C# ILMerge Unable to cast object of type 'ClassY' to type 'ClassX'

Question

Here's the situation: I am using ILMerge to merge an assembly & all it's references into 1 .dll file using this method (custom.dll). I have an application which dynamically loads this assembly at runtime (program.exe). Both application & assembly use a common library (common.dll). Abstract class ClassX is defined in common.dll whilst implementation ClassY is defined in custom.dll. When program.exe tries to load ClassY from custom.dll it throws the exception:

Unable to cast object of type 'ClassY' to type 'ClassX'.

The code is like this, but foo is dynamically loaded rather than just a straight new ClassY();

object foo = new ClassY();
ClassX bar = (ClassX)foo;

Does anyone have any idea why it would do this? ClassY definitely implements ClassX, all assembly/library versions are exactly the same.

Answer 1

All the DLLs must be marked COM visible. Without it, the compiler could not resolved different assembly types -- even they are named the same.

Answer 2

Have you checked the inner exception? It's possible that since your ClassY is dynamically loaded that when you do the cast it's trying to load one of the pre-merged assemblies. The inner exception would be "Unable to fin assembly '{you're pre-merged assembly name}'.