DLL won't work in C#

Question

Whenever I try to reference my own DLL in C# through Visual Studio, it tells me it was unable to make a reference to the DLL as it's not a COM library.

I've searched around the internet to find a solution to this with no clear answer or help any where really. It's a rather "simple" DLL which captures the raw picture data from a Fingerprint Scanner. I have tested that the C++ code worked just fine before I tried to make it into a DLL, just so you know.

I followed Microsofts guide on how to make a DLL and here is what I ended up with:

• JTBioCaptureFuncsDll.h
• JTBioCaptureFuncsDll.cpp
• JTBioCapture.cpp

JTBioCaptureFuncsDll.h

#ifdef JTBIOCAPTUREFUNCSDLL_EXPORTS
#define JTBIOCAPTUREFUNCSDLL_API __declspec(dllexport)
#else
#define JTBIOCAPTUREFUNCSDLL_API __declspec(dllimport)
#endif

using byte = unsigned char*;

struct BioCaptureSample {
    INT32 Width;
    INT32 Height;
    INT32 PixelDepth;
    byte Buffer;
};

JTBioCaptureFuncsDll.cpp

// JTBioCapture.cpp : Defines the exported functions for the DLL application.
//

#include "stdafx.h"

namespace JTBioCapture
{
    using byte = unsigned char*;

    class JTBioCapture
    {
    public:
        // Returns a Struct with Information Regarding the Fingerprint Sample
        static JTBIOCAPTUREFUNCSDLL_API BioCaptureSample CaptureSample();
    };
}

JTBioCapture.cpp

/*
* Courtesy of WinBio God Satish Agrawal on Stackoverflow
*/
BioCaptureSample CaptureSample()
{
    HRESULT hr = S_OK;
    WINBIO_SESSION_HANDLE sessionHandle = NULL;
    WINBIO_UNIT_ID unitId = 0;
    WINBIO_REJECT_DETAIL rejectDetail = 0;
    PWINBIO_BIR sample = NULL;
    SIZE_T sampleSize = 0;

    // Connect to the system pool. 
    hr = WinBioOpenSession(
        WINBIO_TYPE_FINGERPRINT,    // Service provider
        WINBIO_POOL_SYSTEM,         // Pool type
        WINBIO_FLAG_RAW,            // Access: Capture raw data
        NULL,                       // Array of biometric unit IDs
        0,                          // Count of biometric unit IDs
        WINBIO_DB_DEFAULT,          // Default database
        &sessionHandle              // [out] Session handle
        );
    if (FAILED(hr))
    {
        wprintf_s(L"\n WinBioOpenSession failed. hr = 0x%x\n", hr);
        goto e_Exit;
    }

    // Capture a biometric sample.
    wprintf_s(L"\n Calling WinBioCaptureSample - Swipe sensor...\n");
    hr = WinBioCaptureSample(
        sessionHandle,
        WINBIO_NO_PURPOSE_AVAILABLE,
        WINBIO_DATA_FLAG_RAW,
        &unitId,
        &sample,
        &sampleSize,
        &rejectDetail
        );
    if (FAILED(hr))
    {
        if (hr == WINBIO_E_BAD_CAPTURE)
        {
            wprintf_s(L"\n Bad capture; reason: %d\n", rejectDetail);
        }
        else
        {
            wprintf_s(L"\n WinBioCaptureSample failed. hr = 0x%x\n", hr);
        }
        goto e_Exit;
    }

    wprintf_s(L"\n Swipe processed - Unit ID: %d\n", unitId);
    wprintf_s(L"\n Captured %d bytes.\n", sampleSize);

    // Courtesy of Art "Messiah" Baker at Microsoft
    PWINBIO_BIR_HEADER BirHeader = (PWINBIO_BIR_HEADER)(((PBYTE)sample) + sample->HeaderBlock.Offset);
    PWINBIO_BDB_ANSI_381_HEADER AnsiBdbHeader = (PWINBIO_BDB_ANSI_381_HEADER)(((PBYTE)sample) + sample->StandardDataBlock.Offset);
    PWINBIO_BDB_ANSI_381_RECORD AnsiBdbRecord = (PWINBIO_BDB_ANSI_381_RECORD)(((PBYTE)AnsiBdbHeader) + sizeof(WINBIO_BDB_ANSI_381_HEADER));
    PBYTE firstPixel = (PBYTE)((PBYTE)AnsiBdbRecord) + sizeof(WINBIO_BDB_ANSI_381_RECORD);
    int width = AnsiBdbRecord->HorizontalLineLength;
    int height = AnsiBdbRecord->VerticalLineLength;

    wprintf_s(L"\n ID: %d\n", AnsiBdbHeader->ProductId.Owner);
    wprintf_s(L"\n Width: %d\n", AnsiBdbRecord->HorizontalLineLength);
    wprintf_s(L"\n Height: %d\n", AnsiBdbRecord->VerticalLineLength);

    BioCaptureSample returnSample;

    byte byteBuffer;
    for (int i = 0; i < AnsiBdbRecord->BlockLength; i++) {
        byteBuffer[i] = firstPixel[i];
    }
    returnSample.Buffer = byteBuffer;
    returnSample.Height = height;
    returnSample.Width = width;
    returnSample.PixelDepth = AnsiBdbHeader->PixelDepth;

    /*
    * NOTE: (width / 3) is necessary because we ask for a 24-bit BMP but is only provided
    * a greyscale image which is 8-bit. So we have to cut the bytes by a factor of 3.
    */
    // Commented out as we only need the Byte buffer. Comment it back in should you need to save a BMP of the fingerprint.
    // bool b = SaveBMP(firstPixel, (width / 3), height, AnsiBdbRecord->BlockLength, L"C:\\Users\\smf\\Desktop\\fingerprint.bmp");
    // wprintf_s(L"\n Success: %d\n", b);

e_Exit:
    if (sample != NULL)
    {
        WinBioFree(sample);
        sample = NULL;
    }

    if (sessionHandle != NULL)
    {
        WinBioCloseSession(sessionHandle);
        sessionHandle = NULL;
    }

    wprintf_s(L"\n Press any key to exit...");
    _getch();

    return returnSample;
}

The idea is that in C# you call "CaptureSample()" and then the code attempts to capture a fingerprint scan. When it does a scan, a struct should be returned to C# that it can work with, holding:

• Byte Buffer
• Image Height
• Image Width
• Image Pixeldepth

But when I try to reference the DLL in my C# project I get the following error:

I have also tried to use the TlbImp.exe tool to make the DLL but to no avail. It tells me that the DLL is not a valid type library.

So I'm a bit lost here. I'm new to C++ so making an Interop/COM Component is not something I've done before nor make a DLL for use in C#.

Answer 1

You cannot reference a library of unmanaged code written in C++ in a .NET Project. So to call code from such library you have to either use DllImport, or use a WrapperClass.

I referred to this answer : https://stackoverflow.com/a/574810/4546874.