passing 2D array from C# to unmanaged C++ using C++/CLI wrapper

Question

I am working in a project that requires optimization and I want to translate one part of the code from C# to C++. I am using a C++\CLI wrapper but I am really new with this approach and I have not come to understand it completely. The following code returns an error when I run the program and I don't know the reason.

The C# program is the following:

int[,] Arr = new int[5, 5];

        for (int i = 0; i < 5; i++)
        {
            for (int j = 0; j < 5; j++)
            {
                Arr[i, j] = i + j;
            }
        }

test.MatrixComputation(Arr, Arr.GetLength(0));

The C++/CLI project as follows:

void MatrixComputation(cli::array<int, 2> ^arr, int size)
    {
        pin_ptr<int> p_arr = &arr[0, 0];
        pu -> ChangeArray((int**)p_arr, size);
    }

The Unmanaged C++ code:

void Unmanaged::MatrixComputation(int** arr, int size)
{
    for (int i = 0; i < size; i++)
    {
        for (int j = 0; j < size; j++)
        {
            arr[i][j] = i + j;     // line 27
        }
    }

    std::cout << arr[2][2] << std::endl;
}

It compiles well but when I run it I get the following error:

*Line 27: System.NullReferenceException: Object reference not set to an instance of an object*

The casting to double pointer is a bad strategy but it is the only thing that came into my mind. Also, I know that C++ doesn't have 2D arrays just like C# has, but I need the multidimensional array in C# and I can not change it to write a jagged array Arr[][].

Thanks in advance.

That cast got you into trouble, the C++ code assumes it is a jagged array and it is not. Declare the argument int*, use arr[i * size + j] to index the array. — Hans Passant
– Hans Passant, Commented Mar 6, 2018 at 11:54
I will do that, do I need to previously fix the array using fixed{} in the C# code? I didn't do this in the code I showed up but the array may be moved to another location inside the memory while my C++ code is reading from it. — Andres J. Sanchez-Fernandez
– Andres J. Sanchez-Fernandez, Commented Mar 7, 2018 at 9:48

kfazi · Accepted Answer · 2018-03-06 11:38:59Z

1

You should copy managed array to unmanaged one and then do the computations:

void MatrixComputation(cli::array<int, 2> ^arr, int size)
{
    std::unique_ptr<int[]> myArray(new int[size * size]);
    for (auto y = 0; y < size; y++)
    {
        for (auto x = 0; x < size; x++)
        {
            myArray[y * size + x] = arr[x, y];
        }
    }

    pu -> ChangeArray(std::move(myArray), size);
}

void Unmanaged::MatrixComputation(std::unique_ptr<int[]> arr, int size)
{
    for (int i = 0; i < size; i++)
    {
        for (int j = 0; j < size; j++)
        {
            arr[i * size + j] = i + j;
        }
    }

    std::cout << arr[2 * size + 2] << std::endl;
}

answered Mar 6, 2018 at 11:38

kfazi

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1 Comment

Andres J. Sanchez-Fernandez Over a year ago

Thanks for the answer, I will try to do it too and see how is its performance!

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