How to use DllImport with c++ functions that take a function pointer as an argument?

I'm trying to understand how dll importing in c# works.
I've managed to get it working for function F1 shown below, however I want to extend this to a function of type F2, which takes a function pointer as an argument

I've look at the example on MSDN, but it is too dissimilar from what i am trying to do and I don't really understand it.

Can someone tell me the c# code that will enable me to import the function F2 into my c# program, or point me to a a more basic tutorial than the MSDN help site, because I really didn't find it useful in helping me solve this particular problem.

This is the .cpp file that i compile into a dll which my c# program then uses. I have successfully imported F1, but all attempts to import F2 have failed...

extern "C"
{
__declspec(dllexport) double F1(double x)
{
return x;
}

__declspec(dllexport) double F2(double (*f) (double), double x)
{
return f(x);
}
}

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How to use DllImport with c++ functions that take a function pointer as an argument?

rhd75

See the EnumWindows example in this article.

Althought Paul DiLascia published it in his C++ Q&A column, actually the starting section is C#. Oops.
vasudupe
I have a C++ function that takes int** as one of it's parameters - say HResult Func(int** x)

I use the [DllImport("<dll name>")] and declare in the C# code as :

public static extern int func( ref int x )

I problem with above is that I doubt if this declararion is is sufficient to represent the "pointer to pointer" aspect of native signature.

Do you know what signature is required in the C# world

thanks,

~S
a. Nova
Passing method pointers around in .NET is done with delegates. I'd have to check to see if you can use a DllImport with a delegate... You could also pinvoke GetProcAddress on F1 and send the IntPtr to F2.
dave45

try out IntPtr

Regards
Erik BN
Thanks Sheng that article looks like it will help. With respect to dealing with importing functions with function pointer arguments.

However I think that the use of GetProcAddress is actually closer to what I ideally want to do.
I want to have the function F2 written in native c++ take another function F1 in native c++ as a parameter and call the function F2 from my c# program, which is what i'm now trying to do.

I've run into a problem though, once I have the address of the function, how do I call it

I have the address in funcAddress as shown below, where funcAddress is a IntPtr.

funcAddress = Win32.GetProcAddress(Win32.GetModuleHandle("TestFunction.dll"), "F1");

Now what do I do with funcAddress and how do I pass it to function F2
Stefander
If you're interested with the purely delegate means, the following is an example that works for me:
C# Code:

delegate double MyDelegate ( double value );

[DllImportAttribute("cppLibraryD", EntryPoint = "F1")]

private static extern double F1 ( double value );

[DllImportAttribute("cppLibraryD", EntryPoint = "F2")]

private static extern double F2 ( MyDelegate fp, double value );

static void Main ( string[] args )

{

double value = F1(5);

MyDelegate myDelegate = new MyDelegate(F1);

value = F2(myDelegate, 42);

XmlDocument locodes = new XmlDocument();

}

C++ Code:

extern "C" {

__declspec(dllexport) double F1(double value)

{

return value + 1.0;

}

__declspec(dllexport) double F2(double (__stdcall*func)(double), double value)

{

return func(value);

}

}

Take note of the __stdcall on the function pointer argument in the F2 function declaration. Since from C# (or any .NET language) this will actually be calling a delegate (which then calls your function pointer) you must use __stdcall because that's the calling conventions uses by delegates.
HarveyC123
How exactly do I use GetProcAddress to do as you suggested
A.Russell
Peter you're a star!

Thats exactly what i needed and it works perfectly without needing to mess about with getting proceedure address.

Thanks a lot!
How to use DllImport with c++ functions that take a function pointer as an argument?

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