I didn't think it was possible, but .NET surprised me yet again with a cool feature I never knew existed: The ObservableCollection. This became available in .NET 3.0.
In essence, an ObservableCollection is a collection with an event you can connect
to. The event fires when the collection changes. As usual, working with the .NET classes is so ridiculously easy, it feels like cheating.
The following is small test program
to illustrate how the ObservableCollection works.
To start, create an ObservableCollection and then store it in the Session
object so it will persist between page post backs. I also added the code
to pull it out of Session state when there is a page post back:
public partial class _Default : System.Web.UI.Page{ public ObservableCollection<int> MyInts; // ---- Page_Load ------------------------------ protected void Page_Load(object sender, EventArgs e) { if (IsPostBack == false) { MyInts = new ObservableCollection<int>(); MyInts.CollectionChanged += CollectionChangedHandler; Session["MyInts"] = MyInts; // store for use between postbacks } else { MyInts = Session["MyInts"] as ObservableCollection<int>; } }
Here's the event handler I hooked up
to the ObservableCollection, it writes status strings
to a ListBox. Note: The event handler fires in a different thread than the IIS process thread.
// ---- CollectionChangedHandler ----------------------------------- // // Something changed in the Observable collection public void CollectionChangedHandler(object sender, NotifyCollectionChangedEventArgs e) { // need
to dig around
to get the current page and control
to write to: // (because this is in a separate thread) Page CurrentPage = System.Web.HttpContext.Current.Handler as Page; ListBox LB = CurrentPage.FindControl("ListBoxHistory") as ListBox; switch (e.Action) { case NotifyCollectionChangedAction.Add: LB.Items.Add("Add: " + e.NewItems[0]); break; case NotifyCollectionChangedAction.Remove: LB.Items.Add("Remove: " + e.OldItems[0]); break; case NotifyCollectionChangedAction.Reset: LB.Items.Add("Reset: "); break; default: LB.Items.Add(e.Action.ToString()); break; } }
Next, add some buttons and code
to exercise the ObservableCollection: <br /> <asp:Button ID="ButtonAdd" runat="server" Text="Add" OnClick="ButtonAdd_Click" /> <asp:Button ID="ButtonRemove" runat="server" Text="Remove" OnClick="ButtonRemove_Click" /> <asp:Button ID="ButtonReset" runat="server" Text="Reset" OnClick="ButtonReset_Click" /> <asp:Button ID="ButtonList" runat="server" Text="List" OnClick="ButtonList_Click" /> <br /> <asp:TextBox ID="TextBoxInt" runat="server" Width="51px"></asp:TextBox> <br /> <asp:ListBox ID="ListBoxHistory" runat="server" Height="255px" Width="195px"> </asp:ListBox> // ---- Add Button -------------------------------------- protected void ButtonAdd_Click(object sender, EventArgs e) { int Temp; if (int.TryParse(TextBoxInt.Text, out Temp) == true) MyInts.Add(Temp); } // ---- Remove Button -------------------------------------- protected void ButtonRemove_Click(object sender, EventArgs e) { int Temp; if (int.TryParse(TextBoxInt.Text, out Temp) == true) MyInts.Remove(Temp); } // ---- Button Reset ----------------------------------- protected void ButtonReset_Click(object sender, EventArgs e) { MyInts.Clear(); } // ---- Button List -------------------------------------- protected void ButtonList_Click(object sender, EventArgs e) { ListBoxHistory.Items.Add("MyInts:"); foreach (int i in MyInts) { // a bit of tweaking
to get the text
to be indented ListItem LI = new ListItem(" " + i.ToString()); LI.Text = Server.HtmlDecode(LI.Text); ListBoxHistory.Items.Add(LI); } }
Here's what it looks like after entering some numbers and clicking some buttons:
An interesting note is that I had
to use: System.Web.HttpContext.Current.Response
to write
to a control on the page. As mentioned earlier, this implies that the notification event is in a thread separate from the IIS thread.
Another interesting note: From the online help:
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed
to be thread safe
What does that mean
to Asp.Net developers?
If you are going
to share an ObservableCollection among different sessions, you'd better make it a static
object.
I hope someone finds this useful.
Steve Wellens
