Calling C from C#

I am from electric engineer background, therefore my knownledge is small about C#,DLL,etc.... I want to use c function into a C#. I know there is a couple post about that but I didn't find one that is enough simple.

Currently, I got C function call windows API to read/write on the USB port. First to create a .dll do I need a header file? Because I got the following function decleration function into a header. The examples that I saw on stack overflow and Internet only use a simple .c file, Can I get rid of the header files?

__declspec(dllexport) LMUSB_HANDLE  __stdcall InitializeDevice(unsigned short usVID,
                                    unsigned short usPID,
                                    LPGUID lpGUID,
                                    BOOL *pbDriverInstalled);
__declspec(dllexport) LMUSB_HANDLE  __stdcall InitializeDeviceByIndex(unsigned short usVID,
                                           unsigned short usPID,
                                           LPGUID lpGUID,
                                           DWORD dwIndex,
                                           BOOL bOpenDataEndpoints,
                                           BOOL *pbDriverInstalled);
__declspec(dllexport) BOOL  __stdcall TerminateDevice(LMUSB_HANDLE hHandle);
__declspec(dllexport) BOOL  __stdcall WriteUSBPacket(LMUSB_HANDLE hHandle,
                          unsigned char *pcBuffer,
                          unsigned long ulSize,
                          unsigned long *pulWritten);
__declspec(dllexport) DWORD  __stdcall ReadUSBPacket(LMUSB_HANDLE hHandle,
                         unsigned char *pcBuffer,
                         unsigned long ulSize,
                         unsigned long *pulRead,
                         unsigned long ulTimeoutMs,
                         HANDLE hBreak);
 __declspec(dllexport) BOOL  __stdcall Endpoint0Transfer(LMUSB_HANDLE hHandle, UCHAR ucRequestType,
                             UCHAR ucRequest, USHORT usValue,
                             USHORT usIndex, USHORT usLength,
                             PUCHAR pucBuffer, PUSHORT pusCount);

Second point, do I need to write __declspec(dllexport) in the cpp file? Here an function from the cpp:

extern "C" __declspec(dllexport) BOOL PASCAL EXPORT TerminateDevice(LMUSB_HANDLE hUSB)

I got no idea what "BOOL PASCAL EXPORT" does, this code is recycled from a furnisher project.

Finaly, when the DLL is properly build. How I import it in the C# project? I tried the following but without success :

[DllImport("lmusbdll.dll")]

I see that you could use right click on the project and add reference but Visual Studio pop-up an error message:

A reference to "DLL path" could not be added. Please make sure that the file is accessible, and that it is a valid assembly or COM component.

[EDIT]

I tried the following solution , but when I tried to reference the header file in my c# project. I still get the message that I cannot reference the file.

You don't need a header file, but you will need to __declspec(dllexport) each function. And if it's C++ (i.e. a .cpp file) you'll need extern "C".

There's a handy program called depends.exe that allows you to look at a DLL and see what functions it exports and if it depends on any other DLLs. That might be why your DllImport is failing.