ViewState in Dynamic Control

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class DynamicControl : System.Web.UI.Page
{
    private const string VIEWSTATEKEY = "ContactCount";

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            //Set the number of default controls
            ViewState[VIEWSTATEKEY] = ViewState[VIEWSTATEKEY] == null ? 2 : ViewState[VIEWSTATEKEY];
            
            //Load the contact control based on Vewstate key
            LoadContactControls();
        }
    }

    /// <summary>
    /// Load the contact control based on Vewstate key
    /// </summary>
    private void LoadContactControls()
    {
        for (int i = 0; i < int.Parse(ViewState[VIEWSTATEKEY].ToString()); i++)
        {
            phContactDetails.Controls.Add(LoadControl("~/UserControl/ContactDetails.ascx"));
        }
    }

    /// <summary>
    /// Handels the click event of AddMore button
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    protected void btnAddMore_Click(object sender, EventArgs e)
    {
        ViewState[VIEWSTATEKEY] = int.Parse(ViewState[VIEWSTATEKEY].ToString()) + 1;
        LoadContactControls();
    }
}