Pass array from C# to C++

Question

I am trying to pass an array from C# to a C++ DLL and then print it from C++.

The C# code is the following:

[DllImport("DLL1.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void GMSH_PassVector([MarshalAs(UnmanagedType.SafeArray,SafeArraySubType = VarEnum.VT_I4)] int[] curveTag);

int[] curveTagArray = { 1, 2, 3, 4 };
GMSH_PassVector(curveTagArray);

The C++ header file code is the following:

extern "C" GMSH_API  void GMSH_PassVector(std::vector<int> *curveTags);

The C++ cpp file code is the following to display the first element of the array:

void GMSH_PassVector(std::vector<int> *curveTags)
{
    printf("Hello %d \n", curveTags[0]);
}

Seems that I'm doing something wrong, a value is displayed but it is the wrong one.

Does this answer your question? How to pass C# array to C++ and return it back to C# with additional items? — 41 72 6c
– 41 72 6c, Commented Jul 5, 2022 at 6:27
I don't think the marshal mechanism is able to translate a int[] to a std::vector. It's better to use language base types. Change the C++ function as void GMSH_PassVector(int * arr, int size) and in C# you have to instantiate a IntPtr to hold the address of the first element of the array. — Marco Beninca
– Marco Beninca, Commented Jul 5, 2022 at 6:32

Marco Beninca · Accepted Answer · 2022-07-05 07:56:06Z

2

I don't think the marshal mechanism is able to translate a int[] to a std::vector. It's better to use language base types. Change the C++ function as

void GMSH_PassVector(int * arr, int size)

and in C# you have to instantiate a IntPtr to hold the address of the first element of the array.

int bufferSize = 4;
int[] buffer = new int[bufferSize];

buffer[0] = 1;
buffer[1] = 2;
buffer[2] = 3;
buffer[3] = 4;

int size = Marshal.SizeOf(buffer[0]) * buffer.Length;
IntPtr bufferPtr = Marshal.AllocHGlobal(size);
Marshal.Copy(buffer, 0, bufferPtr, size);

GMSH_PassVector(bufferPtr, size);

Marshal.FreeHGlobal(bufferPtr);

answered Jul 5, 2022 at 7:56

Marco Beninca

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