Pass multi - dimensional array from managed code to unmanaged code

Question

I would like to do the following:

Create three dimesinal array in c# code like this:

var myArray = new short[x,y,z];
UnanagedFunction(myArray);

Pass it to unmanaged code (c++) like this:

void UnmanagedFunction(short*** myArray)
{
    short first = myArray[0][0][0];
}

UPDATED When I try the following code I have runtime error:

Attempted to read or write to protected memory.

Thank you!!!

the first part of code is in c# the second is in c++ and i tried it now compiler allows me the c++ code — Sergey Kucher, Aug 22 '11 at 21:12
@Simon, how is an array of triples going to help? Do you mean quadruples (x, y, z and the value)? — svick, Aug 25 '11 at 06:06
I think the problem is that you have `short[,,]` (which doesn't have C++ equivalent), but you treat it as `short[][][]`. — svick, Aug 25 '11 at 06:13
But there is no marshaling support for nested (short[][][]) arrays — Sergey Kucher, Aug 25 '11 at 07:01
You can't even do that in pure C++. You must write all-but-one dimensions on the function prototype. Read this: http://c-faq.com/~scs/cclass/int/sx9a.html (so for example `void UnmanagedFunction(short myArray[][10][10]`) ) — xanatos, Aug 25 '11 at 07:20

score 8 · Accepted Answer · answered Aug 25 '11 at 07:50

IntPtr Array3DToIntPtr(short[, ,] Val)
        {
            IntPtr ret = Marshal.AllocHGlobal((Val.GetLength(0) + Val.GetLength(1) + Val.GetLength(2)) * sizeof(short));

            int offset = 0;
            for (int i = 0; i < Val.GetLength(0); i++)
            {

                for (int j = 0; j < Val.GetLength(1); j++)
                {
                    for (int k = 0; k < Val.GetLength(2); k++)
                    {
                        Marshal.WriteInt16(ret,offset, Val[i, j, k]);
                        offset += sizeof(short);


                    }
                }
            }

            return ret;
        }

This has been tested and it works, the only limitation is that you have to call Marshal.FreeHGlobal on the array pointer when you are done with it or you will get a memory leak, I would also suggest that you change your c++ function so that it accepts the array dimensions or you will only be able to use 3d arrays of specific size

score 2 · Answer 2 · edited May 23 '17 at 12:07

I'm writing it in pure C#, but if you take away the unsafe static from Func, the Func should work in C/C++. Be aware that I'm note sure sure it's ok ok to write this :-) I'm using this Indexing into arrays of arbitrary rank in C#

static unsafe void Main(string[] args) {
    var myArray = new short[5, 10, 20];

    short z = 0;

    for (int i = 0; i < myArray.GetLength(0); i++) {
        for (int j = 0; j < myArray.GetLength(1); j++) {
            for (int k = 0; k < myArray.GetLength(2); k++) {
                myArray[i, j, k] = z;
                z++;
            }
        }
    }

    // myArray[1, 2, 3] == 243

    fixed (short* ptr = myArray) {
        Func(ptr, myArray.GetLength(0), myArray.GetLength(1), myArray.GetLength(2));
    }
}

// To convert to C/C++ take away the static unsafe
static unsafe void Func(short* myArray, int size1, int size2, int size3) {
    int x = 1, y = 2, z = 3;
    int el = myArray[x * size2 * size3 + y * size3 + z]; // el == 243
}

Pass multi - dimensional array from managed code to unmanaged code

2 Answers2

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