public ListView myList;
List < string>Contacts = new List< string>();

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.Control);
        myList = FindViewById<ListView>(Resource.Id.clientsList);
        myList.ItemLongClick += MyList_ItemLongClick;
    }

    public void insertContact(string ContactInfo)
    {
        list =FindViewById<ListView>(Resource.Id.contactssList);
        contacts.Add(clientInfo);
        ArrayAdapter<string> myArray = new ArrayAdapter<string>(Application.Context, Android.Resource.Layout.SimpleListItem1, contacts);
        list.Adapter = null;
        list.Adapter = myArray;
        myList.DeferNotifyDataSetChanged();
    }

    public void startAsync(){
        Contacts con = new Contacts();
        Thread thread = new Thread(()=> con.Start());
        thread.Start();
    }

MainAct main = new MainAct();
main.insertContact("new Contact");

public ListView getListView()
        {
            ListView myList = FindViewById<ListView>(Resource.Id.contactssList);
            return myList;
        }

main.insertClient("what", main.getListView());

main.RunOnUIThread(()=>     main.insertClient("what", main.getListView()));