Is there a way to unload or reload the currently loaded runtime?

Question

Ever since we started upgrading some of our projects to .NET framework 4.0, I've been running into a lot of issues regarding the following error:

This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.

It's now happening with our installer application. The installer goes through a list of DLL's and installs the necessary ones on a target machine. Each assembly is loaded into context using

Assembly.LoadFile

The first assemblies in the queue are 3.5. Once the installer picks up a 4.0 DLL and tries to load it using Assembly.LoadFile, I get that error. It seems as though loading the first DLL sets the precedent as to which "runtime" is currently loaded.

Is there a way I can unload this currently loaded runtime before processing a 4.0 DLL in order to avoid this error?

Answer 1

This will depend on what CLR is the executable that is loading the assemblies running. If it runs on CLR 2.0 you won't be able to load a .NET 4.0 assembly. So make sure that if you try to load a .NET 4.0 assembly you are running the application on CLR 4.0. You mention the installer application. There's a possibility to set a launch condition requirement for .NET 4.0:

---

UPDATE:

You could also force your executable to be loaded by the CLR 4.0 by putting this in the app.config file:

<startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>

Because the CLR 4.0 supports loading assemblies compiled against previous versions this will allow you to work around the problem without recompiling the executable against .NET 4.0.