Xamarin Component Store を眺めてみる ref: http://qiita.com/items/811cdd8ab3d1243045b6

MainActivity.cs

<省略>
using Xamarin.Controls;

namespace AlertCenterSample
{
    [Activity (Label = "AlertCenterSample", MainLauncher = true)]
    public class Activity1 : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);

            // Get our button from the layout resource,
            // and attach an event to it
            Button button = FindViewById<Button>(Resource.Id.myButton);

            button.Click += delegate
            {
                AlertCenter.Default.Init (Application);

                AlertCenter.Default.PostMessage ("Knock knock!", "Who's there?", Resource.Drawable.Icon);
                AlertCenter.Default.PostMessage ("Interrupting cow.", "Interrupting cow who?",
                                                 Resource.Drawable.Icon, () => {
                    Console.WriteLine ("Moo!");
                });
            };
        }
    }
}