Can you use a .NET 4.0 dll in a 3.5 project?

Question

You can if you don't mind creating a COM wrapper/adapter and calling the .NET 4 DLL through that http://stackoverflow.com/a/9508452/74585 — Matthew Lock, Feb 29 '12 at 23:50
Please check my other answer to the same question: http://stackoverflow.com/questions/16038442/how-can-i-use-net-4-0-code-in-c-sharp-project-which-is-built-using-net-framewo/16040309#16040309 — Aidin, Apr 17 '13 at 19:58

score 26 · Accepted Answer · answered Sep 24 '10 at 21:19

26

Nope. You can use a .Net 3.5 assembly in a 4.0 project, but not the other way around.

answered Sep 24 '10 at 21:19

NotMe

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score 17 · Answer 2 · answered Sep 24 '10 at 21:20

17

No you can't. An assembly compiled against .NET 4.0 can be loaded only by the CLR 4.0. On the other hand the CLR 4.0 can load assemblies compiled against .NET 3.5.

answered Sep 24 '10 at 21:20

Darin Dimitrov

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CLR 4.0 loading a 3.5 assembly surprised and confused me the other day. Scratched my head over that one for a bit. – Jim Mischel Sep 24 '10 at 21:29
1

@Jim, why? If this wasn't possible I cannot even imagine ever migrating to .NET 4.0. Microsoft have always done a good job at keeping backwards compatibility when releasing new versions of the framework/CLR. – Darin Dimitrov Sep 24 '10 at 21:36
I don't know why it surprised me. In retrospect, it shouldn't have. But for some reason at the time I didn't expect it. – Jim Mischel Sep 24 '10 at 21:41

score 0 · Answer 3 · edited Jun 21 '17 at 12:42

https://code.msdn.microsoft.com/Using-a-NET-4-Based-DLL-bb141db3/

Use our .NET 4 DLL via COM

using System; 
using Net4ToNet2Adapter; 

namespace Net2Assembly 
{ 
    class Program 
    { 
        static void Main(string[] args) 
        { 
            Console.WriteLine("CLR version from EXE: {0}", Environment.Version); 
            Type myClassAdapterType = Type.GetTypeFromProgID("Net4ToNet2Adapter.MyClassAdapter"); 
            object myClassAdapterInstance = Activator.CreateInstance(myClassAdapterType); 
            IMyClassAdapter myClassAdapter = (IMyClassAdapter)myClassAdapterInstance; 
            myClassAdapter.DoNet4Action(); 
        } 
    } 

}

Why is this not the accepted answer? I haven't tested it yet, but I will tomorrow. — Arlen Beiler, Nov 19 '19 at 02:57

Can you use a .NET 4.0 dll in a 3.5 project?

3 Answers3

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