Tool Tip is not showing after paging

Page_Load
==========

foreach (GridViewRow item in GridView1.Rows)
{
item.Cells[2].BackColor = Color.FromName("#6698FF");


string StudentCode = item.Cells[2].Text;
StudentCode = StudentCode.Substring(0, 2);

string sc = getstudentcode(StudentCode);
item.Cells[2].ToolTip = sc;



public string getStudentcode(string Studentcode)
{
string result = "Test";
SqlConnection con = new SqlConnection("Data Source=Ranjith-pc;Initial Catalog=StudentsDB;User Id=sa;Password=123;");

SqlCommand cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandText = "select StudentName from StudentsCode where Code='" + StudentCode + "'";

using (con)
{
con.Open();
SqlDataReader reader = cmd.ExecuteReader();
if (reader.Read())
{
result = reader["StudentName"].ToString();
}

}
return result;
}

// Paging

protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{

GridView1.PageIndex = e.NewPageIndex;
GridView1.DataBind();
}

This is my code
================

protected void Page_Load(object sender, EventArgs e)
{
foreach (GridViewRow item in GridView1.Rows)
{
item.Cells[2].BackColor = Color.FromName("#6698FF");


string StudentCode = item.Cells[2].Text;
StudentCode = StudentCode.Substring(0, 2);

string sc = getstudentcode(StudentCode);
item.Cells[2].ToolTip = sc;
}


public string getStudentcode(string Studentcode)
{
string result = "Test";
SqlConnection con = new SqlConnection("Data Source=Ranjith-pc;Initial Catalog=StudentsDB;User Id=sa;Password=123;");

SqlCommand cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandText = "select StudentName from StudentsCode where Code='" + StudentCode + "'";

using (con)
{
con.Open();
SqlDataReader reader = cmd.ExecuteReader();
if (reader.Read())
{
result = reader["StudentName"].ToString();
}

}
return result;
}

// Paging

protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{

GridView1.PageIndex = e.NewPageIndex;
GridView1.DataBind();
}

This is my perfect code
=========================
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
using System.Web.Configuration;
using System.Drawing;
using System.Collections.Specialized;
using System.Text.RegularExpressions;

public partial class Users_StudentDetails : System.Web.UI.Page
{
private static readonly string _connString = String.Empty;
SqlConnection con = new SqlConnection(_connString);

protected void Page_Load(object sender, EventArgs e)
{

foreach (GridViewRow item in GridView1.Rows)
{
item.Cells[2].BackColor = Color.FromName("#6698FF");


string StudentCode = item.Cells[2].Text;
StudentCode = StudentCode.Substring(0, 2);

string sc = getstudentcode(StudentCode);
item.Cells[2].ToolTip = sc;
}
}


public string getStudentcode(string Studentcode)
{
string result = "Test";
SqlConnection con = new SqlConnection("Data Source=Ranjith-pc;Initial Catalog=StudentsDB;User Id=sa;Password=123;");

SqlCommand cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandText = "select StudentName from StudentsCode where Code='" + StudentCode + "'";

using (con)
{
con.Open();
SqlDataReader reader = cmd.ExecuteReader();
if (reader.Read())
{
result = reader["StudentName"].ToString();
}

}
return result;
}

// Paging

protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{

GridView1.PageIndex = e.NewPageIndex;
GridView1.DataBind();
}

static Users_StudentDetails()
{
_connString = WebConfigurationManager.ConnectionStrings["StudentsDBConnectionString"].ConnectionString;
}


}