ExecutionEngineException in diagnostic source listener

[rynowak]

You can cause the CLR to crash by using an invalid combination of types.

new { bar = new Guid() }

...

[DiagnosticName(...)]
public void OnFoo (string bar)
{
    ...
}

This is not valid, but the failure mode should be throwing an exception explaining the mistake, not shutting down the CLR 😆

---

The issue here is that the generated thunk for splatting the type isn't doing the right thing when the result of accessing the anonymoustype<>.bar property is a value type.

Adding the following at ProxyMethodEmitter@L160 fixing the issue

if (mapping.PropertyType.GetTypeInfo().IsValueType)
{
    il.Emit(OpCodes.Box, mapping.PropertyType);
}

Additionally, we should see if we can do anything to surface errors about this with context as it is today when you fix the bug, the exception message you get doesn't mention the method on the adapter which caused the problem.

[Eilon]

Please add tests as well.

[ryanbrandenburg]

The error message part of this turns out to be more complicated than fixing the issue, so I split it off into #55 to avoid confusing the issue.