Main.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">
    <TextView
        android:text="Please enter name"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textView1" />
    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/nameEditText" />
    <Button
        android:text="Go"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/goButton" />
</LinearLayout>

MainActivity.cs

using System;

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

namespace TestApp01
{
  [Activity (Label = "TestApp01", 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 Android.App;
using Android.Views;
using Android.Widget;
using Android.OS;

namespace TestApp01
{
  public class MainFragment : Fragment 
	{
		private Button GoButton { get; set; }
		private EditText NameEditText { get; set; }

		public override Android.Views.View OnCreateView (LayoutInflater inflater, 
		                                                 ViewGroup container, 
		                                                 Bundle savedInstanceState)
		{
			View mainView = inflater.Inflate(Resource.Layout.Main, container);

			NameEditText = mainView.FindViewById<EditText> (Resource.Id.nameEditText);
			GoButton = mainView.FindViewById<Button>(Resource.Id.goButton);

			GoButton.Click += (sender, e) => {
				var bundle = new Bundle();
				bundle.PutString("name", NameEditText.Text);

				var transaction = FragmentManager.BeginTransaction();
				var fragment = SubFragment.Instantiate(Activity, "SubFragment", bundle);
				transaction.Replace(Resource.Id.fragment1, fragment);
				transaction.Commit();
				Activity.FragmentManager.ExecutePendingTransactions();
			};

			return mainView;
		}
	}
}

Sub.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"
    android:minWidth="25px"
    android:minHeight="25px">
    <TextView
        android:text="Medium Text"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/messageText" />
</LinearLayout>

SubFragment.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 TestApp01
{
  public class SubFragment : Fragment
	{
		private TextView MessageText { get; set; }

		public override void OnCreate (Bundle savedInstanceState)
		{
			base.OnCreate (savedInstanceState);
		}

		public override Android.Views.View OnCreateView (LayoutInflater inflater, 
		                                                 ViewGroup container, 
		                                                 Bundle savedInstanceState)
		{
			View mainView = inflater.Inflate(Resource.Layout.Sub, container);

			MessageText = mainView.FindViewById<TextView> (Resource.Id.messageText);

			var name = Arguments.GetString("Name");
			MessageText.Text = string.Format("Hello! {0}!", name);

			return mainView;
		}
	}
}