Date: 05/03/07 (C Sharp)    Keywords: no keywords

Hey guys. I'm having a problem writing a Composite Server Control, so I figured I'd actually try to post a question here. The Composite Control is basically a Label and a DropDownList. My problem is that anytime the a PostBack occurs, I lose the SelectedIndex. Anyone have any suggestions? I'll provide part of the code behind the cut :

		public class AddNewDropDownListEventArgs : EventArgs {
			private String m_sNewValue;

			public String NewValue {
				get { return m_sNewValue; }
			}

			public AddNewDropDownListEventArgs( String newValue ) {
				m_sNewValue = newValue;
			}

			/// 

			/// Keeps the user from having to cast it out from being an EventArgs
			/// 
			/// this.NewValue
			public override string ToString() {
				return m_sNewValue;
			}
		}

		/// 

		/// A DropDownList and optional Label that contains an special element that allows the user to add new elements to the list
		/// The Special elements are not added until the OnPreRender phase, so the list will act normal until that point.
		/// 
		public class AddNewDropDownList : UserControl, INamingContainer, IPostBackEventHandler {
			protected Label m_oLabel;
			protected DropDownList m_oDropDownList;
			public event EventHandler AddNew;

			public String Label {
				get { return (String)ViewState["Label"]; }
				set { ViewState["Label"] = value; }
			}

			public ListItemCollection Items {
				get { return m_oDropDownList.Items; }
			}

			public int SelectedIndex {
				get { return m_oDropDownList.SelectedIndex; }
				set { m_oDropDownList.SelectedIndex = value; }
			}

			public ListItem SelectedItem {
				get { return m_oDropDownList.SelectedItem; }
			}

			public bool IncludeBlankItem {
				get {
					if( ViewState["IncludeBlankItem"] == null ) {
						return true;
					}
					return (bool)ViewState["IncludeBlankItem"];
				}
				set { ViewState["IncludeBlankItem"] = value; }
			}

			public AddNewDropDownList() {
				m_oLabel = new Label();
				m_oDropDownList = new DropDownList();
			}

			public void RaisePostBackEvent( string eventArgument ) {
				if( AddNew != null ) {
					AddNew( this, new AddNewDropDownListEventArgs( eventArgument ) );
				}
			}

			#region AddNewItems
			// There are a lot of overloads for this function
			// basically, it added the new item right before the [ Add New ] Item
			// optionally, it selects it
			#endregion

			protected void CreateSpecialItems( bool bCreateBlankItem ) {
				if( bCreateBlankItem ) Items.Insert( 0, new ListItem( String.Empty, String.Empty ) );
				Items.Add( new ListItem( "[ Add New ]", "[ Add New ]" ) );
			}

			protected override void CreateChildControls() {
				// null means don't render, String.Empty means render
				// This way, the label can be created for client-side dynamic updating
				if( Label != null ) {
					m_oLabel.Text = Label;
					Controls.Add( m_oLabel );
				}

				Controls.Add( m_oDropDownList );

				base.CreateChildControls();
			}

			protected override void OnInit( EventArgs e ) {
				if( !IsPostBack ) {
					CreateSpecialItems( IncludeBlankItem );
				}

				base.OnInit( e );
			}

			protected override void LoadViewState( object savedState ) {
					if( IsPostBack ) {
						ArrayList aItems = (ArrayList)ViewState["anddlItems"];
						foreach( ArrayList aItem in aItems ) {
							Items.Add( new ListItem( aItem[0].ToString(), aItem[1].ToString() ) );
						}
					}
				}

			}

			protected override void OnPreRender( EventArgs e ) {
				m_oLabel.ID = "Label";

				m_oDropDownList.ID = "DropDownList";
				m_oDropDownList.Attributes["onchange"] = "getNewItem( this, '" + this.UniqueID + "' );";

				StringBuilder sGetNewItemScript = new StringBuilder();
				sGetNewItemScript.Append( "function getNewItem( oControl, sUniqueID ) {\n" );
				sGetNewItemScript.Append( "\tif( oControl.options[oControl.selectedIndex].value == '[ Add New ]' ) {\n" );
				sGetNewItemScript.Append( "\t\tvar retVal = prompt( 'Please input the new value.' );\n" );
				sGetNewItemScript.Append( "\t\tif( ( retVal != null ) && ( retVal != '' ) ) {\n" );
				sGetNewItemScript.Append( "\t\t\t__doPostBack( sUniqueID, retVal );\n" );
				sGetNewItemScript.Append( "\t\t}\n" );
				sGetNewItemScript.Append( "\t}\n" );
				sGetNewItemScript.Append( "}\n" );

				Page.ClientScript.RegisterClientScriptBlock( this.GetType(), "GetNewItem", sGetNewItemScript.ToString(), true );

				base.OnPreRender( e );
			}

			protected override object SaveViewState() {
				ArrayList aItems = new ArrayList();
				foreach( ListItem oItem in Items ) {
					ArrayList aItem = new ArrayList();
					aItem.Add( oItem.Text );
					aItem.Add( oItem.Value );
					aItems.Add( aItem );
				}

				ViewState["anddlItems"] = aItems;
				return base.SaveViewState();
			}
		}

	}
}

Source: http://community.livejournal.com/csharp/84955.html