whenever sending a parameter (using queerystring ) to the another page that parameter is invalid

See more:

ASP.NET

actually i have sending parameter to the another page using queery string.in this we have using
2 tables

table1:Dept
Did varchar(50)-primarykey notnull

Dname varchar(50)-null

Dproduct varchar(50)-null

table2:Emp

Empid varchar(50) null

Ename varchar(50) null

Esal varchar(50) null

i have taken gridview on page and whenever select value using index postion that value(parameter) will passing to the another page that page have another gridview.my intation is another gridview taken parameter and display gridview.we have using two tables for each gridview. but my probelm is parameter will sending to the another gridview.error will be there


"parameter is in valid"


my code like as

first gridview page

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

public partial class varbyparam : System.Web.UI.Page
{
    SqlConnection con;
    SqlCommand cmd;
    SqlDataReader dr; 
    protected void Page_Load(object sender, EventArgs e)
    {
        bind();
    }
    public void bind()
    {
        con = new SqlConnection("Data Source=SANKALPA-PC;Initial Catalog=EmpDept;Integrated Security=True");
        con.Open();
        cmd = new SqlCommand("select * from Dept", con);
        dr = cmd.ExecuteReader();      
        Gridview1.DataSource = dr;
        Gridview1.DataBind();
        Gridview1.DataKeyNames = new string[] { "Did" };
        con.Close();
        dr.Close();
        
    }

    protected void Gridview1_SelectedIndexChanging(object sender, GridViewSelectEventArgs e)
    {
        Gridview1.SelectedIndex = e.NewSelectedIndex;
        string Did= Gridview1.SelectedValue.ToString();
        Response.Redirect("~/param1.aspx?Did="+Did);
    }
}

second gridview page

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

public partial class param1 : System.Web.UI.Page
{
    SqlConnection con;
    SqlCommand cmd;
    SqlDataReader dr;
    protected void Page_Load(object sender, EventArgs e)
    {
        //string Deptid = Request.QueryString["Did"].ToString();
        con = new SqlConnection("Data Source=SANKALPA-PC;Initial Catalog=EmpDept;Integrated Security=True");
        con.Open();
        string sql = "select * from Emp where Did=" + Request.QueryString["Did"].ToString();
        cmd = new SqlCommand(sql,con);
        dr = cmd.ExecuteReader();
        Gridview2.DataSource = dr;
        Gridview2.DataBind();       
        con.Close();
    }
}