AutoComplete with IFilterable, Mono for Android sample http://screencast.com/t/7rWbUaVY (based on https://github.com/xamarin/monodroid-samples/tree/master/ContentControls)

AutoAdapter.cs

using System;
using System.Collections.Generic;
using Android.App;
using Android.Views;
using Android.Widget;

namespace ContentControls {
	public class AutoAdapter : ArrayAdapter, IFilterable
	{
		LayoutInflater inflater;
		Filter filter;
		Activity context;
		public string[] AllItems;
		public string[] MatchItems;

		public AutoAdapter (Activity context, int txtViewResourceId, string[] items)
			: base(context, txtViewResourceId, items)
		{
			inflater = context.LayoutInflater;
			filter = new SuggestionsFilter(this);
			AllItems = items;
			MatchItems = items;
		}
		public override int Count {
			get {
				return MatchItems.Length;
			}
		}
		public override Java.Lang.Object GetItem (int position)
		{
			return MatchItems[position];
		}
		public override View GetView (int position, View convertView, ViewGroup parent)
		{
			View view = convertView;
			if (view == null)
				view = inflater.Inflate(Android.Resource.Layout.SimpleDropDownItem1Line, null);

            view.FindViewById<TextView>(Android.Resource.Id.Text1).Text = MatchItems[position];

            return view;
		}
	
		public override Filter Filter {
			get {
				return filter;
			}
		}

		class SuggestionsFilter : Filter
		{
			AutoAdapter a;
			public SuggestionsFilter (AutoAdapter adapter) : base() {
				a = adapter;
			}
			protected override Filter.FilterResults PerformFiltering (Java.Lang.ICharSequence constraint)
			{
				FilterResults results = new FilterResults();
				if (constraint != null) {
					var searchFor = constraint.ToString ();
Console.WriteLine ("searchFor:" + searchFor);
					var matchList = new List<string>();
					
					var matches = from i in a.AllItems
								where i.IndexOf(searchFor) >= 0
								select i;
	
					foreach (var match in matches) {
						matchList.Add (match);
					}
		
					a.MatchItems = matchList.ToArray ();
Console.WriteLine ("resultCount:" + matchList.Count);

Java.Lang.Object[] matchObjects;
matchObjects = new Java.Lang.Object[matchList.Count];
for (int i = 0; i < matchList.Count; i++) {
	matchObjects[i] = new Java.Lang.String(matchList[i]);
}

					results.Values = matchObjects;
					results.Count = matchList.Count;
				}
				return results;
			}
			protected override void PublishResults (Java.Lang.ICharSequence constraint, Filter.FilterResults results)
			{
				a.NotifyDataSetChanged();
			}
		}
	}
}

AutoCompleteTextViewScreen.cs

using System.Collections.Generic;
using System.IO;
using Android.App;
using Android.OS;
using Android.Widget;

namespace ContentControls {

    [Activity(Label = "AutoCompleteTextView")]
    public class AutoCompleteTextViewScreen : Activity {
       
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            SetContentView(Resource.Layout.AutoCompleteTextView);

            AutoCompleteTextView act = FindViewById<AutoCompleteTextView>(Resource.Id.AutoCompleteInput);


            Stream seedDataStream = Assets.Open(@"WordList.txt");
            //StreamReader reader = new StreamReader(seedDataStream);

            List<string> lines = new List<string>();
            using (StreamReader reader = new StreamReader(seedDataStream)) {
                // 3
                // Use while != null pattern for loop
                string line;
                while ((line = reader.ReadLine()) != null) {
                    // 4
                    // Insert logic here.
                    // ...
                    // "line" is a line in the file. Add it to our List.
                    lines.Add(line);
                }
            }

            string[] wordlist = lines.ToArray();

            act.Adapter = new AutoAdapter(this, Android.Resource.Layout.SimpleDropDownItem1Line, wordlist);
        }
    }
}

I'm trying to implement an autocomplete textview as same like you but item list is keep on updating at run time.. can you please tell me how it will work