Test.cs

using Xamarin.Forms;
using System.Diagnostics;

namespace TBTest
{
	public class App : Application
	{
		public App ()
		{
			// The root page of your application
			var contentPage = new ContentPage {
				Content = new StackLayout {
					VerticalOptions = LayoutOptions.Center,
					Children = {
						new Label {
							XAlign = TextAlignment.Center,
							Text = "Welcome to Xamarin Forms!"
						}
					}
				}
			};
			contentPage.ToolbarItems.Add (
				new ToolbarItem { Text = "One", Order = ToolbarItemOrder.Primary }
			);

			var enabledButton = new ToolbarItem { Text = "Enabled", Order = ToolbarItemOrder.Secondary,
				Command = new Command (() => Debug.WriteLine ("Enabled Button Clicked"), () => true)};

			var disabledButton = new ToolbarItem { Text = "Disabled", Order = ToolbarItemOrder.Secondary,
				Command = new Command (() => Debug.WriteLine ("Disabled Button Clicked"), () => false)};


			contentPage.ToolbarItems.Add (enabledButton);
			contentPage.ToolbarItems.Add (disabledButton);

			MainPage = new NavigationPage (contentPage);
		}
	}
}