Unable to use my .dll in Visual C# project

Question

I created a DLL in visual studio 2013, and it built fine. It is a Visual C++ project, but written in C. However, I'm unable to add the .dll I built as a reference to my C# project. I get:

A reference to 'C:\...\FileGuidUtils.dll' could not be added. Please make sure that the file is accessible, and that it is a valid assembly or COM component.

I followed a bunch of different tutorials online and same result. The only MSDN tutorial I found doesn't account for C code, only C++.

Here is my FileGuidUtils.cpp file:

#include "stdafx.h"
#include <stdio.h>
#include <tchar.h>
#include <ole2.h>
#include <errno.h>

extern "C" __declspec(dllexport) WCHAR* getFileIdFromPath(_TCHAR* path);
extern "C" __declspec(dllexport) WCHAR* getPathFromFileId(_TCHAR* guid);

__declspec(dllexport) WCHAR* getFileIdFromPath(_TCHAR* path) {
    HANDLE hFile;
    FILE_OBJECTID_BUFFER buf;
    DWORD cbOut;
    GUID guid;
    WCHAR *szGuid = (WCHAR *)malloc(sizeof(WCHAR) * 39);
    BOOL result;

    hFile = ::CreateFile(path, 0,
        FILE_SHARE_READ |
        FILE_SHARE_WRITE |
        FILE_SHARE_DELETE,
        NULL,
        OPEN_EXISTING,
        0, NULL);
    if (hFile == INVALID_HANDLE_VALUE) {
        fprintf(stderr, "CreateFile() Error: 0x%0.8x\n", ::GetLastError());
        ::CloseHandle(hFile);
        return NULL;
    }
    result = ::DeviceIoControl(hFile,
        FSCTL_CREATE_OR_GET_OBJECT_ID,
        NULL, 0,
        &buf, sizeof(buf),
        &cbOut, NULL);
    if (!result) {
        fprintf(stderr, "DeviceIoControl() Error: 0x%0.8x\n", ::GetLastError());
        ::CloseHandle(hFile);
        return NULL;
    }

    ::CopyMemory(&guid, &buf.ObjectId, sizeof(GUID));
    ::StringFromGUID2(guid, szGuid, 39);

    return szGuid;
}

__declspec(dllexport) WCHAR* getPathFromFileId(_TCHAR* guid) {
    struct FILE_NAME_INFO_AND_BUF {
        FILE_NAME_INFO fni;
        TCHAR buf[260];
    };
    HANDLE hRoot;
    FILE_ID_DESCRIPTOR desc;
    HANDLE hFile;
    FILE_NAME_INFO_AND_BUF fnib = { 0 };
    HRESULT hr;
    TCHAR szRoot[] = _T("C:");
    BOOL result;

    hRoot = ::CreateFile(szRoot, 0, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
    if (hRoot == INVALID_HANDLE_VALUE) {
        _tprintf(_T("CreateFile() Error: 0x%0.8x\n"),
            ::GetLastError());
        return NULL;
    }

    desc.dwSize = sizeof(desc);
    desc.Type = ObjectIdType;

    hr = ::CLSIDFromString(guid, &desc.ObjectId);
    if (FAILED(hr)) {
        fprintf(stderr, "CLSIDFromString() Error: 0x%0.8x\n", ::GetLastError());
        ::CloseHandle(hRoot);
        return NULL;
    }

    hFile = ::OpenFileById(hRoot, &desc, GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, 0);
    if (hFile == INVALID_HANDLE_VALUE) {
        fprintf(stderr, "OpenFileById() Error: 0x%0.8x\n", ::GetLastError());
        ::CloseHandle(hFile);
        return NULL;
    }

    result = ::GetFileInformationByHandleEx(hFile, FileNameInfo, &fnib, sizeof(fnib));
    if (!result) {
        fprintf(stderr, "GetFileInformationByHandleEx Error %d\n", ::GetLastError());
        ::CloseHandle(hFile);
        return NULL;
    }

    wchar_t *path = (wchar_t *)malloc(sizeof(wchar_t) * 512);
    wcscpy_s(path, 256, szRoot);
    wcscat_s(path, 256, fnib.fni.FileName);

    return path;
}

There were other autogenerated files but I did not modify them, save for commenting out the #define WIN32_LEAN_AND_MEAN line in stdafx.h since my project needs the full version of Windows.h.

Answer 1

and that it is a valid assembly or COM component

It is neither. You'll have to write [DllImport] declarations in your C# project to use it.

If you want Add Reference to work then the simplest way is to create a C++/CLI project with a public ref class . File > Add > New Project > Visual C++ > CLR > Class Library template. Minimal knowledge of C++/CLI syntax is required to bring it to a good end.