Accept a handle 'inside' a .NET class...

gistfile1.cs

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
struct Foo
{
    private IntPtr _realFoo;

    public override string ToString()
    {
        return "My real value is at: " + _realFoo;
    }
}

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