Receive an object passed from a C function to a callback. (Alternate definition using struct)

csharpcode.cs

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
struct Foo
{
    public int Value;
}

class Program
{
    // Declare type of callback function:
    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
    public delegate void FooCallback(ref Foo passedObject);

    // Declare native C function:
    [DllImport("NativeDLL.dll")]
    public static extern void InvokeCallbackWithObject(FooCallback callback);

    static void Main(string[] args)
    {
        InvokeCallbackWithObject(MyCallback);
    }

    // The function receiving the object:
    static void MyCallback(ref Foo passedObject)
    {
        Console.WriteLine("Got: " + passedObject.Value);

        // mutate via pointer:
        unsafe
        {
            fixed (Foo* fooPtr = &passedObject)
            {
                fooPtr->Value += 100;
            }
        }

        Console.WriteLine("Got: " + passedObject.Value);
    }
}

nativecode.cpp

// Sorry, only have MSVC lying around...

#ifdef NATIVEDLL_EXPORTS
#define NATIVEDLL_API __declspec(dllexport)
#else
#define NATIVEDLL_API __declspec(dllimport)
#endif

extern "C"
{
    struct Foo
    {
        int Value;
    };

    void NATIVEDLL_API InvokeCallbackWithObject(void (*func)(Foo*))
    {    
        Foo myFoo;
        myFoo.Value = 123;
        func(&myFoo);
    }
}