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Silverlight Chat WrapPanel Crash / Bug

- by Matt

I've been given the task to create a simple Silverlight chat box for two people. My control must adhere to the following requirements Scrollable Text must wrap if it's too long When a new item / message is added it must scroll that item into view Now I've successfully made a usercontrol to meet these requirements, but I've run into a possible bug / crash that I can't for the life of me fix. I'm looking for either a fix to the bug, or a different approach to creating a scrollable chat control. Here's the code I've been using. We'll start with my XAML for the chat window <ListBox x:Name="lbChatHistory" Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" ScrollViewer.VerticalScrollBarVisibility="Visible" ScrollViewer.HorizontalScrollBarVisibility="Disabled" > <ListBox.ItemTemplate> <DataTemplate> <Grid Background="Beige"> <Grid.ColumnDefinitions> <ColumnDefinition Width="70"></ColumnDefinition> <ColumnDefinition Width="Auto"></ColumnDefinition> </Grid.ColumnDefinitions> <TextBlock x:Name="lblPlayer" Foreground="{Binding ForeColor}" Text="{Binding Player}" Grid.Column="0"></TextBlock> <ContentPresenter Grid.Column="1" Width="200" Content="{Binding Message}" /> </Grid> </DataTemplate> </ListBox.ItemTemplate> </ListBox> The idea is to add a new Item to the listbox. The Item (as layed out in the XAML) is a simple 2 column grid. One column for the username, and one column for the message. Now the "items" that I add to the ListBox is a custom class. It has three properties (Player, ForeColor, and Message) that I using binding on within my XAML Player is a string of the current user to display. ForeColor is just a foreground color preference. It helps distinguish the difference between messages. Message is a WrapPanel. I programmatically break the supplied string on the white space for each word. Then for each word, I add a new TextBlock element to the WrapPanel Here is the custom class. public class ChatMessage :DependencyObject, INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; public static DependencyProperty PlayerProperty = DependencyProperty.Register( "Player", typeof( string ), typeof( ChatMessage ), new PropertyMetadata( new PropertyChangedCallback( OnPlayerPropertyChanged ) ) ); public static DependencyProperty MessageProperty = DependencyProperty.Register( "Message", typeof( WrapPanel ), typeof( ChatMessage ), new PropertyMetadata( new PropertyChangedCallback( OnMessagePropertyChanged ) ) ); public static DependencyProperty ForeColorProperty = DependencyProperty.Register( "ForeColor", typeof( SolidColorBrush ), typeof( ChatMessage ), new PropertyMetadata( new PropertyChangedCallback( OnForeColorPropertyChanged ) ) ); private static void OnForeColorPropertyChanged( DependencyObject d, DependencyPropertyChangedEventArgs e ) { ChatMessage c = d as ChatMessage; c.ForeColor = ( SolidColorBrush ) e.NewValue; } public ChatMessage() { Message = new WrapPanel(); ForeColor = new SolidColorBrush( Colors.White ); } private static void OnMessagePropertyChanged( DependencyObject d, DependencyPropertyChangedEventArgs e ) { ChatMessage c = d as ChatMessage; c.Message = ( WrapPanel ) e.NewValue; } private static void OnPlayerPropertyChanged( DependencyObject d, DependencyPropertyChangedEventArgs e ) { ChatMessage c = d as ChatMessage; c.Player = e.NewValue.ToString(); } public SolidColorBrush ForeColor { get { return ( SolidColorBrush ) GetValue( ForeColorProperty ); } set { SetValue( ForeColorProperty, value ); if(PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs( "ForeColor" )); } } public string Player { get { return ( string ) GetValue( PlayerProperty ); } set { SetValue( PlayerProperty, value ); if ( PropertyChanged != null ) PropertyChanged( this, new PropertyChangedEventArgs( "Player" ) ); } } public WrapPanel Message { get { return ( WrapPanel ) GetValue( MessageProperty ); } set { SetValue( MessageProperty, value ); if ( PropertyChanged != null ) PropertyChanged( this, new PropertyChangedEventArgs( "Message" ) ); } } } Lastly I add my items to the ListBox. Here's the simple method. It takes the above ChatMessage class as a parameter public void AddChatItem( ChatMessage msg ) { lbChatHistory.Items.Add( msg ); lbChatHistory.ScrollIntoView( msg ); } Now I've tested this and it all works. The problem I'm getting is when I use the scroll bar. You can scroll down using the side scroll bar or arrow keys, but when you scroll up Silverlight crashes. FireBug returns a ManagedRuntimeError #4004 with a XamlParseException. I'm soo close to having this control work, I can taste it! Any thoughts on what I should do or change? Is there a better approach than the one I've taken? Thanks in advance. UPDATE I've found an alternative solution using a ScrollViewer and an ItemsControl instead of a ListBox control. For the most part it's stable.

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