MainActivity.cs

using System;

using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;

namespace Test01a
{
  [Activity (Label = "Test01a", MainLauncher = true)]
	public class Activity1 : Activity
	{
		protected override void OnCreate (Bundle bundle)
		{
			base.OnCreate (bundle);
			SetContentView (Resource.Layout.MainContainer);
		}
	}
}

MainContainer.axml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <fragment
        class="TestApp01.MainFragment"
        android:id="@+id/fragment1"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

MainFragment.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Util;
using Android.Views;
using Android.Widget;

namespace Test01a
{
  public class MainFragment : Fragment
	{
		int count = 1;

		public override View OnCreateView (LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
		{
			var rootView = inflater.Inflate(Resource.Layout.Main, container);
			var button = rootView.FindViewById<Button> (Resource.Id.myButton);
			
			button.Click += delegate {
				button.Text = string.Format ("{0} clicks!", count++);
			};

			return button;
		}
	}
}