Here is what I want: I have a huge legacy C/C++ codebase written for POSIX, including some very POSIX specific stuff like pthreads. This can be compiled on Cygwin/GCC and run as an executable under Windows with the Cygwin DLL.

What I would like to do is build the codebase itself into a Windows DLL that I can then reference from C# and write a wrapper around it to access some parts of it programatically.

I have tried this approach with the very simple "hello world" example at http://www.cygwin.com/cygwin-ug-net/dll.html and it doesn't seem to work.

#include <stdio.h>
extern "C" __declspec(dllexport) int hello();

int hello()
{
  printf ("Hello World!\n");
    return 42;
}

I believe I should be able to reference a DLL built with the above code in C# using something like:

[DllImport("kernel32.dll")]
public static extern IntPtr LoadLibrary(string dllToLoad);

[DllImport("kernel32.dll")]
public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

[DllImport("kernel32.dll")]
public static extern bool FreeLibrary(IntPtr hModule);


[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
private delegate int hello();

static void Main(string[] args)
{
    var path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "helloworld.dll");
    IntPtr pDll = LoadLibrary(path);
    IntPtr pAddressOfFunctionToCall = GetProcAddress(pDll, "hello");

    hello hello = (hello)Marshal.GetDelegateForFunctionPointer(
                                                                pAddressOfFunctionToCall,
                                                                typeof(hello));

    int theResult = hello();
    Console.WriteLine(theResult.ToString());
    bool result = FreeLibrary(pDll);
    Console.ReadKey();
}

But this approach doesn't seem to work. LoadLibrary returns null. It can find the DLL (helloworld.dll), it is just like it can't load it or find the exported function.

I am sure that if I get this basic case working I can reference the rest of my codebase in this way. Any suggestions or pointers, or does anyone know if what I want is even possible? Thanks.