Have you ever wonder what happens when you create and use breakpoints in .NET? Here’s a little picture that answers that question (if you don’t like the font, you have a different version at the bottom).

We have the main actors here as follows:

  • .NET Application – our regular .NET application that we want to debug. Methods, as provided by the compiler in the Intermediate Language form (IL) are Just-in-Time compiled to a native code when called. So, imagine our “e8 50 ff ff” represents an example binary code of a line we want to debug (no matter what it does now)
  • Debugger Runtime Control Thread (hereinafter referred to as Debugger RC Thread) – it is a special thread inside every .NET process for debugging purposes and serves as a bridge between the CLR and an external debugger. It consists of a so-called “debugger loop”, listening on events coming from the Debug Port (supported by the OS). Please note that in case of native debugging, such a special thread is typically injected into the debuggee process. But we don’t need to do that here, as .NET runtime provides it. And moreover, this thread understand the CLR data structures, so it is able to cooperate with JIT and so.
  • external Debugger – it is our external process that we cooperate with. Imagine it as a tooling part of Visual Studio or other IDE you use. It is using a set of COM objects that are able to communicate via Inter-process communication (IPC) mechanism with Debugger Runtime Control Thread.

Continue reading