It's true. I recently explored In-process side-by-side, a feature where you can call legacy .NET 3.5 code running in the 3.5 VM from code running in .NET 4.5. I

It's true. I recently explored In-process side-by-side, a feature where you can call legacy .NET 3.5 code running in the 3.5 VM from code running in .NET 4.5. It does this by spinning up two copies of the CLR. We figured this would be awesome, as we wouldn't have to port the old libraries to .NET 4.5 (and yes - porting would be required. Sadly that library was written very poorly and used reflection on private methods that didn't exist in 4.5, among other uglyness). The long and short of it though is the way you "call into" the .NET 3.5 code is through COM. This had lots of implications, such as no support for generics, lots of difficulty debugging, etc. We decided to just bite the bullet and upgrade the old lib instead, and I think we made the right choice. COM is a mess.