The Length property is currently missing from the Xamarin.iOS API for IUILayoutSupport (TopLayoutGuide, BottomLayoutGuide). Here's a quick hack to fix it temporarily...

IUILayoutSupportExtensions.cs

using MonoTouch.UIKit;
using MonoTouch.ObjCRuntime;

namespace Shield
{
    public static class TopLayoutGuideExtensions
	{
		public static float Length(this IUILayoutSupport layoutSupport)
		{
			float length = 0f;// = 0;

			try
			{
				var selector = new Selector ("length");
				length = MonoTouch.ObjCRuntime.Messaging.float_objc_msgSend (layoutSupport.Handle, selector.Handle);
			}
			catch { }

			return length;
		}
	}
}