The Problem
So you have embedded a Dropdownlist on your page, you have bound a lot of data to this Dropdownlist (such as a country list) and have an OnSelectedIndexChanged event associated with it. It works perfectly until you set enableviewstate="false" then your event doesn't get fired. You desperately want to disable the ViewState for this control because it has appended this to your ViewState.
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Although it flies down on your localhost, the dropdownlist’s ViewState will greatly increase your page download time on the web especially if you have a number of these controls. You want a simple solution to be able to remove this ViewState added by your Dropdownlist while maintaining your postback events, i.e. A Dropdownlist without the ViewState.
The Reason
The reason this happens is simple. The primary use of ViewState is to maintain control “instances” when posting back. When you bound your DropdownList you most likely did something like follows.
public void BindCountryDropDown()
{
if (!Page.IsPostBack)
{
ddlCountrySelect.DataSource = CountryDAL.AllCountries;
ddlCountrySelect.DataBind();
if (ddlCountrySelect.Items.FindByValue(CurrentCountry.CountryId.ToString()) != null)
{
ddlCountrySelect.ClearSelection();
ddlCountrySelect.Items.FindByValue
CurrentCountry.CountryId.ToString()).Selected = true;
}
}
}
You specified that when posting back, do not populate the DropdownList.
You did this for 1 reason. If you didn’t put the famous !Page.IsPostBack your DropdownList’s SelectedValue would never change.
There is actually a second reason...you don’t have to bind the data on postback.
Look at my Watch above on postback. Even though I didn’t bind the control on postback, I still have my 241 ListItems. As you have probably guessed, this is down to the ViewState. ViewState (transparently) gives us the impression that the application has been waiting for your response.
It does this by remembering what data you had to prior to postback. Why does it remember?
Well take your DropdownList. On Postback the first thing your application does is De-Serialise your DropdownList from the ViewState. The result is the exact Dropdownlist that you rendered.
Then the application retrieves
string selectedValue = HttpContext.Current.Request.Form[UniqueID];
This would retrieve your submitted value.
From here the DropdownList just does a
ClearSelection();
Items.FindByValue(selectedValue).Selected = true;
Now you have a new SelectedValue , SelectedIndex and SelectedItem. Therefore your selected Index has changed and your events will be fired.
With ViewState disabled your DropdownList is never De-Serialised/Populated and your Items collection will be always be empty. Your SelectedIndex will always be Zero which means your OnSelectedIndexChanged event will never be fired. Even if it was fired your SelectedIndex will be 0 and your SelectedItem will always be null.
The Solution
There are many solutions to the problem most of them are a bit messy and time consuming. The best solution is to simply create your own DropDownList WebControl. Don’t worry its not as daunting as you think...
This is the complete code you need.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.Security.Permissions;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.IO;
using System.Collections.Specialized;
using System.Xml.Serialization;
using System.Collections;
using System.Reflection;
using PixelPress.Common.Web;
namespace PixelPress.Interface.WebControls
{
public class PixelDropDownList : DropDownList, IPostBackEventHandler
{
public override string SelectedValue
{
get
{
if (!EnableViewState && Page.IsPostBack)
{
string selectedValue = HttpContext.Current.Request.Form[UniqueID];
if (selectedValue != null)
{
// Then we get our selected Item
return selectedValue;
}
else
{
return base.SelectedValue;
}
}
else
{
return base.SelectedValue;
}
}
set
{
base.SelectedValue = value;
}
}
public void RaisePostBackEvent(string eventArgument)
{
// If the Viewstate is disabled we retrieve our selected index from the Form
if (!EnableViewState && Page.IsPostBack)
{
OnSelectedIndexChanged(new EventArgs());
}
}
}
All we are doing essentially is interfering with the ASP: DROPDOWNLIST when the ViewState is disabled. The DropdownList will work as normal when the ViewState is enabled.
When the ViewState is disabled we simply make sure that the OnSelectedIndexChanged event is fired when a Postback event is raised. To do this we add interface member RaisePostBackEvent to accompany RaisePostDataChangedEvent. In RaisePostBackEvent we just make sure that our OnSelectedIndexChanged is called.
Then we ensure that the SelectedValue returns the submitted value.
However, your ListItems will still be empty and SelectedIndex and SelectedItem will still be null, you will just be able to refer to SelectedValue.
That’s it, simply
- Create a new CS file in your class library, name it what you like.
- Paste the above into your file and compile your Class Library
- Go to the page/control using the new DropdownList
- Register your new control. See below PixelPress_Lib is the name of your Assembly; PixelPress.Interface.WebControls is the namespace that your control is compiled under.
- To use on your page just do as you did with a ASP:DROPDOWNLIST
An example of it in use on your page
<%@ register
tagprefix="pixel"
namespace="PixelPress.Interface.WebControls"
assembly="PixelPress_Lib"
%>
<h1> Country and Region Manager</h1>
The following are all of teh regions associated with
<pixel:pixeldropdownlist
id="ddlCountrySelect"
runat="server"
enableviewstate="false"
autopostback="true"
datavaluefield="CountryId"
datatextfield="CountryTitle"
onselectedindexchanged="ddlCountrySelect_SelectedIndexChanged" />
On a final note, I have heard suggestions to use the SessionState to persist the state of the DropdownList. This would be a huge waste of SessionState and server memory. Don’t consider it as an option..
Thanks for reading.
Published by
Paul Tierney
Senior Software Engineer
Harvard Development Galway (HDG)