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05/05/2009 at 12:40PM PDT, ID: 24382611
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9.3

DropdownList items are cleared on postback

Asked by smacca in Programming for ASP.NET, Microsoft Visual C#.Net, C# Programming Language

Tags: dropdownlist postback

Hi,

I have created a custom dropdownlist that gets its values from a datasource (database).
My problem exists, when the page is posted back to itself - the items in the list are not persisted.

In my code, I can see why this happens - it is because I am not refilling the list automatically on postback (common practice).
However, how does the list refill itself.


     Q: Are the items in the list, supposed to be maintained across postback (must I call some methods like CreateChildControls)????
     Q: Do I have to rebind the list in the lists SelectedIndexChanged event handler (like you would with a GridView/DataList etc)????


This has been an issue for sometime now - to resolve it, I have just removed the !Page.IsPostBack condition (however, as this suggests, the code gets run when leaving pages etc which is inefficient - this is the main reason for it being there).

Hopefully, this is a 101 question - and I have missed this lesson early in my career.

Any help appreciated - thanks for your time.

Cheers.
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using System;
using System.Web.UI.WebControls;
using System.Collections;
using System.ComponentModel;
 
namespace MWeb.Web.UI.WebControls
{
	/// <summary>
	/// Base Class for Extra Item <code>DropDownList</code>s
	/// </summary>
	abstract public class ExtraItemDropDownList : System.Web.UI.WebControls.DropDownList, IExtraItemList
    {
        #region Declarations
 
        #region Constants
 
        public const string DEFAULT_EXTRAITEM_TEXT = "None";
		public const string DEFAULT_EXTRAITEM_VALUE = "0";
 
		public const string DEFAULT_DATATEXT_FIELD = "DataTextField";
		public const string DEFAULT_DATAVALUE_FIELD = "DataValueField";
 
        public const int DEFAULT_CACHE_TIME_IN_MINS = 0;
 
		#endregion
 
        #region Member Variables
 
        //private bool _hasDataBind = false;
 
        #endregion
 
        #endregion
 
        #region Constructor
 
        public ExtraItemDropDownList():
                base()
		{
            this.Init += new EventHandler(this.Control_Init);
            this.Load += new EventHandler(this.Control_Load);
            this.PreRender += new EventHandler(this.Control_PreRender);
        }   
 
		#endregion
 
		#region Abstract and Virtuals
 
		protected abstract void Fill();
		public virtual void Refresh()
		{
            //clear all items.
            this.Items.Clear();
 
            //refill.
            this.Fill();
 
            //clean data.
            this.IsDirtyData = false;
		}
 
		#endregion
 
		#region Control Events
 
		///<summary>
		///Fills the DropDownList with items from the database on the first request
		///If PopulateOnLoad is true. Will populate itself on any request where
		///IsDirtyData is true.
        ///</summary>       
        private void Control_Init(object sender, System.EventArgs e)
        {
            if ((!Page.IsPostBack && this.PopulateOnLoad) || this.IsDirtyData)
            {
                this.Refresh(); ////////////////*************THIS IS NOT EXECUTED ON POSTBACK (AS EXPECTED) - BUT IT FILLS THE LIST!!!! QUESTION: Are the list items suppossed to be maintained across postback or am I supposed to rebind the list!
            }
        }
 
        private void Control_Load(object sender, System.EventArgs e)
        {
 
        }
 
        private void Control_PreRender(object sender, System.EventArgs e)
        {
            //disable if only single item.
            if (this.DisableSingleItem && this.Enabled)
            {//must already be enabled!
 
                this.Enabled = this.IncludeExtraItem ? (this.Items.Count > 2) : (this.Items.Count > 1);
            }
 
            //remove excluded values.
            if (!string.IsNullOrEmpty(this.ExcludedValueList))
            {
                string[] excludedValues = this.ExcludedValueList.Split(new char[] { ',', ' ' });
                foreach (string excludedValue in excludedValues)
                {
                    this.Items.Remove(this.Items.FindByValue(excludedValue));
                }
            }
        }
 
        public override void DataBind()
        {
            //bind data.
            //try
            //{
                //base.DataBind();
            //}
            //catch { ;} //TODO: THIS IS A HACK - a selected index exception was being thrown when I was filtering/refreshing lists. Couldnt find a solution so this was a temp fix.
 
            //create new list item
            if (this.IncludeExtraItem)
            {
                ListItem newItem = new ListItem();
                newItem.Text = this.ExtraItemText;
                newItem.Value = this.ExtraItemValue;
                newItem.Selected = false;
 
                //add list item to specified position
                if ((this.ExtraItemPosition == ExtraItemPosition.Top || this.ExtraItemPosition == ExtraItemPosition.Start))
                {
                    this.Items.Insert(0, newItem);
                }
                else if ((ExtraItemPosition == ExtraItemPosition.Bottom || this.ExtraItemPosition == ExtraItemPosition.End))
                {
                    this.Items.Add(newItem);
                }
            }
 
            // now select the default item
            if (this.InitialValue.ToString().Length > 0)
            {
                for (int i = 0; i < this.Items.Count;i++)
                {
                    ListItem li = this.Items[i];
                    if (li.Value == this.InitialValue)
                    {
                        if (!li.Selected)
                            li.Selected = true;
                        break;
                    }
                }
            }
 
            //set binded flag.
            //this._hasDataBind = true;
        }		
		
		#endregion
 
		#region Properties
 
        [Description("Sets the cache time in minutes for the list."), Category("Loading")]
        public int CacheTimeInMins
        {
            get
            {
                if (this.ViewState["CacheTimeInMins"] == null)
                {
                    this.ViewState["CacheTimeInMins"] = DEFAULT_CACHE_TIME_IN_MINS;
                }
                return (int)this.ViewState["CacheTimeInMins"];
            }
            set
            {
                this.ViewState["CacheTimeInMins"] = value;
            }
        }
 
        [Description("If you set this value to true before the Load event occurs then the control will reload its self."), Category("Loading")]
        public bool IsDirtyData
        {
            get
            {
                if (this.ViewState["IsDirtyData"] == null)
                {
                    this.ViewState["IsDirtyData"] = false;
                }
                return (bool)this.ViewState["IsDirtyData"];
            }
            set
            {
                this.ViewState["IsDirtyData"] = value;
            }
        }
 
        [Description("If you set this value to true then this control will pre-populate itself when it loads."), Category("Loading")]
        public bool PopulateOnLoad
        {
            get
            {
                if (this.ViewState["PopulateOnLoad"] == null)
                {
                    this.ViewState["PopulateOnLoad"] = true;
                }
                return (bool)this.ViewState["PopulateOnLoad"];
            }
            set
            {
                this.ViewState["PopulateOnLoad"] = value;
            }
        }
 
        [Description("The value of the initially selected item"), Category("Data")]
        public string InitialValue
        {
            get
            {
                if (this.ViewState["InitialValue"] == null)
                {
                    this.ViewState["InitialValue"] = string.Empty;
                }
                return (string)this.ViewState["InitialValue"];
            }
 
            set
            {
                this.ViewState["InitialValue"] = value;
            }
        }
 
		[Description("bool:  Include extra item in list?"),Category("Extra")]
		public bool IncludeExtraItem
		{
			get
			{
				if(this.ViewState["IncludeExtraItem"]==null)
				{
					this.ViewState["IncludeExtraItem"] = false;
				}
				return (bool) this.ViewState["IncludeExtraItem"];
			}
			set
			{
				this.ViewState["IncludeExtraItem"] = value;
			}
		}
 
		[Description("bool:  Should the extra item always been shown at the top of the list"),Category("Extra")]
		public ExtraItemPosition ExtraItemPosition
		{
			get
			{
				if(this.ViewState["ExtraItemPosition"]==null)
				{
					this.ViewState["ExtraItemPosition"] = ExtraItemPosition.Top;
				}
				return (ExtraItemPosition) this.ViewState["ExtraItemPosition"];
			}
			set
			{
				this.ViewState["ExtraItemPosition"] = value;
			}
		}
 
		[Description("string:  The text string displayed for extra item"),Category("Extra")]
		public string ExtraItemText
		{
			get
			{
				if(this.ViewState["ExtraItemText"]==null)
				{
					this.ViewState["ExtraItemText"] = string.Empty;
				}
				return (string) this.ViewState["ExtraItemText"];
			}
			set
			{
				this.ViewState["ExtraItemText"] = value;
			}
		}		
 
		[Description("string:  The value used for the extra item"),Category("Extra")]
		public string ExtraItemValue
		{
			get
			{
                if (this.ViewState["ExtraItemValue"] == null)
				{
                    this.ViewState["ExtraItemValue"] = string.Empty;
				}
                return (string)this.ViewState["ExtraItemValue"];
			}
			set
			{
                this.ViewState["ExtraItemValue"] = value;
			}
        }
 
        [Description("If you set this to true, the list will disabled when their is only one item in the list."), Category("Loading")]
        public bool DisableSingleItem
        {
            get
            {
                if (this.ViewState["DisableSingleItem"] == null)
                {
                    this.ViewState["DisableSingleItem"] = true;
                }
                return (bool)this.ViewState["DisableSingleItem"];
            }
            set
            {
                this.ViewState["DisableSingleItem"] = value;
            }
        }
 
        [Description("Excludes the values specified from list."), Category("Loading")]
        public string ExcludedValueList
        {
            get
            {
                return (string)this.ViewState["ExcludedValueList"];
            }
            set
            {
                this.ViewState["ExcludedValueList"] = value;
            }
        }
	
		#endregion
 
	}
}
[+][-]05/05/09 07:55 PM, ID: 24311008

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Zones: Programming for ASP.NET, Microsoft Visual C#.Net, C# Programming Language
Tags: dropdownlist postback
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Solution Provided By: M3mph15
Participating Experts: 1
Solution Grade: A
 
 
 
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