Passing C# Multidimensional Array to C++

Question

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C++

  // C++

namespace whatever
{

    public ref class Class1
    {

        public:
            static void Test(double **a, int nRow, int nCol)
            {
                           // Do something with 2D array.

            }

    };

}
//========================================================

C#

// C#

      using whatever

       private void button1_Click(object sender, EventArgs e)
        {

            double[,] a = new double[4, 3];
            a[0, 0] = 1; a[1, 0] = 1; a[2, 0] = 1; a[3, 0] = 0;
            a[0, 1] = 2; a[1, 1] = 1; a[2, 1] = 1; a[3, 1] = 1;
            a[0, 2] = 1; a[1, 2] = 2; a[2, 2] = 1; a[3, 2] = 15;


            unsafe
            {
                fixed (double** ppa = a) // What is the correct syntax?
                {

                    Class1.Test(ppa, a.GetLength(0), a.GetLength(1));
                }
            }

    }
// +++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The C# syntax fixed (double** ppa = a) generated the following error message in VS2010 C#.

Error 1 Cannot implicitly convert type double* to double**. An explicit conversion exists (are you missing a cast?)

What is the correct way to pass the 2D array generated in the C# part over to the C++ ref class?

Posted 23-Oct-10 8:53am

firmwaredsp

Updated 23-Oct-10 9:17am

Henry Minute

v2

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Nish Nishant · Answer 1 · 2010-10-23T10:41:00

Unfortunately a managed multi-dimensional array is not equivalent to a pointer to pointer in native code. You could convert your 2-dimensional array into a doubly jagged array (an array of arrays) and then convert that to a pointer to a pointer and then pass that to C++.

But a more important question here is why you need to do this at all. Your C++ class is managed and can easily and trivially handle a managed array (including a 2-d array). So what do you need to resort to pointers for?

Nish Nishant · Answer 2 · 2010-10-23T11:11:00

>> Would you show me how

This is an article I wrote some years back:

http://www.codeproject.com/KB/mcpp/cppcliarrays.aspx[^]

Scroll down to the section titled "Direct manipulation of CLI arrays using native pointers", and then the 2nd sub-section "Natively accessing a jagged array". You may need to change things just a little to fit your specific needs but that example should tell you what you need to do.

Nish Nishant · Answer 3 · 2010-10-23T15:13:00

The whole point is to avoid doing the unsafe code in C#. Pass the managed type (2-d or jagged array) to the C++/CLI ref class. The C++/CLI ref class converts this to the native pointer type and calls the native function. That's how this is typically done.

[Update (this comment vs answer thing is kinda messy!)]
----------------------------------------------------------------

Here's a sample showing how you can pass a 2-d array to a native function expecting a pointer to pointer.

C++

void Foo(double** nums, int xLen, int yLen)
{
    for (int i=0; i<xLen; i++)
    {
        for (int j=0; j<yLen; j++)
        {
            double d = *(double*)(nums[i] + j);
        }
    }
}

int main(. . .)
{
    array<double, 2>^ numbers = gcnew array<double, 2>(2,3);
    numbers[0,0] = 12.3;
    numbers[0,1] = 2.1;
    numbers[0,2] = 4.3;
    numbers[1,0] = 23.2;
    numbers[1,1] = 7.7;
    numbers[1,2] = 17.6;

    pin_ptr<double> p1 = &numbers[0,0];
    pin_ptr<double> p2 = &numbers[1,0];
    double* p3[2];
    p3[0] = p1;
    p3[1] = p2;
    double** p4 = p3;

    Foo(p4, 2, 3);

    return 0;
}

firmwaredsp · Answer 4 · 2010-10-23T17:17:00

Solution 6

On the C# side :

double[,] ar = new double[nRow, nCol];

// Load values into ar.

// From C# pass to C++/CLI ref class.
Class1.Test(ar);

// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++

On C++ side:

void Class1::Test(array<double,>^ ar)
{
/*
How do I pass the 'ar' array to Native(double** a)?

Your article showed how to use a pinned ptr for a 1D and jagged arrays, but not for a 2D array.

Is there a way to use a pinned pointer on a 2D array?
*/

}

void Class1::Native(double** a) { // do something }

Posted 23-Oct-10 17:17pm

firmwaredsp

Updated 23-Oct-10 21:52pm

v3

Comments

Nish Nishant 24-Oct-10 7:53am

No, you cannot comprehensively access a 2-d array via a pinned pointer. It's got to be a jagged array.

Nish Nishant 24-Oct-10 7:56am

With some hacking around, you may technically be able to have a double** pointing to the 2-d array. What you do is point the double* members in the double** to each dimension (by doing the math).

Nish Nishant 24-Oct-10 8:13am

I've updated one of my answers above with some sample code.

Toli Cuturicu 24-Oct-10 14:49pm

Convert your fake answers to appropriate comments by tomorrow or I will delete them!

Passing C# Multidimensional Array to C++

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