Question

If I have a 2.0 CLR assembly (pure managed code, no mixed mode issues) that I need to link to from a 4.0 CLR Application, does the 2.0 code run on the 2.0 CLR or 4.0.

Basically, is there any risk of 4.0 breaking changes affecting the 2.0 code?

Answer 1

The answer above is incorrect. You do get side by side with the full frameworks. A .Net 2 APPLICATION (note that means EXE, not library) will not auto promote to .Net 4.

But if a .Net 4 application loads a .Net 2 assembly it is loaded into the same runtime (otherwise how could they share information). The .Net 2 assembly is loaded into the .net 4 runtime using a compatibility mode that is supposed to minimize breakage in changes (mostly for security changes in .Net 4).

A .Net 2 assembly cannot reference a .Net 4 assembly because it would not have features.

The ONLY exception to this that I know of is if you load a .Net assembly from a C++ app. The C++ application can load and host two runtimes. It could have a .Net 2 assembly and a .Net 4 assembly loaded, but they would not be able to talk to each other directly. This is how CLR Procs work in SQL Server. You can have a .Net 2 CLR Proc and a .Net 4 CLR Proc that do not communicate, but are both loaded on the server.

There was a great article on MSDN Magazine about hosting the .Net framework recently, but I can’t find it now. Maybe someone else can post the link.

So you should be able to load just about any .Net 2 assembly into a .Net 4 executable without much problem. The only problems I have seen are with security permissions.