C# Discussion Boards - CodeProject

14-Apr-09 8:01

Hi all,

I'm having a problem using DllImport in C# to call a C++ function.

C++ Header File:

DLLEXPORT int drawMarkupOnLine (int jobId, int lineNumber, short* line, int drawMode);

C++ Source File:

DLLEXPORT int drawMarkupOnLine (int jobId, int lineNumber, short* line, int drawMode)
{
...
}

==========================================

C# Source File:

[DllImport("vdrdll.dll", EntryPoint = @"?drawMarkupOnLine@@YAHHHPAFH@Z")]

unsafe static extern int drawMarkupOnLine(int jobId, int lineNumber, short* line, int drawMode);

...

short* scan0 = (short*)bmd.Scan0;	// Start of image.
short* nextScanLine = scan0;		// First scan line.
drawMarkupOnLine (0, i, nextScanLine, lca[i]);

==========================================

When I try to call the function, I get an EntryPointNotFoundException, with message, "Unable to find an entry point named '?drawMarkupOnLine@@YAHHHPAFH@Z' in DLL 'vdrdll.dll'."

I've tried various permutations of IntPtr, string pointers, and void pointers for the third parameter (which I'm assuming is causing the problem), but nothing seems to work. (Other functions with only int parameters work just fine.) Can someone suggest a reliable way to get this to link?

Thanks,
Alan

Luc 64801114-Apr-09 8:12

14-Apr-09 8:12

Hi Alan,

in C# one does not need unsafe nor short*, instead ref short would do.

since native C++ is mangling names, I suggest you try DllImport("vdrdll.dll", EntryPoint="...", ExactSpelling=true), since maybe the marshaling is adding an A or W because of the pointer.

Smile | :)

Alan Balkany14-Apr-09 8:25

14-Apr-09 8:25

Thanks for the suggestion Luc, but I'm still getting the same result. It even happens when I use a simple local variable for the 3rd parameter.

Alan Balkany14-Apr-09 8:31

14-Apr-09 8:31

It gets stranger: The problem still shows up even with all int parameters.

Luc 64801114-Apr-09 8:42

14-Apr-09 8:42

OK, I suggest you use DUMPBIN or some other tool to verify the symbols exported by your DLL file.

BTW: you are not getting confused (or confusing your tools) by having several copies of the DLL in different places, other folders, GAC, ...?

FWIW: I tend to use C, not C++, for native code, avoiding all name mangling. Never had such problems.

Smile | :)

Alan Balkany14-Apr-09 8:46

14-Apr-09 8:46

Thanks Luc. I got the symbol from the VdrDll.exp file (exports library file), which seems to work for the other exported functions.

I tried renaming the generated DLL, and got the expected missing module error message.

Yes, C would be simpler, but I'm trying to leverage a massive existing code base already in C++.

Luc 64801114-Apr-09 9:36

14-Apr-09 9:36

Hi Alan,

I did some experiments using VS2008 Express for both C++ and C# and all went well, here is the relevant code:

#define NATIVECPPEXPERIMENT_API __declspec(dllexport)

NATIVECPPEXPERIMENT_API int fint1(int arg1) {
	return arg1*arg1;
}

NATIVECPPEXPERIMENT_API int fint4(int arg1,int arg2, short* arg3, int arg4) {
	*arg3=(short)(arg1*arg2);
	return arg1*arg4;
}

namespace ConsoleApplication1 {
	class Program {
		static void log(string s) {Console.WriteLine(s);}

		static void Main(string[] args) {
			int res1=cppfint1(12);
			log("res1="+res1);
			short shrt=14;
			int res4=cppfint4(12,13,ref shrt, 15);
			log("res4="+res4);
			log("shrt="+shrt);
			Console.ReadLine();

		}

		[DllImport("nativeCppExperiment.dll", EntryPoint="?fint1@@YAHH@Z")]
		static extern int cppfint1(int arg1);

		[DllImport("nativeCppExperiment.dll", EntryPoint="?fint4@@YAHHHPAFH@Z")]
		static extern int cppfint4(int agr1, int arg2, ref short arg3, int arg4);
	}
}

So either you are overlooking something, or your system is ill. I suggest you try closing and reopening Studio, and consider rebooting.

Smile | :)

Alan Balkany14-Apr-09 10:00

14-Apr-09 10:00

Thanks again for the help Luc! Your code looks perfect. This is what I tried to do with the ref parameter.

I've been experimenting, and taking out the EntryPoint clause allowed the linker to find the function:

[DllImport("vdrdll.dll")]
static unsafe extern void drawMarkupOnLine(int jobId, int lineNumber, short* line, int drawMode);

I don't understand why this worked. Now I'm getting a read/write-protected-memory exception, but this may just be a "normal" bug.

Luc 64801114-Apr-09 10:19

14-Apr-09 10:19

Alan Balkany wrote:
taking out the EntryPoint clause allowed the linker to find the function

That is amazing, if not impossible. I've never read that should work.

I tried it for my second call, and I immediately got an EntryPointNotFoundException, as I was expecting.

namespace ConsoleApplication1 {
	class Program {
		static void log(string s) {Console.WriteLine(s);}
		static void Main(string[] args) {
			int res1=cppfint1(12);
			log("res1="+res1);
			short shrt=14;
			int res4=fint4(12,13,ref shrt, 15);
			log("res4="+res4);
			log("shrt="+shrt);
			Console.ReadLine();

		}

		[DllImport("nativeCppExperiment.dll", EntryPoint="?fint1@@YAHH@Z")]
		static extern int cppfint1(int arg1);

		[DllImport("nativeCppExperiment.dll")]
		static extern int fint4(int agr1, int arg2, ref short arg3, int arg4);
	}
}

Your system is acting weird if you ask me.

Alan Balkany14-Apr-09 10:35

14-Apr-09 10:35

I took your advice and restarted the system, and did a Rebuild Solution on both halves. The results were the same; it links perfectly with the Entry Point left out for only that function.

"Your system is acting weird if you ask me."

I blame Microsoft. Wink | ;)

Luc 64801114-Apr-09 10:57

14-Apr-09 10:57

I blame Microsoft.[^]

Smile | :)

Luc 64801114-Apr-09 11:10

14-Apr-09 11:10

could it be you have a C-version of that function somewhere?
or a hidden extern "C" { ... } around it?

Smile | :)

Alan Balkany14-Apr-09 11:20

14-Apr-09 11:20

Neither of those are present. (Looking around, I can see you've been busy today!)

Thanks again for the help!

Luc 64801114-Apr-09 11:22