2 dropdownlists in webform

Question

User156871696 posted

Hello, am learning asp.net . İ am making dropdownlist , i like to explain my problem via a sample. İ have two dropdownlist ,dropdownlist1 and dropdownlist2 , i am choosing Turkey from dropdownlist1 ,Ankara from dropdownlist2 , when i reload page values are changing. Thank you for your helping

Answer 1

User409696431 posted

Please provide more information.  How are you providing the values for the dropdowns?  When are you providing them?  What are the values changing to after postback?

Answer 2

User156871696 posted
Hello,

1-) i have acces database , there are two table cities and town , i am pulling data to dropdownlist from my database
2-) when i open my web page , i am seeking my city via dropdownlist1 , town dropdownlist2
3-) after postback nothing change , everything is ok

My problem is this,

when i open page everytime. i want to see same places.
Changing places when i close the webpage and then reopen it again

Thank you very much for your help

Regards

Answer 3

User409696431 posted

It is not typical behavior for controls to have the same value as the last time you opened the page.  In order to see the same value when you open the page again at a later time, you have to store the values somewhere, look for that value on pageload, and set the dropdowns to those values after you databind them. Are the values you want to save going to be associated with each visitor to the site?  If so, you have to have a way of identifying the visitor and storing the value in a way that associates it with the visitor.

As an example, if this is a site with membership, you could use a table in the database that associates the values with the member id.

Answer 4

User839733648 posted

Hi AhmetOzer,

According to your description, I suggest that you could use database to do this.

Page will refresh to default when you reload it.

So when you selecting the item, then save the result to database.

And when you open the page again, you could let it read the data saving last time.

Best Regards,

Jenifer

Answer 5

User156871696 posted
Dear Jenifer,

Thank you very much for information, can you please provide
a simple sample , if this don’t take much time.
Because i tried this way , i couldn’t do it.

This is my code



protected void save_Click(object sender, EventArgs e) *** this button for saving *****
{
OleDbConnection conn = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + Server.MapPath("~/App_Data/cities.accdb") + ";Persist
Security Info=True");
conn.Open();

string id = DropDownList1.SelectedValue; ****(note : this id for selected city)****
string town = DropDownList2.SelectedItem.Text;


string connstr = "INSERT INTO selectedcitytown (id,town) VALUES (''" + @id + "',''" + @town + "')";
OleDbCommand com = new OleDbCommand(connstr, conn);
com.Parameters.AddWithValue("@id", DropDownList1.SelectedValue);
com.Parameters.AddWithValue("@town", DropDownList2.SelectedItem.Text);
com.ExecuteNonQuery();
Response.Redirect("~/PURSAKLAR/TumHaritaIlanlari.aspx");
}


database :name cities
inside database :there are 3 tables
table 1: cities ( id ,city) / (numeric, text) ****note for DropDownList1 ****
table2: towns (id, town, city)/( numeric,text,numeric) ***note for DropDownList2 ****
table3: selectedcitytown (city,town)/(text,text)

Answer 6

User409696431 posted

Ahmet,Ozer,

The code you show is only for saving the values to the database (does that part work?).  It does not populate the dropdowns with those values the next time you come to the page, and it has no way to tell who the visitor is.   Unless you want the dropdowns to be set to the values the last visitor set, not the values each visitor set the last time they were at that page, you have to store different values for each visitor.   The first thing you have to decide is: How will you identify that a returning visitor has been to the site before and has saved values?   That's why I mentioned membership in my earlier reply, and as a way (not the only way) to do this.

Answer 7

User156871696 posted

Dear Kathy,

You are right reallly , now i understood . This way is not a way to solve my problem.

Thank you so much

Best Regards

Ahmet

Answer 8

User839733648 posted

Hi AhmetOzer,

I have another better suggestion that you could use session to store the selected item and read it when reloading.

Here is my demo.

.aspx

            <asp:DropDownList ID="DropDownList1" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"  AutoPostBack="true" OnDataBound="DropDownList1_DataBound" >
            </asp:DropDownList>

code-behind.

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                BindData();
            }
        }
        string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        private void BindData()
        {
            SqlConnection con = new SqlConnection(constr);
            con.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = con;
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "select Eid,Ename from tb_info";
            SqlDataAdapter adp = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            adp.Fill(ds, "tb_info");
            DropDownList1.DataSource = ds;
            DropDownList1.DataTextField = "Ename";
            DropDownList1.DataValueField = "Eid";
            DropDownList1.DataBind();

        }

        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {//save to session
            Session["dd1"] = DropDownList1.SelectedValue;
        }

        protected void DropDownList1_DataBound(object sender, EventArgs e)
        {//read session
            DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Convert.ToString(Session["dd1"])));
        }

result:

Best Regards,

Jenifer

Answer 9

User-2054057000 posted

This situation arises when you bind the drop down in code behind by not using !postback. The correct solution is:

protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostBack)
    {
       // Bind your drop downs
    }
}

Answer 10

User156871696 posted

Dear Jenifer ,

Thank you so much , i will check and back to you to give information.

Hava good day.

Regards

Ahmet

Answer 11

User156871696 posted

Thank you.

Regards

Ahmet

Answer 12

User156871696 posted

Dear Jenifer ,

I have tried and tested below codes , codes are giving some mistake as you see below.

Thank you so much for your helping

Have good day

Ahmet OZER

TEST RESULT AND ERRORS

******************************************************************************************************************************************************
Server Error in '/sil8' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 27: }
Line 28: }
Line 29: string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
Line 30: private void BindData()
Line 31: {

Source File: c:\Users\OZER\Desktop\sil test sil\sil dropdown test\sil8\PURSAKLAR\TumHaritaIlanlari.aspx.cs Line: 29

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
Default..ctor() in c:\Users\OZER\Desktop\sil test sil\sil dropdown test\sil8\PURSAKLAR\TumHaritaIlanlari.aspx.cs:29
ASP.pursaklar_tumharitailanlari_aspx..ctor() in c:\Users\OZER\AppData\Local\Temp\Temporary ASP.NET Files\sil8\e0cec965\b312c8e0\App_Web_f1j0j10q.0.cs:0
__ASP.FastObjectFactory_app_web_f1j0j10q.Create_ASP_pursaklar_tumharitailanlari_aspx() in c:\Users\OZER\AppData\Local\Temp\Temporary ASP.NET Files\sil8\e0cec965\b312c8e0\App_Web_f1j0j10q.2.cs:0
System.Web.Compilation.BuildResultCompiledType.CreateInstance() +31
System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp) +100
System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath) +31
System.Web.UI.PageHandlerFactory.System.Web.IHttpHandlerFactory2.GetHandler(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath) +64
System.Web.HttpApplication.MapHttpHandler(HttpContext context, String requestType, VirtualPath path, String pathTranslated, Boolean useAppConfig) +191
System.Web.MapHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +145
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

******************************************************************************************************************************************************

WEB SITE CODES
***************

.aspx
*******************************************************************************************************************************************************

<asp:DropDownList ID="DropDownList1" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true" OnDataBound="DropDownList1_DataBound" >
</asp:DropDownList>

.aspx.cs
********************************************************************************************************************************************************

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
BindData();
}
}
string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
private void BindData()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select id,city from cities";
OleDbDataAdapter adp = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
adp.Fill(ds, "cities");
DropDownList1.DataSource = ds;
DropDownList1.DataTextField = "city";
DropDownList1.DataValueField = "id";
DropDownList1.DataBind();

}

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{//save to session
Session["dd1"] = DropDownList1.SelectedValue;
}

protected void DropDownList1_DataBound(object sender, EventArgs e)
{//read session
DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Convert.ToString(Session["dd1"])));
}

********************************************************************************************************************************************************
database name : cities
inside database :there are 3 tables
table 1: cities ( id ,city) / (numeric, text) ****note for DropDownList1 ****
table2: towns (id, town, city)/( numeric,text,numeric) ***note for DropDownList2 ****
table3: selectedcitytown (city,town)/(text,text)

Answer 13

User839733648 posted

Hi AhmetOzer,

Line 29: string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;

You could see from the error message that the ConnectionString you've used is wrong so you could not get the data succesdfully.

This string is related to the name of connectionStrings in your Web.config file.

Here is my Web.config file. Pay attention to the highlight content.

<?xml version="1.0"?>
<configuration>
  <connectionStrings>
    <add name="EmployeeManagementConnectionString" connectionString="Data Source=(localdb)\mssqllocaldb;Initial Catalog=EmployeeManagement;Integrated Security=True" providerName="System.Data.SqlClient"/>
  </connectionStrings>
...
</configuration>

So my constr is written like:

string constr = ConfigurationManager.ConnectionStrings["EmployeeManagementConnectionString"].ConnectionString;

I suggest that you could check your Web.config file.

Best Regards,

Jenifer

Answer 14

User409696431 posted

Amet,

Are you trying to store values when people go back to the site later?  Or just when they are staying on your site during one visit?  Session will not work in the first case.  If you make your requirement clear it would help.

Answer 15

User156871696 posted

Dear Jenifer

I have checked  my Web.config as you said  , i have compared your code and my code , and finally i did.

Of course  my influence on this work  may be %10 , actually you solved this problem.

Thank you

Ahmet OZER

Answer 16

User156871696 posted

Dear Kathy,

Thank you very much for your interesting , my respond is as below,

Are you trying to store values when people go back to the site later? ****(My answer is no)***** Or just when they are staying on your site during one visit?**(my answer is yes)****  Session will not work in the first case.  If you make your requirement clear it would help.

Thank you very much

Have good day

Regards

Ahmet

Answer 17

User156871696 posted

Dear Jenifer ,

I want to thank you very much again.

Have a good day

Ahmet

Answer 18

User156871696 posted
Dear jenifer ,

I am trying to make to dropdownlist (dropdownlist1 and dropdownlist2 ) as follow. First dropdownlist is ok (dropdownlist1)

But secon dropdownlist (dropdownlist2) is not showing list of towns

Have a good day

Ahmet

******** .aspx.cs ****

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Data.OleDb;
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;

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

string constr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\\Bolgeler.accdb;Persist Security Info=True";


public void Bind_DropDownList1()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select CityNo,City from Cities";
OleDbDataAdapter adp = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
adp.Fill(ds, "Cities");
DropDownList1.DataSource = ds;
DropDownList1.DataTextField = "City";
DropDownList1.DataValueField = "CityNo";
DropDownList1.DataBind();

}




public void Bind_DropDownList2()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select TownNo,Town from Towns where CityNo='" + DropDownList1.SelectedValue + "';
OleDbDataAdapter adp = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
adp.Fill(ds, "Towns");
DropDownList2.DataSource = ds;
DropDownList2.DataTextField = "Town";
DropDownList2.DataValueField = "TownNo";
DropDownList2.DataBind();

}

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Bind_DropDownList1();
}
}


protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{//save to session
Session["dd1"] = DropDownList1.SelectedValue;
}

protected void DropDownList1_DataBound(object sender, EventArgs e)
{//read session
DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Convert.ToString(Session["dd1"])));
}


protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{//save to session
Session["dd2"] = DropDownList2.SelectedValue;
}

protected void DropDownList2_DataBound(object sender, EventArgs e)
{//read session
DropDownList2.SelectedIndex = DropDownList2.Items.IndexOf(DropDownList2.Items.FindByValue(Convert.ToString(Session["dd2"])));
}

*** .aspx *******

<asp:DropDownList ID="DropDownList1" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true" OnDataBound="DropDownList1_DataBound" >
</asp:DropDownList>


<asp:DropDownList ID="DropDownList2" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged" AutoPostBack="true" OnDataBound="DropDownList2_DataBound" >
</asp:DropDownList>

Answer 19

User839733648 posted

Hi AhmetOzer,

According to your description, it seems that you did not callback the Bind_DropDownList2() event.

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Bind_DropDownList1();
            }

        }

        string constr = ConfigurationManager.ConnectionStrings["EmployeeManagementConnectionString"].ConnectionString;
        private void Bind_DropDownList1()
        {
            SqlConnection con = new SqlConnection(constr);
            con.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = con;
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "select * from tb_Province";
            SqlDataAdapter sda = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            sda.Fill(ds, "tb_Province");
            DropDownList1.DataSource = ds;
            DropDownList1.DataTextField = "proName";
            DropDownList1.DataValueField = "proID";
            DropDownList1.DataBind();

        }

        private void Bind_DropDownList2()
        {
            SqlConnection con = new SqlConnection(constr);
            con.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = con;
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "select * from tb_City where proID=" + DropDownList1.SelectedValue.ToString();
            SqlDataAdapter adp = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            adp.Fill(ds, "tb_City");
            DropDownList2.DataSource = ds;
            DropDownList2.DataTextField = "cityName";
            DropDownList2.DataValueField = "cityID";
            DropDownList2.DataBind();

        }
        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList2.Items.Clear();
            Bind_DropDownList2();
            Session["dd1"] = DropDownList1.SelectedValue;
        }

        protected void DropDownList1_DataBound(object sender, EventArgs e)
        {
            DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Convert.ToString(Session["dd1"])));
        }

        protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
        {
            Session["dd2"] = DropDownList2.SelectedValue;
        }

        protected void DropDownList2_DataBound(object sender, EventArgs e)
        {
            DropDownList2.SelectedIndex = DropDownList2.Items.IndexOf(DropDownList2.Items.FindByValue(Convert.ToString(Session["dd2"])));
        }

result:

Best Regards,

Jenifer

Answer 20

User156871696 posted

Dear Jenifer,

Thank you very much for your respond ,

First  and second step is ok, i need finalize final step i mean third step.

dropdownlist1  listing is ok and also cities not changing this is also ok.

dropdownlist2 listing is ok  , this is good.  But when i refresh the page towns are  changing , but i dont want it changed when i refresh page.

Best Regards

.aspx ****

<asp:DropDownList ID="DropDownList1" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" OnDataBound="DropDownList1_DataBound" AutoPostBack="True" >
</asp:DropDownList>

<asp:DropDownList ID="DropDownList2" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged" AutoPostBack="true" OnDataBound="DropDownList2_DataBound" >
</asp:DropDownList>

aspx.cs ****

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Data.OleDb;
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;

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

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Bind_DropDownList1();
}

}

string constr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\\CitiesTowns.accdb;Persist Security Info=True";
private void Bind_DropDownList1()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select * from Cities";
OleDbDataAdapter sda = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
sda.Fill(ds, "Cities");
DropDownList1.DataSource = ds;
DropDownList1.DataTextField = "City";
DropDownList1.DataValueField = "CityId";
DropDownList1.DataBind();

}

private void Bind_DropDownList2()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select * from Towns where CityId=" + DropDownList1.SelectedValue.ToString();
OleDbDataAdapter adp = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
adp.Fill(ds, "Towns");
DropDownList2.DataSource = ds;
DropDownList2.DataTextField = "Town";
DropDownList2.DataValueField = "TownId";
DropDownList2.DataBind();

}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{

DropDownList2.Items.Clear();
Bind_DropDownList2();
Session["dd1"] = DropDownList1.SelectedValue;
}

protected void DropDownList1_DataBound(object sender, EventArgs e)
{
DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Convert.ToString(Session["dd1"])));
}

protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
Session["dd2"] = DropDownList2.SelectedValue;
}

protected void DropDownList2_DataBound(object sender, EventArgs e)
{
DropDownList2.SelectedIndex = DropDownList2.Items.IndexOf(DropDownList2.Items.FindByValue(Convert.ToString(Session["dd2"])));
}

Answer 21

User839733648 posted

Hi AhmetOzer,

I suggest you just need to add Bind_DropDownList2() to !IsPostBack condition.

            if (!IsPostBack)
            {
                Bind_DropDownList1();
                Bind_DropDownList2();
            }

result:

Best Regards,

Jenifer

Answer 22

User156871696 posted

Dear Jenifer,

Thank you. 

Super,  codes are working as i wanted, and i learned this subject very well.

Thank you very much again , because I asked you many questions and you have always answered to my questions.

Thank you very much again

Have a good day

Ahmet OZER

aspx.cs *****

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Data.OleDb;
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;

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

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Bind_DropDownList1();
Bind_DropDownList2();
}

}

string constr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\\CitiesTowns.accdb;Persist Security Info=True";
private void Bind_DropDownList1()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select * from Cities";
OleDbDataAdapter sda = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
sda.Fill(ds, "Cities");
DropDownList1.DataSource = ds;
DropDownList1.DataTextField = "City";
DropDownList1.DataValueField = "CityId";
DropDownList1.DataBind();

}

private void Bind_DropDownList2()
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select * from Towns where CityId=" + DropDownList1.SelectedValue.ToString();
OleDbDataAdapter adp = new OleDbDataAdapter(cmd);
DataSet ds = new DataSet();
adp.Fill(ds, "Towns");
DropDownList2.DataSource = ds;
DropDownList2.DataTextField = "Town";
DropDownList2.DataValueField = "TownId";
DropDownList2.DataBind();

}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{

DropDownList2.Items.Clear();
Bind_DropDownList2();
Session["dd1"] = DropDownList1.SelectedValue;
}

protected void DropDownList1_DataBound(object sender, EventArgs e)
{
DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Convert.ToString(Session["dd1"])));
}

protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
Session["dd2"] = DropDownList2.SelectedValue;
}

protected void DropDownList2_DataBound(object sender, EventArgs e)
{
DropDownList2.SelectedIndex = DropDownList2.Items.IndexOf(DropDownList2.Items.FindByValue(Convert.ToString(Session["dd2"])));
}

}

aspx  ****

<asp:DropDownList ID="DropDownList1" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" OnDataBound="DropDownList1_DataBound" AutoPostBack="True" >
</asp:DropDownList>

<asp:DropDownList ID="DropDownList2" AppendDataBoundItems="true" runat="server" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged" AutoPostBack="true" OnDataBound="DropDownList2_DataBound" >
</asp:DropDownList>