Libuv bindings with static callbacks, no extra c layer, and magic numbers!

PrepareWatcher.cs

// Compile on Mac with Mono as 'dmcs PrepareWatcher.cs'
// With a libuv clone, after 'make', do:
// 	ar -x uv.a
// 	gcc -o libuv.dylib -dynamiclib -m32 *.o -framework CoreServices
// 	rm __.SYMDEF\ SORTED *.o
//
// Then run with 'mono PrepareWatcher.exe'
using System;
using System.Runtime.InteropServices;

namespace Libuv {
	public class PrepareWatcher {
		[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
		public delegate void uv_prepare_cb(IntPtr prepare, int status);
		[DllImport("uv")]
		internal static extern int uv_prepare_init(IntPtr prepare);
		[DllImport("uv")]
		internal static extern int uv_prepare_start(IntPtr prepare, uv_prepare_cb cb);
		[DllImport("uv")]
		internal static extern int uv_prepare_stop(IntPtr prepare);

		private static uv_prepare_cb unmanaged_callback;

		static PrepareWatcher()
		{
			unmanaged_callback = StaticCallback;
		}
		
		private Action<int> callback;
		private IntPtr _handle;
		private GCHandle me;
		public PrepareWatcher(Action<int> callback)
		{
			this._handle = Marshal.AllocHGlobal(64);
			uv_prepare_init(this._handle);
			var handle = (uv_handle_t)Marshal.PtrToStructure(this._handle, typeof(uv_handle_t));
			this.me = GCHandle.Alloc(this, GCHandleType.Pinned);
			handle.data = GCHandle.ToIntPtr(this.me);
			this.callback = callback;
		}
		private static void StaticCallback(IntPtr watcher, int status)
		{
			var handle = (uv_handle_t)Marshal.PtrToStructure(watcher, typeof(uv_handle_t));
			var instance = GCHandle.FromIntPtr(handle.data);
			var watcher_instance = (PrepareWatcher)instance.Target;
			watcher_instance.callback(status);
		}
		public void Start()
		{
			uv_prepare_start(this._handle, unmanaged_callback);
		}
		public void Stop()
		{
			uv_prepare_stop(this._handle);
		}
		~PrepareWatcher()
		{
			Cleanup();
		}
		private void Cleanup()
		{
			me.Free();
			Marshal.FreeHGlobal(this._handle);
		}
	}
	public struct uv_handle_t {
		public uv_handle_type type;
		public uv_close_cb close_cb;
		public IntPtr data;
	}
	[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
	public delegate void uv_close_cb(IntPtr handle);
	public enum uv_handle_type {
		UV_UNKNOWN_HANDLE = 0,
		UV_TCP,
		UV_NAMED_PIPE,
		UV_TTY,
		UV_FILE,
		UV_TIMER,
		UV_PREPARE,
		UV_CHECK,
		UV_IDLE,
		UV_ASYNC,
		UV_ARES_TASK,
		UV_ARES_EVENT,
		UV_GETADDRINFO,
		UV_PROCESS
	}
	public class MainClass {
		[DllImport("uv")]
		internal static extern int uv_init();
		[DllImport("uv")]
		internal static extern int uv_run();
		public static void Main() {
			uv_init();
			PrepareWatcher prepare = new PrepareWatcher((status) => {
					Console.WriteLine("Prepare Watcher Called");
			});
			prepare.Start();
			uv_run();
		}
	}
}