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
