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

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