c# create unmanaged object* array

Question

i need to create a variable length array of unmanaged object, but i need the array to be unmanaged too, so i can't use List<> or other classes but i have to malloc/realloc it in the c way.

this is my code:

unsafe class A{
    unsafe private int a1, a2;
    unsafe public byte* arr;
}

and on a different class method i would like to do something like this:

private unsafe A* arr = Marshal.AllocHGlobal(array_size * Marshal.SizeOf<A>());

Now i these questions:

1. How to convert IntPtr to A*? OR does exist a proper c# malloc/realloc function? OR if i .dll import the real c/c++ realloc function would be safe to use it?
2. How do i get a A* pointer to an A unmanaged object? if i try with '&' it says "cannot take the adress of [...] a managed object" but it shouldn't be managed. Am I doing something wrong in class declaration?

optional: why sizeof(A) doesn't work but Marshal.SizeOf() it's fine?

Answer 1

You cannot use C# class for such things, but you can use struct:

[StructLayout(LayoutKind.Sequential)]
struct A
{
    public int A1;
    public int A2;
}

int array_size = 5;
// just cast IntPtr to A*
A* arr = (A*) Marshal.AllocHGlobal(array_size * Marshal.SizeOf<A>());
// set first element of unmanaged array
arr[0] = new A {A1 = 1, A2 = 2};
// set second
arr[1] = new A {A1 = 3, A2 = 4};

// alternative syntax:
// set first element
*arr = new A { A1 = 5, A2 = 6 };
// set second element
*(arr + 1) = new A { A1 = 5, A2 = 6 };
// get first element
var first = *arr;
var second = *(arr + 1);
Console.WriteLine(first.A1 + " " + first.A2);
Marshal.FreeHGlobal((IntPtr)arr);