectedly.)
I write a bit about how Fiddler can "magically" fix things here: http://blogs.telerik.com/fiddler/posts/13-02-28/help!-running-fiddler-fixes-my-app-
The issue you're encountering is actually a bug in the .NET Framework itself. The rules of HTTP are such that the server may close a KeepAlive connection at any time after sending the first response (e.g. it doesn't need to accept another request on the connection, even if the client requested KeepAlive behavior).
.NET has a bug where it expects that the server will include a Connection: close response header if it will close the connection after the response is complete. If the server closes the connection without the Connection: Close header (entirely valid per RFC2616), .NET will encounter the closed connection when attempting to send the next request on the connection and it will throw this exception. What .NET should be doing is silently creating a new connection and resending the request on that new connection.
Fiddler resolves this problem because it doesn't care if the server closes the connection, and it keeps the connection to the client alive. When the client sends its second request, Fiddler attempts to reuse its connection to the server, notices that it's closed, and silently creates a new connection.
You can mitigate this problem in your code by:
- Disabling keepalive on the request (this hurts performance)
- Catching the exception and retrying automatically
- Changing the server to keep connections alive longer
Approach #3 only works if you control the server and because the client may be behind a gateway/proxy that closes connections after use, you should probably use approach #2 as well.