Can I pass a C# array to a C pointer via a C++/CLI wrapper?

Question

Here is my C function definition:

//C
int FXForwardRate(
    long         *riskZCDates,      /* (I) Risk ZC dates */ 
    double       *fwdRate);         /*(O) Output */

the first parameter is actually pointing to an array of long. In C# I would ideally like to call it like this:

//C#    
int[] array = {1,2,3}; //if i'm not mistaking long in C is int in :NET
double fwdRate = 0;

Class1.FXForwardRate(array, ref fwdRate);

Now for my C++/CLI code I don't know how to declare the function in a Class1 class (I didn't put the whole code to be concise):

//C++/CLI
int Class1::FXForwardRateW(int[] premium, double %fwdRate)
{
     double _tempFwdRate = 0.0;
     int _status = FXForwardRate(premium, &_tempFwdRate);
     fwdRate = _tempFwdRate;
     return _status;
}

I know from another function I wrote that what I did for the second parameter is right (not sure it's the best method though). But how about the first one? Should I do the same thing? ie: define a pointer pass it to the C function and then copy the values back to the array? Regards

Do you have to "pin" the array/object?

user166390
– user166390

2011-09-16 15:23:25 +00:00
Commented Sep 16, 2011 at 15:23 — user166390
– user166390, Commented Sep 16, 2011 at 15:23

Ben Voigt · Accepted Answer · 2011-09-16 15:45:59Z

3

The correct solution (both Lirik and pst gave pointers in the right direction) would be:

int Class1::FXForwardRateW(array<int>^ premium, double %fwdRate)
{
     double tempFwdRate = fwdRate;
     pin_ptr<int> premiumPtr = &premium[0];
     int status = FXForwardRate(premiumPtr, &tempFwdRate);
     fwdRate = tempFwdRate;
     return status;
}

edited Sep 16, 2011 at 15:45

answered Sep 16, 2011 at 15:28

Ben Voigt

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Comments

ildjarn · Accepted Answer · 2011-09-16 15:48:09Z

0

using System::Runtime::InteropServices::OutAttribute;

int Class1::FXForwardRateW(array<int>^ premium, [Out] double% fwdRate)
{
    double tempFwdRate;
    pin_ptr<int> pin = &premium[0];
    int status = FXForwardRate(reinterpret_cast<long*>(pin), &tempFwdRate);
    fwdRate = tempFwdRate;
    return status;
}

This will appear to C# to have the signature:

int FXForwardRateW(int[] premium, out double fwdRate);

(If fwdRate is truly output-only and not input/output then semantically you want the argument type to be out double rather than ref double.)

However, note that using C++/CLI for this may be overkill unless you have more complicated marshaling to do elsewhere; this could be done more easily using P/Invoke directly from C#:

[DllImport("c-library-name.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int FXForwardRate(int[] premium, out double fwdRate);

No manual marshaling necessary.

edited Sep 16, 2011 at 15:48

answered Sep 16, 2011 at 15:40

ildjarn

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1 Comment

nche Over a year ago

Thanks. But I'm wrapping a library with 400 functions and some have return or take in complex Data Structures. I had actually set out with PInvoke but in the end, decided to drop it. But I'm actually thinking about a mixed solution now... What do you think?

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