IntPtr to three dimensional array

Question

I am using a C++ dll into my C# project. Using pInvoke (Platform Invoke).

My C++ code returns a double***, and the C# function returns a IntPtr.

I have the dimensions of the matrix on both sides, they are just not constant.

On C# side I have:

[DllImport("mylib.dll")]
public static extern IntPtr GetMatrix(int width, int height, int depth);

And on C++ side I have:

extern "C" {
    __declspec(dllexport) double*** GetMatrix(int width, int height, int depth) {
        // do stuff
        return matrix;
    }
}

Now I need to convert this IntPtr to a double[,,], but, I have no idea how to do that.

Basicaly I need the oposite to the answer of this question.

Answer 1

I've never tried this with these kinds of arrays, but this first solution may work and I'm lead to believe so by this MSDN article:

(Note this only works if you can change the function definition on the C# side) In your C# function, change your IntPtr parameter to a double[,,] parameter and add this MarshalAs attribute to it, similar to how the above MSDN article does it.

[DllImport("FooBar.dll")]
extern void Foo([MarshalAs(UnmanagedType.LPArray, SizeConst=**YOUR ARRAY SIZE HERE**)] double[,,] bar);

This also assumes that you have a constant-sized array, and I'm not sure how this would exactly work with a 3d array, but if you'd like to investigate this type of solution further, here's the full documentation for MarshalAsAttribute.

---

The other solution is very similar to the opposite of that other question. You would initialize your double[,,] to whatever size you needed, then start filling in rows with Marshal.Copy (this overload). You could also marshal over a single double with the same function. Something like this:

double[,,] arr = new double[25, 25, 200];
int il = arr.GetLength(0), jl = arr.GetLength(1), kl = arr.GetLength(2);

IntPtr values = DllFoo();

for (int i = 0; i < il; i++)
{
    for (int j = 0; j < jl; j++)
    {
        for (int k = 0; k < kl; k++)
        {
            double[] temp = new double[1];
            arr[i,j,k] = Marshal.Copy(values, temp, 1, k + j * jl + k * jl * kl);
        }
    }    
}

You can definitely squeeze a lot more performance out of that snippet, but it should still work.