C# DllImport library written on C++

Question

I'm pretty new in C#. I have Dll written on C++ by myself and I'm going to use functions from that dll in C# app.

So, I do the following when declare functions in C++ project:

public static __declspec(dllexport) int captureCamera(int& captureId);

Then I'm trying to import this method in C# app:

[DllImport("MyLib.dll")]
public static extern int captureCamera(ref int captureId);

But I have the exception:

Unable to find an entry point named 'captureCamera' in DLL 'MyLib.dll'.

The task is to do dllimport without specifying EntryPoint parameter. Could anyone help me?

Answer 1

public static __declspec(dllexport) int captureCamera(int& captureId);

is that method? If it is function, it cannot be static, as static and dllexport are mutually exclusive.

And the name is mangled. See http://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B_Name_Mangling . If you can get the mangled name, and then provide the DllImport with it ( EntryPoint=MANGLED_NAME ), it should work.

You can provide linker with .def file, containing definition of exported functions, and their names aren't mangled then:

Project.def:

EXPORTS
    captureCamera @1

Answer 2

Are you declaring

extern "C" {
    __declspec(dllexport) int captureCamera(int& captureId);
}

inside your c++ code - C# can only access C, not C++ functions.

Answer 3

You are defining a C++ function without an extern "C" block. As C++ allows you to overload functions (ie create many captureCamera() functions with different sets of arguments), the actual function name inside the DLL will be different. You can check it by opening Visual Studio Command prompt, going to your binary directory and running this:

dumpbin /exports YourDll.dll

You will get something like this:

Dump of file debug\dll1.dll

File Type: DLL

  Section contains the following exports for dll1.dll

    00000000 characteristics
    4FE8581B time date stamp Mon Jun 25 14:22:51 2012
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 00011087 ?captureCamera@@YAHAAH@Z = @ILT+130(?captureCamera@@YAHAAH@Z)

  Summary

        1000 .data
        1000 .idata
        2000 .rdata
        1000 .reloc
        1000 .rsrc
        4000 .text
       10000 .textbss

The ?captureCamera@@YAHAAH@Z is the mangled name that actually encodes the arguments you've specified.

As mentioned in other answers, simply add extern "C" to your declaration:

extern "C" __declspec(dllexport) int captureCamera(int& captureId)
{
    ...
}

You can recheck that the name is correct by rerunning dumpbin:

Dump of file debug\dll1.dll

File Type: DLL

  Section contains the following exports for dll1.dll

    00000000 characteristics
    4FE858FC time date stamp Mon Jun 25 14:26:36 2012
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 000110B4 captureCamera = @ILT+175(_captureCamera)

  Summary

        1000 .data
        1000 .idata
        2000 .rdata
        1000 .reloc
        1000 .rsrc
        4000 .text
       10000 .textbss