Using C# DLL in managed C++ (command line)

Question

I've looked over a few questions with a title just like this one, but they either do not talk about command line, or don't seem to work for me for some reason. From what I have read, it seemed as if I could "simply" do the following:

The dummy code (C#):

using System;

public static class Foo {
    public static void Bar() {
        Console.WriteLine("o3o");
    }
}

More dummy code (Visual C++):

#using <test.dll>

int main() {
    Foo::Bar();
    return 0;
}

C# DLL compiled using:

csc /out:test.dll /t:library src\cs\Foo.cs

Visual C++ object file compiled using:

cl /Ox /clr /AI. /c src\vc\test.cpp

Executable compiled using:

link /out:test.exe test.obj

The following exception is thrown upon running the executable:

Unhandled Exception: System.TypeLoadException: Could not load type 'Foo' from assembly 'test, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
   at main()
   at mainCRTStartup()

I have a strong feeling that I was supposed to also reference the DLL in the link phase, but I couldn't find any option for linking a DLL similar to GCC's -l option. Attempting to pass the DLL along with the .obj to link causes it to tell me that linking assemblies is not supported. Interestingly, though, if I build a .netmodule instead of a DLL, i.e. by:

csc /out:test.dll /t:module src\cs\Foo.cs

and changing the #using directive to

#using <test.netmodule>

the executable runs without error. This feels a bit weird, for I don't think anybody packages code in .netmodules (what are .netmodules, anyway?).

Think of a .netmodule as the equivalent of a .lib in plain C++, that's why you can link with it. Managed code is somewhat different, you don't just link .dll files together. Why are you trying to link them in the first place? Your C++/CLI exe can just reference the C# dll without linking with it, just put the dll in the same directory as the exe. — Lucas Trzesniewski
– Lucas Trzesniewski, Commented Feb 6, 2017 at 16:20
The DLL does reside in the same directory as the executable, but the aforementioned exception is thrown upon running it. — Mona the Monad
– Mona the Monad, Commented Feb 6, 2017 at 16:24
Ok I looked at the issue and posted an answer. I just wanted to make sure you didn't get caught by the #1 trap when mixing C# and C++/CLI first :) (the default configuration of the IDE is kind of broken - it makes it hard to do this for no reason by not putting the output files in the same place - as you have used directories in your commands I figured you could have had the same issue, but then I realized what /AI. was) — Lucas Trzesniewski
– Lucas Trzesniewski, Commented Feb 6, 2017 at 16:50

Lucas Trzesniewski · Accepted Answer · 2017-02-06 16:45:19Z

All of your steps should have worked, but one very simple issue is preventing your program from running.

Namely: Your C# DLL assembly name is test, and your C++/CLI exe assembly has the same name. They both have the same identity.

So when looking for Foo::Bar in the test assembly, the loader first checks if the assembly is loaded in the AppDomain. It is - it's your C++/CLI exe, and you can't have several assemblies with the same identity loaded simultaneously within the same AppDomain. Your C# dll wasn't even given a try.

Just change either one of them and everything will work fine:

link /out:test2.exe test.obj

As for what's a .netmodule, it's the format used for linking managed code statically, that's why you managed to link your C# code with your C++/CLI code without issues. It's roughly the equivalent of a .lib file for managed code.
And you're right, it's not used very often.

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