Calling C++ Library from C# with C++/CLI Wrapper

Question

I have a C++ library, header of which looks like:

void NotMyFun(double * a, int * b);

The function reads from a, and writes to b. To call the library I have created a C++/CLI wrapper, where below function is defined:

static void MyWrapperFun(double * a, int * b)
{
    NotMyFun(a,b);
}

and works OK. From C# code, say, I have two managed arrays, i.e. double[] ma and double[] mb, where ma already holds some meaningful data, and mb is -meaningfully- filled when wrapper is called. Is below an OK way to call the wrapper function?

unsafe
{
    fixed (double* pma = ma)
    {
        fixed (int* pmb = mb)
        {
            MyWrapperNS.MyWrapperClass.MyWrapperFun(pma,pmb);
        }
    }
}

Are the unsafe pointers a fast way? Is any data copying involved here while passing and retrieving to/from C++/CLI wrapper? Or pointers are already pointing to a continuous memory space in C# arrays?

Besides, do I need any manual memory cleaning here? If the pointers are tied to the memory of managed C# arrays, I guess they are properly garbage collected after, but just want to be sure.

That's not a real C++/CLI wrapper. You need to expose managed types in your C++/CLI function. So that the C# function can call it. You can tell that it's no good because your C++/CLI function does nothing. That function needs to adapt the C++ interface to be a .net interface. A word of advice, don't use unsafe code in the C#. You simply don't need to if you get the C++/CLI right. I wonder if you need the extra layer of C++/CLI. Did you consider p/invoke? — David Heffernan
– David Heffernan, Commented Dec 27, 2012 at 17:26
I am trying to decrease data copy as much as possible. What you mean is I guess copying from managed arrays to unmanaged arrays in the C++/CLI wrapper but I am really dealing with huge amounts of data. That's why I am using unsafe pointers. — user192936
– user192936, Commented Dec 27, 2012 at 17:30

David Heffernan · Accepted Answer · 2012-12-27 17:51:52Z

2

Personally I think you are over-complicating things. I'd avoid the unsafe code and skip the C++/CLI layer. I'd use a simple p/invoke declared like this:

[DllImport(@"mylib.dll")]
static extern void NotMyFun(double[] a, int[] b);

Because double and int are blittable types, no copying is necessary. The marshaller just pins the arrays for the duration of the call.

answered Dec 27, 2012 at 17:51

David Heffernan

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