Call C++ function in C# from DLL - strange paramet

Question

I have a function writen in C++ with the next header:

void EncodeFromBufferIN(void* bufferIN,int bufferINSize, unsigned char*  &bufferOUT, int &bufferOUTSize);

I've edited the .h and .cpp files like this, to be able calling the function by importing the DLL in C#:

**EncodeFromBufferIN.h**
extern "C" {
     __declspec(dllexport) void EncodeFromBufferIN(void* bufferIN, int bufferINSize, unsigned char*  &bufferOUT, int &bufferOUTSize);
}
**EncodeFromBufferIN.cpp**
extern void EncodeFromBufferIN(void* bufferIN, int bufferINSize, unsigned char*  &bufferOUT, int &bufferOUTSize){
    // stuff to be done
}

But now my problem is that I don't know how to call the function in C#. I've added the next code in C# but not sure how to pass the parameters to the function.

[DllImport("QASEncoder.dll")]
        unsafe public static extern void EncodeFromBufferIN(void* bufferIN, int bufferINSize, out char[] bufferOUT, out int bufferOUTSize);

The bufferIN and bufferOUT should be strings but if I'm calling the function like this:

public string prepareJointsForQAS()
{
   string bufferIN = "0 0 0 0 0";
   char[] bufferOUT;
   int bufferOUTSize;
   EncodeFromBufferIN(bufferIN, bufferIN.Length, bufferOUT, bufferOUTSize);
}

I get this error: "The best overloaded method matrch for ... has some invalid arguments". So how should the parameters be passed?

Answer 1

Marshalling works best with C Style calls. So it is best to use pure C on your public interface. If it is at all feasible to change the native code to

void EncodeFromBufferIN(
    unsigned char* bufferIN, 
    int bufferINSize, 
    unsigned char* bufferOUT,     
    int* bufferOUTSize);

Then the call in C# can be thus

[DllImport("QASEncoder.dll")]
public static extern void EncodeFromBufferIN(
    String bufferIN, 
    int bufferINSize, 
    StringBuilder bufferOUT, 
    ref int bufferOUTSize);

String inStr = new String(255);
int inSize = 255;
// make an educated estimate for the output size 
// and preallocate in C# (I am guessing 255)
StringBuilder outStr = new StringBuilder(255);
int outSize = 255;

EncodeFromBufferIN(inStr, inSize, outStr, outSize);

This way you can avoid memory allocations in unmanaged code which (although feasible) can get messy.

Hope this gets you going.

Answer 2

A few corrections that worked for me:

        string inStr = "Value to pass";
        int inSize = inStr.Length;
        StringBuilder outStr = new StringBuilder(255);
        int outSize = 255;

        EncodeFromBufferIN(inStr, inSize, outStr, ref outSize);

regards.