Can application built with .NET 4.5 run on .NET 4.0?

Question

My project is targeting to .NET 4.5. It doesn't use any new 4.5 methods, so it actually works fine on the machine with only .NET 4.0 installed.

This is all good until I added some extension methods and reflection. Then when I ran this .NET 4.5 program on the 4.0 machine, it failed with "System.TypeLoadException: Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly mscorlib". The famous ExtensionAttribute program that has been documented well here.

Another quick way to test this is to add the following line. Then the program will throw the exception when running on .NET 4.0 only.

Console.WriteLine(typeof(System.Runtime.CompilerServices.ExtensionAttribute).Assembly.FullName);

I'm wondering if there is a way to work around it. For example, the ILMerge (when using the correct /targetplatform option as documented in the link) actually changes the ExtensionAttribute from mscorlib to System.Core if the project is target to .NET 4.0 (with 4.5 installed). But it doesn't seem to work on the project targeted to .NET 4.5.

I know this is a long shot. But just want to see if anyone has other ideas since it was so close.

Thanks.

Answer 1

In general, this will not work. It does work in some cases since 4.5 is an in place replacement for 4.0, but it's not going to work in general. I've, personally, seen problems with types that have moved into different assemblies, and the bindings aren't setup correctly, just like you're seeing. The reflection types aren't the only types that were moved in 4.5.

My project is targeting to .NET 4.5. It doesn't use any new 4.5 methods, so it actually works fine on the machine with only .NET 4.0 installed.

If this is the case, you could just change your application to target .NET 4.0. This should allow it to run safely on a machine with only .NET 4 installed.