use GridView1_RowDataBound event of gridView
Add Following Line DataKeyNames="Serial Number,Internal Serial Number,Waranty"
<asp:gridview runat="server" id="GridView1" backcolor="White" xmlns:asp="#unknown">DataKeyNames="Serial Number,Internal Serial Number,Waranty"></asp:gridview>
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
TextBox txtvalue;
if (e.Row.RowType == DataControlRowType.DataRow)
{
txtvalue = (TextBox)e.Row.FindControl("txtSNo");
txtvalue.Text=GridView1.DataKeys[e.Row.RowIndex]["txtSNo"].ToString();
}
}