Xamarin.iOS で特定の画面だけ向きを固定する時ハマったこと

//MyNavigationController.cs
// This file has been autogenerated from a class added in the UI designer.

using System;
using MonoTouch.Foundation;
using MonoTouch.UIKit;
using System.Linq;

namespace MyApp
{
	public partial class MyNavigationController : UINavigationController
	{
		public MyNavigationController (IntPtr handle) : base (handle) { }

        public override UIInterfaceOrientation PreferredInterfaceOrientationForPresentation()
        {
            return this.ViewControllers.Last().PreferredInterfaceOrientationForPresentation();
        }

        public override UIInterfaceOrientationMask GetSupportedInterfaceOrientations()
        {
            return this.ViewControllers.Last().GetSupportedInterfaceOrientations();
        }

        public override bool ShouldAutorotate()
        {
            return this.ViewControllers.Last().ShouldAutorotate();
        }

	}
}

//BViewController.cs
// This file has been autogenerated from a class added in the UI designer.

using System;

using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace MyApp
{
	public partial class BViewController : UIViewController
	{
		public PhotoPreviewViewController (IntPtr handle) : base (handle)
		{
		}

        public override UIInterfaceOrientation PreferredInterfaceOrientationForPresentation()
        {
            return UIInterfaceOrientation.LandscapeLeft;
        }

        public override UIInterfaceOrientationMask GetSupportedInterfaceOrientations()
        {
            return UIInterfaceOrientationMask.LandscapeLeft;
        }

        public override bool ShouldAutorotate()
        {
            return false;
        }
	}
}