C# – Calling Excel/DLL/XLL functions from C#


I have a particular function in an Excel addin(xll).
The addin is proprietary and we do not have access to the source code. However we need to call some functions contained within the addin and we would like to call it from a C# program.

Currently, I was thinking of writing a C++ interface calling the Excel function with xlopers, then calling this C++ interface from C#.

Does anybody who has prior experience of this kind of issues know what would be the best solution for that ?


Best Solution

You need to create a fake xlcall32.dll, put it in the same directory as your XLL (do not put excel's own xlcall32.dll in the PATH). Here is some code:

# include <windows.h>

typedef void* LPXLOPER;

extern "C" void __declspec(dllexport) XLCallVer ( ) {}

extern "C" int __declspec(dllexport) Excel4 (int xlfn, LPXLOPER operRes, int count,... ) { return 0; }

extern "C" int __declspec(dllexport) Excel4v(int xlfn, LPXLOPER operRes, int count, LPXLOPER far opers[]) {return 0;}

Now suppose I have an XLL called xll-dll.xll with a function called (use "depends.exe" to find out the names of the exported functions) xlAdd that well adds two doubles: extern "C" __declspec(dllexport) XLOPER * __cdecl xlAdd(XLOPER* pA, XLOPER* pB);

The following code calls it:

# include <windows.h>
# include <iostream>

// your own header that defines XLOPERs
# include <parser/xll/xloper.hpp>

// pointer to function taking 2 XLOPERS
typedef XLOPER * (__cdecl *xl2args) (XLOPER* , XLOPER* ) ;

void test(){
/// get the XLL address
HINSTANCE h = LoadLibrary("xll-dll.xll");
if (h != NULL){
xl2args myfunc;
/// get my xll-dll.xll function address
myfunc = (xl2args) GetProcAddress(h, "xlAdd");
if (!myfunc) { // handle the error
FreeLibrary(h); }
else { /// build some XLOPERS, call the remote function
XLOPER a,b, *c;
a.xltype = 1; a.val.num = 1. ;
b.xltype = 1; b.val.num = 2. ;
c = (*myfunc)(&a,&b);
std::cout << " call of xll " << c->val.num << std::endl; }
FreeLibrary(h); }

int main()

My exe actually works (to my own surprise), and output 3 as expected. You must have some knowledge of what your XLL actually expects for parameters. If it allocates some memory, you must check if the #define xlbitDLLFree 0x4000 is set on your XLOPER c->type, and call back "xlAutoFree".

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