Pinvoking a native function with array arguments

Question

I am completely confused with how to go about calling functions in native dll with array arguments.

Example:

The function is defined in the C# project as:

Answer 1

Your C++ code is broken. The caller allocates the array, and the callee populates it. Like this:

extern "C" _declspec(dllexport) void modifyArray(int* x, int len)
{   
    for (int i=0; i<len; i++)
        x[i] = i;
}

As far as your p/invoke call goes, SetLastError should not be true. The function is not calling SetLastError. It should be:

[DllImport("Project2.dll", CallingConvention = CallingConvention.Cdecl)]
static extern void modifyArray(int[] x, int len);

Answer 2

This has nothing to do with PInvoke, this is just a plain old C issue. You would have the exact same problem if you called modifyArray from C

int* pArray = NULL;
modifyArray(pArray, len);
pArray == NULL;  // true! 

In modifyArray you are trying to change where x points to. This change won't be visible to the calling function because the pointer is passed by value. In order to change where it points to you need to pass a double pointer

void modifyArray(int** x, int len) { 
  *x = ...;
}

Note that you are currently trying to return stack allocated memory instead of heap allocated memory. That is incorrect and will lead to problems down the line