Paging GridView with ROW_NUMBER()

using System;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class PagingGridView : System.Web.UI.Page
{

    public int PageNum
    {
        get { return Convert.ToInt16(ViewState["PageNum"]); }
        set { ViewState["PageNum"] = value; }
    }

    public int PageSize
    {
        get { return Convert.ToInt16(ViewState["PageSize"]); }
        set { ViewState["PageSize"] = value; }
    }

    
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            PageNum = 1;
            PageSize = 3;
            BindGridView();
        }

    }

    private string ConnectionString
    {
        get { return @"Data Source=(local);Initial Catalog=AdventureWorks;Integrated Security=True"; }
    }

    private void BindGridView()
    {

        DataSet objDs = new DataSet();
        System.Data.SqlClient.SqlConnection
        myConnection = new SqlConnection(ConnectionString);
        System.Data.SqlClient.SqlDataAdapter myCommand;
        myCommand = new System.Data.SqlClient.SqlDataAdapter("sp_GridView_RowNumber", myConnection);
        myCommand.SelectCommand.CommandType = CommandType.StoredProcedure;

        // PageNum
        myCommand.SelectCommand.Parameters.Add(new SqlParameter("@PageNum", SqlDbType.Int, 4));
        myCommand.SelectCommand.Parameters["@PageNum"].Value = PageNum;

        // PageSize
        myCommand.SelectCommand.Parameters.Add(new SqlParameter("@PageSize", SqlDbType.Int, 4));
        myCommand.SelectCommand.Parameters["@PageSize"].Value = PageSize;

        // TotalRowsNum
        SqlParameter TotalRowsNum = new SqlParameter("@TotalRowsNum", SqlDbType.Int);
        TotalRowsNum.Value = null;
        TotalRowsNum.Direction = ParameterDirection.Output;
        myCommand.SelectCommand.Parameters.Add(TotalRowsNum);
        myCommand.SelectCommand.Parameters["@TotalRowsNum"].Value = null;

        myConnection.Open();
        myCommand.Fill(objDs);

        GridView1.DataSource = objDs;
        GridView1.DataBind();

        Navigation(Convert.ToInt32(TotalRowsNum.Value.ToString()));

    }

    private void Navigation(int totalRecords)
    {
        Double totalPages = Math.Ceiling(((double)totalRecords / PageSize));

        if ((totalRecords == 1) || (totalPages == 0))
        {
            totalPages = 1;
        }

        if (PageSize > totalRecords)
        {
            PageSize = (int)totalPages;
        }

        GoToPageTxt.Text = PageNum.ToString();
        CurrentPage.Text = PageNum.ToString();
        TotalPages.Text =  totalPages.ToString();
    }


    protected void NavigationLink_Click(Object sender, CommandEventArgs e)
    {
        switch (e.CommandName)
        {
            case "First":
                PageNum = 1;
                break;
            case "Last":
                PageNum = Convert.ToInt16(TotalPages.Text);
                break;
            case "Next":
                PageNum = Convert.ToInt16(CurrentPage.Text) + 1;
                break;
            case "Prev":
                PageNum = Convert.ToInt16(CurrentPage.Text) - 1;
                break;
        }
        BindGridView();
    }

    protected void PageSizeImb_Click(object sender, ImageClickEventArgs e)
    {
        PageNum = 1;
        PageSize = Convert.ToInt16(PageSizeTxt.Text);
        BindGridView();
    }


    protected void GoToPageImb_Click(object sender, ImageClickEventArgs e)
    {
        if (GoToPageTxt.Text != "")
        {
            int maxPage  = Convert.ToInt32(TotalPages.Text);
            int goToPage = Convert.ToInt32(GoToPageTxt.Text);

            if (goToPage <= maxPage)
            {
                PageNum = goToPage;
                BindGridView();
            }
        }
    }


}