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I am working on project that required to pass array of objects from c# to c++ by reference and when I pass one object by reference it return by the new value successfully but when I pass an array it return with the old value so what is wrong with my code?

C++ code

extern "C" __declspec(dllexport) int solve(Cell*(&xx)[5][5]) {
xx[0][0]->side = 55;
return xx[0][0]->side;
}

and C# code

internal static class UnsafeMethods
{
[DllImport(@"E:\Cs\4th Year\HPC\Parallel_calculations\Debug\Parallel_calculations.dll", EntryPoint = "solve", CallingConvention = CallingConvention.Cdecl)]
public static extern int solve([MarshalAs(UnmanagedType.SafeArray)] ref Cell[,] x);
}

 Cell[,] arr = new Cell[5, 5];
        for (int i = 0; i < arr.GetLength(0); i++)
        {
            for (int y = 0; y < arr.GetLength(1); y++)
            {
                arr[i, y] = new Cell(0, 0, 0, false, false, false, 50);
            }
        }
        arr[0, 0].side = 4;
        int x = UnsafeMethods.solve(ref arr);
        Console.WriteLine(x + " " + arr[0, 0].side);

x is with new value but arr[0,0].side return with old value

Improve this question
10
  • Possible duplicate of Pass multi - dimensional array from managed code to unmanaged code Commented Dec 22, 2016 at 18:53
  • no i can send it arleady but without reference ! Commented Dec 22, 2016 at 18:58
  • I am not sure that one can correctly pass multidimensional array to native function other then by how it is described in the linked question, but I may be wrong. In any case, arrays by themselves are passed as [In] parameters - msdn.microsoft.com/en-us/library/z6cfh6e6(v=vs.110).aspx. So you should at least try to add [InOut] attribute on the array parameter. If that won't work, then you should try the proposed solution and then manually reconstruct the array - stackoverflow.com/questions/6747112/… Commented Dec 22, 2016 at 19:05
  • till now no one of the two answers solve my problem and i can only pass the array by value !, any help Commented Dec 22, 2016 at 20:02
  • Another problem with your code is that you specify UnmanagedType.SafeArray, but the C++ code is using a reference to an array (basically a pointer to pointer to the first element of the array). The correct C++ type to use is the SAFEARRAY structure. Otherwise, leave the C++ code as is, and specify UnmanagedType.LPArray in C#. Also, as the other poster mentioned, arrays are always marshaled as one-dimensional between managed and unmanaged code. Commented Dec 22, 2016 at 21:10
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