async sample

gistfile1.txt

using System;
using System.Threading;
using System.Threading.Tasks;
using Android.App;
using Android.Widget;
using Android.OS;

namespace MonoAndroidApplication18
{
    [Activity(Label = "MonoAndroidApplication18", MainLauncher = true, Icon = "@drawable/icon")]
    public class Activity1 : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            
            SetContentView(Resource.Layout.Main);

            var button = FindViewById<Button>(Resource.Id.MyButton);

            button.Text = "Before task starts";

            Task.Factory.StartNew(() =>
            {
                Thread.Sleep(5000);

                RunOnUiThread(() => 
                    button.Text = "Task is complete");
            });

            button.Text = "After task starts";
        }
    }
}