Add a Reference from a C# App to a DLL compiled without /clr?

Question

I'm using Visual Studio 2008 to build a Solution with two Projects: a C# Console App and a C++ DLL. I want the app to call a function from the dll using P/Invoke. Therefore I'm trying to add the dll as a Reference to the C# app. But when I try the Add Reference command, Visual Studio won't let me do it unless I set the /clr property on the dll (under Configuration Properties:General). Now, I thought that P/Invoke could handle plain-old win32 dlls. Indeed, if I build my dll without /clr and just copy it by hand to bin/Debug, then the app runs fine. So why is /clr required to add the dll as a reference? And if VS won't let me add it, is there some (clean) workaround so that my app finds the dll?

I see that someone had a similar issue here (though with a 3rd-party dll): http://stackoverflow.com/questions/1377019/unable-to-add-a-dll-reference-to-vs-2008 The answer he got was to build a wrapper. But this isn't really necessary, since the app can use the dll just fine; it's just the Add Reference step that doesn't work. And besides, won't the wrapper code need a reference to the dll, raising the same problem as before? I'd really like an answer that doesn't involve writing a wrapper at all.

Answer 1

When using PInvoke on a C++ DLL, it is not necessary to add a reference. References are only needed when you are calling managed code in another DLL. Simply put the C++ DLL in the same directory and add it's name to the DllImport attribute

Answer 2

Why not just add a post-build step to copy your unmanaged DLL to your project directory? You don't need a "reference" to be able to refer to an unmanaged DLL, and it sounds like the only problem you're experiencing is due to the file not being automatically copied into the search path.

Answer 3

Theoretically you could add the C++-DLL as a linked resource to your C#-DLL. This would make .NET copy your C++-DLL wherever it copies the C#-DLL even into the GAC. Theoretically means that there are some disadvantages:

• You could instruct the C# compiler (csc.exe) via command line options to add the linked resource but I never found a way to do it via .csproj file or even Visual Studio
• If the C#-DLL contains WinForms user controls and you want to use it in the designer it wont work because the Winforms-Designer copies the C#-DLL to a temporary location where it loads it but it ignores the linked resource.
• I don't know if it works if you put your C#-DLL into the GAC (yes the C++-DLL would also go into as a resource, but I don't know if it is found when running your C#-DLL!)

So if none of the above is a no-go for you you can call csc.exe the following:

csc.exe ... /linkresource:cpp.dll

Hope this may help!