Smuggling a GCHandle as an IntPtr.

smuggle.cs

using System;
using System.Runtime.InteropServices;

class Foo
{
}

class Program
{
    static IntPtr AllocAndGetPointer()
    {
        var foo = new Foo();
        var gcHandle = GCHandle.Alloc(foo);
        return (IntPtr)gcHandle;
    }

    static Foo RetrieveObjectFromPointer(IntPtr ptr)
    {
        var handle = GCHandle.FromIntPtr(ptr);
        var foo = (Foo) handle.Target;
        handle.Free(); // depending on if you need it later
        return foo;
    }

    static void Main(string[] args)
    {
        var ptr = AllocAndGetPointer();
         
        // pass ptr into some native method, then in callback:
        var foo = RetrieveObjectFromPointer(ptr);

        Console.WriteLine(foo);
    }

}