Updating a C# array inside C++ without Marshal.Cop

Question

I am wanting to update an array that was created inside C#, and then pass a pointer to that array over to C++ and let C++ populate the indexes, to be used back in C#. Right now I am Using a Marshal.Copy() to accomplish this task, but I would like to avoid the potentially unnecessary copy, and call back to c++ to release the array. Is this even possible?

These array are floats and ints, for geometric mesh data.

My current usage (working and not what I want to use) C#

    IntPtr intptr=new IntPtr();
    int count = 0;
    PopulateArray(ref intptr, ref count);
    float[] resultVertices = new float[count];
    Marshal.Copy(intptr, resultVertices, 0, count);

C++

extern "C" __declspec(dllexport) bool PopulateArray(float** resultVerts, int* resultVertLength){

    *resultVerts = new float[5]{0.123f, 3.141529f, 127.001f, 42.42f, 0};
    int myX = 5;
    *resultVertLength = myX;
    return true;
}

Answer 1

If you are willing to allow C# to allocate the array (probably a safer alternative) then you could do this behavior with standard PInvoke attributes.

Change your C++ declaration to:

extern "C" __declspec(dllexport) bool PopulateArray(float resultVerts[], int resultVertLength)

and your C# declaration to:

[DllImport("Win32Library.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern bool PopulateArray([MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)] float[] resultVerts, int resultVertLength);

Your usage from the C# side would then change to:

var resultVertices = new float[5];
PopulateArray(resultVertices, resultVertices.Length);

Answer 2

The only safe way to have C++ code update a managed C# array is to pin the array. Otherwise, it's possible for the garbage collector to try to move the array while the native code is running. You can do this with a GCHandle object.

int count = 5; 
float[] resultVertices = new float[count];

GCHandle handle = GCHandle.Alloc(resultVertices, GCHandleType.Pinned);
IntPtr address = handle.AddrOfPinnedObject();

PopulateArray(address, count);

handle.Free();

---

It can also be done with unsafe code, which is somewhat more intuitive to read and remember:

int count = 5; 
float[] resultVertices = new float[count];
unsafe 
{
    fixed(float* ptr = resultVertices)
    {
        PopulateArray(ptr, count);
    }
}

---

Another alternative is to have C# allocate an unmanaged chunk of memory and pass that to the C++ method. This is better than what you're doing because you are not placing the responsibility of allocation/deallocation in the C++ library code and instead keeping that all in your C#. I know you want to avoid the coy but sometimes doing the copy is more performant than pinning objects, but it depends on how large they are. I recommend you do performance testing to determine which is best for your situation.

int count = 5; 
float[] resultVertices = new float[count];
IntPtr unmanagedMemory = Marshal.AllocHGlobal(count * Marshal.SizeOf(typeof(float)));
PopulateArray(unmanagedMemory, count);
Marshal.Copy(unmanagedMemory, resultVertices, 0, count);

In all these scenarios you should set your C++ code to operate like this:

extern "C" __declspec(dllexport) bool PopulateArray(float* resultVerts, int vertLength)
{
    resultVerts[0] = 0.123f;
    // fill out the rest of them any way you like.
    return true;
}

If the array size is variable, then I recommend having a separate C++ method that calculates the size and returns it rather than having the C++ method allocate the memory.