Using unmanaged library

Question

So in visual studio i have my solution with two projects, first one is managed c++ code and second one is unmanaged c++ library (waffles). I want to use classes from library in my managed code.

If i simply add 'include "GMacros.h"', then i get 'cannot compile with /clr' error. Tried to wrap include in #pragma unmanaged/managed, but it doesnt seem to work.

Is there anything i can do without editing external library code or writing any wrappers?

Answer 1

The generic solution is to wrap the library calls in thin wrapper functions/classes whose header files you can include in managed code. Not very pretty but will get you there.

Answer 2

Unmanaged code can't be called directly in managed .NET. You need to add __declspec(dllexport) to your functions' declarations that should be visible outside the unmanaged library:

public:
    void __declspec(dllexport) MyUnmanagedMethod();

And then in your managed code write a simple wrapper like this:

public ref class Wrapper
{
public:
    [DllImport("MyUnmanagedLibrary.dll")]
    static extern void MyUnmanagedMethod();
}

Now you can call Wrapper.MyUnmanagedMethod like any other static method from you managed code.

Answer 3

P/Invoke with the DLLImport attribute also requires you to get down and dirty with marshalling the function parameters, if it has any, to CLR types. So for example a DWORD becomes an int, IN HANDLE can become an IntPtr, LPDWORD becomes an out int, LPVOID can usually be marshalled as a byte[]... and so on. See a decent summary about it here.

An example pulled out of my recent project where I had to interface with a DLL for an old digital output box:

//This function's header in the DLL was:
//BOOL _stdcall fnPerformaxComSendRecv(IN HANDLE pHandle, IN LPVOID wBuffer, IN DWORD dwNumBytesToWrite, IN DWORD dwNumBytesToRead, OUT LPVOID rBuffer);
[DllImport("PerformaxCom.dll", SetLastError = true, CharSet = CharSet.Auto)]
private static extern bool fnPerformaxComSendRecv(IntPtr pHandle, byte[] wBuffer, int dwNumBytesToWrite, int dwNumBytesToRead, byte[] rBuffer);