How Moles Isolation framework is implemented?

Question

Moles is an isolation framework created by Microsoft. A cool feature of Moles is that it can "mock" static/non-virtual methods and sealed classes (which is not possible with frameworks like Moq). Below is the quick demonstration of what Moles can do:

Assert.AreNotEqual(new DateTime(2012, 1, 1), DateTime.Now);

// MDateTime is part of Moles; the below will "override" DateTime.Now's behavior
MDateTime.NowGet = () => new DateTime(2012, 1, 1); 
Assert.AreEqual(new DateTime(2012, 1, 1), DateTime.Now);

Seems like Moles is able to modify the CIL body of things like DateTime.Now at runtime. Since Moles isn't open-source, I'm curious to know which mechanism Moles uses in order to modify methods' CIL at runtime. Can anyone shed any light?

Answer 1

Moles implements a CLR profiler (in particular the ICorProfilerCallback interface) that allows to rewrite MSIL method bodies before they are compiled into assembly code by the .NET runtime. This is done in particular through the JitCompileStarted callback.

In each method, Moles introduces a detour that looks like this:

static struct DateTime 
{
    static DateTime Now
    {
        get 
        {
            Func<DateTime> d = __Detours.GetDelegate(
                null, // this point null in static methods
                methodof(here) // current method token
                );
            if(d != null)
                return d();
            ... // original body
        }
    }
}

When you set a mole, your delegate is stored in the underlying __Detours dictionary which gets looked up whenver the method is executed.