Debugging error "The Type 'xx' is defined in an assembly that is not referenced"
The full error is as follows:
The type 'System.Windows.Forms.Control' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
and it points at the very first statement (an Debug.Assert line) in the very first class in a library project that doesn't need System.Windows.Forms (or so I thought). I know how to solve it: add the mentioned reference. But how do I find out what library is causing this error, or better, what part of the code triggers using the WinForms library?
Normally, you can add libraries that reference others, but you only need to add references to these others when they're actually used.
EDIT: Alternative solution
This or similar problems can also be resolved using the Binding Log Viewer Fuslogvw.exe from Microsoft's Framework Tools. It shows all attempts and successes of assemblies your application binds to.
Answer
I suspect there's no line of your code that's causing this, since you say you aren't making use of the System.Windows.Forms types and the compiler error isn't pointing to a (useful) line of your code.
What I think is happening is that you're referencing a library which has a publicly-visible method or property that either returns a System.Windows.Forms.Control or takes one as a parameter. It doesn't matter whether you actually end up calling that method/property, the fact that it's publically visible means that your own code has to be able to resolve all the types that the library is using. If the library only used System.Windows.Forms internally, you wouldn't be experiencing this.
It also means just looking at the dependencies of the assemblies you're depending on may merely narrow down the list of suspects, since there could be some assemblies that depend on System.Windows.Forms internally (no problem) and the one troublemaking assembly that has a public parameter / return value of a type from the S.W.Forms assembly.
My suggestion is you just set up an empty project without a reference to S.W.Forms, then add each of your dependencies in turn and try to compile after each one.