C# P/Invoke: native output parameter value won't reach managed code

Question

I'm marshalling a native C++ DLL to a C# DLL. I'm not very acknowledged in C/C++, but I've managed to make it work until I got stuck on this problem. Here's a very simplistic code example:

C++

PROASADLL __declspec(dllexport) void outTest(int* number){
    int temp = *number + 10;
    number = &temp; //*number = 12
}

C#

[DllImport("ProAsaNativeDll.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern void outTest(ref int number);

public static int OutTest()
{
    int number = 2;
    outTest(ref number);
    return number; //number = 2
}

Please notice that I'm my real scenario, I'm trying to get this to work with a pointer to pointer to struct but I decided to leave it out as it is not a marshalling issue; not even this simple code will work. The C++ code works but I wouldn't rule out being dumb and having a problem there: like I said, I don't know much C/C++.

The number variable value in the C# wrapper method won't change. Please help.

It is bad C++ code. That never gets better when you pinvoke it. Make it work from a C++ test program first. — Hans Passant
– Hans Passant, Commented Mar 23, 2016 at 18:57

Mihai Caracostea · Accepted Answer · 2016-03-23 18:51:57Z

2

I think you are getting a bad result because in your C++ code you are actually changing the parameter number by setting it to another pointer.

I believe that your change will only be visible in your function outTest's scope.

However, if you change the value where the pointer ... points... that's should be a different story. Pretty much like this:

*number = *number + 10;

EDIT: This bit is not tested. Oh... and also... I haven't written anything in C++ for ages. Might as well be totally wrong.

answered Mar 23, 2016 at 18:51

Mihai Caracostea

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4 Comments

Jacobr365 Over a year ago

Like riding a bike

Mihai Caracostea Over a year ago

@Jacobr365 Yeah... All's well and neat... Until you fall down and "bend" your nose :)

makoshichi Over a year ago

Nice! Like I said, C++ isn't my forte, but I see both "what you did there" and where I was wrong. Many thanks!

Mihai Caracostea Over a year ago

@S.O. That's how we all learn... Keep up the good work! Cheers!

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