Fiddler Internals

Fiddler's source is still not public, but there were two columns in August's MSDN magazine that describe a lot of the interesting techniques used by Fiddler.

See: Take the Burden Off Users with Automatic Configuration in .NET for information on using Script Engines in .NET, as well as detailed information on how the Framework decides what proxy to use.

See: Get Closer to the Wire with High-Performance Sockets in .NET for techniques on writing a server (even a proxy server) in .NET. Fiddler uses the background-threadpool technique described, which works fine for the load Fiddler needs to handle.