Question

this is the 2nd part of the 1st question using c# pointers

so pointers in c# are ‘unsafe’ and not managed by the garbage collector while an IntPtr is a managed object. but why use pointers then? and when it is possible to use both approaches interchangeably?

Answer 1

The CLI distinguishes between managed and unmanaged pointers. A managed pointer is typed, the type of the pointed-to value is known by the runtime and only type-safe assignments are allowed. Unmanaged pointers are only directly usable in a language that supports them, C++/CLI is the best example.

The equivalent of an unmanaged pointer in the C# language is IntPtr. You can freely convert a pointer back and forth with a cast. No pointer type is associated with it even though its name sounds like “pointer to int”, it is the equivalent of void* in C/C++. Using such a pointer requires pinvoke, the Marshal class or a cast to a managed pointer type.

Some code to play with:

using System;
using System.Runtime.InteropServices;

unsafe class Program {
    static void Main(string[] args) {
        int variable = 42;
        int* p = &variable;
        Console.WriteLine(*p);
        IntPtr raw = (IntPtr)p;
        Marshal.WriteInt32(raw, 666);
        p = (int*)raw;
        Console.WriteLine(*p);
        Console.ReadLine();
    }
}

Note how the unsafe keyword is appropriate here. You can call Marshal.WriteInt64() and you get no complaint whatsoever. It corrupts the stack frame.