gridview paging and sorting

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" OnPageIndexChanging="GridView1_PageIndexChanging"
                                PageSize="5" AllowPaging="True" AllowSorting="true" OnRowEditing="GridView1_RowEditing"
                                OnSorting="gridView1_Sorting">
                                <Columns>
                                    <asp:TemplateField SortExpression="Tital" >
                                        <HeaderTemplate>
                                            Tital</HeaderTemplate>
                                        <ItemTemplate>
                                            <asp:Label ID="lbltital" runat="server" Text='<%#Eval("Tital")%>'></asp:Label>
                                        </ItemTemplate>
                                    </asp:TemplateField>



 protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        GridView1.PageIndex = e.NewPageIndex;
        Obj.Fillgrid(GridView1, "Select PK_ID,Tital From Aboutus");
    }
    protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
    {

    }


    protected void gridView1_Sorting(object sender, GridViewSortEventArgs e)
    {
        //(GridView1.DataSource.GetType()); //Add this line

        DataTable m_DataTable = GridView1.DataSource as DataTable;

        if (m_DataTable != null)
        {
            DataView m_DataView = new DataView(m_DataTable);
            m_DataView.Sort = e.SortExpression + " " + e.SortDirection;

            GridView1.DataSource = m_DataView;
            GridView1.DataBind();
        }
    }
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  DataTable dt = Obj.getdata("Select PK_ID,Tital From Aboutus");

       if (ViewState["dir"] != null)
       {
           if (ViewState["dir"].ToString() == "ASC")
           {
               ViewState["dir"] = "DESC";
           }
           else { ViewState["dir"] = "ASC"; }
       }
       else
       {
           ViewState["dir"] = "ASC";
       }

       dt.DefaultView.Sort = e.SortExpression + " " + ViewState["dir"].ToString();

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