How did Microsoft create assemblies that have circular references?

Question

In the .NET BCL there are circular references between:

• System.dll and System.Xml.dll
• System.dll and System.Configuration.dll
• System.Xml.dll and System.Configuration.dll

Here's a screenshot from .NET Reflector that shows what I mean:

How Microsoft created these assemblies is a mystery to me. Is a special compilation process required to allow this? I imagine something interesting is going on here.

Answer 1

I can only tell how the Mono Project does this. The theorem is quite simple, though it gives a code mess.

They first compile System.Configuration.dll, without the part needing the reference to System.Xml.dll. After this, they compile System.Xml.dll the normal way. Now comes the magic. They recompile System.configuration.dll, with the part needing the reference to System.Xml.dll. Now there's a successful compilation with the circular reference.

In short:

• A is compiled without the code needing B and the reference to B.
• B is compiled.
• A is recompiled.

Answer 2

RBarryYoung and Dykam are onto something. Microsoft uses internal tool which uses ILDASM to disassemble assemblies, strip all internal/private stuff and method bodies and recompile IL again (using ILASM) into what is called 'dehydrated assembly' or metadata assembly. This is done every time public interface of assembly is changed.

During the build, metadata assemblies are used instead of real ones. That way cycle is broken.