foo.cs

using System;
using AppKit;
using Foundation;

namespace SavePanelTest
{
	public static class NSSavePanelPatch
	{
		[System.Runtime.InteropServices.DllImport ("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
		extern static IntPtr IntPtr_objc_msgSend (IntPtr receiver, IntPtr selector);

		public static NSSavePanel GetSavePanel ()
		{
				return ObjCRuntime.Runtime.GetINativeObject<NSSavePanel> (IntPtr_objc_msgSend (ObjCRuntime.Class.GetHandle ("NSSavePanel"), ObjCRuntime.Selector.GetHandle ("savePanel")), true);
		}
	}

	[Register ("AppDelegate")]
	public class AppDelegate : NSApplicationDelegate
	{
		public override void DidFinishLaunching (NSNotification notification)
		{
			NSSavePanel panel = NSSavePanelPatch.GetSavePanel ();
			panel.RunModal ();
		}
	}
}