# Monday, 15 December 2014

Strangest bug of the day

Could not load type 'System.Diagnostic.Debug'
from assembly 'MyAssembly'

Uhm... Hello?
Why are you looking for 'System.Diagnostic.Debug' inside my assembly? Go look inside System!

To be fair, I was fiddling with the CLR Hosting APIs, in particular IHostAssemblyStore.
Using this interface, you can override the CLR assemby probing logic, and load you own assemblies from wherever you want.
This is particularly helpful if you are using your own AppDomainManager, and you want to load it inside your sandboxed domain, which you carefully configured to disallow loading of any external assembly.
If you try to do that, binding fails, obviously. And since the AppDomainManager is created and loaded by the unmanaged portion of the CLR, you don't get a chance to use AssemblyResolve (tried, didn't work).

But that does not really justify this behavior: the Fusion log viewer was reporting no errors, and the exception was wrong (or, at least, not one of the usual exceptions you should look for when debugging binging failures).

There is a tiny parameter, called pAssemblyId. The docs are quite clear: it is used internally to see if that assembly was already loaded.
If it is, do not try to load it again. Sounds like a good optimization BUT:

The docs don't say it, but if you pass back "0", something very peculiar happens.
In my case, I had the Jitter looking for types inside my assembly. Others had different issues.

That was definitely the strangest bug of the day :)