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CheckBox Checked event fires before bound collection updates

I have a custom control to show items with checkboxes inside a ComboBox. To realize this, I used a DataTemplate with a CheckBox. The ItemSource of the ComboBox uses a binding to a ObserableCollection<FilterValue> which contains my filter values. FilterValue is a custom class implementing INotifyPropertyChanged. The properties Content and IsChecked of the CheckBox use bindings as well to use the values of my list. This control will be used in Silverlight.
Binding itself works fine, as seen here:
The problem appears when I register the Checked or Unchecked event.
As soon as one of the check boxes changed its state, the event is fired as expected but at this moment, the value in the bound list is still not updated.
What I saw while debugging is that the Checked/Unchecked events are firing before the PropertyChanged event of the FilterValue.
This means that at the time the event is firing, I can't ask the list for all active (checked) filters. What could I do to achieve this?
FilterControl.xaml:
<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"
xmlns:local="clr-namespace:Controls" x:Class="Controls.FilterControl"
mc:Ignorable="d"
d:DesignHeight="45" d:DesignWidth="140">
<StackPanel x:Name="LayoutRoot">
<sdk:Label x:Name="LblFilterDescription" Content="-" />
<ComboBox x:Name="Filter" Width="120" ItemsSource="{Binding AvailableFilters, RelativeSource={RelativeSource FindAncestor, AncestorType=local:FilterControl}}">
<ComboBox.ItemTemplate>
<DataTemplate>
<CheckBox Content="{Binding Path=Text}" IsChecked="{Binding Path=IsChecked, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Checked="FilterChanged" Unchecked="FilterChanged" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</StackPanel>
</UserControl>
FilterControl.xaml.cs:
public partial class FilterControl : UserControl
{
public delegate void FilterChangedHandler(object sender);
public event FilterChangedHandler OnFilterChanged;
public ObservableCollection<FilterValue> AvailableFilters { get; set; }
public List<string> AppliedFilters
{
get
{
return new List<string>(AvailableFilters.Where(filter => filter.IsChecked).Select(filter => filter.Text));
}
}
public FilterControl()
{
InitializeComponent();
AvailableFilters = new ObservableCollection<FilterValue>();
}
public bool AddFilterValue(string filterValue)
{
bool found = false;
foreach (FilterValue f in AvailableFilters)
{
if (f.Text == filterValue)
{
found = true;
break;
}
}
if (!found)
AvailableFilters.Add(new FilterValue() { IsChecked = false, Text = filterValue });
return found;
}
private void FilterChanged(object sender, RoutedEventArgs e)
{
//Here if I check AvailableFilters, the value is not changed yet.
//PropertyChanged allways fires after this, what makes me unable to
//get all currently applied filters (checked items)...
}
}
FilterValue:
public class FilterValue : INotifyPropertyChanged
{
private bool _IsChecked;
private string _Text;
public bool IsChecked
{
get { return _IsChecked; }
set
{
_IsChecked = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("IsChecked"));
}
}
public string Text
{
get { return _Text; }
set
{
_Text = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Text"));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}

So, as I tried to reproduce this behavior, I realized that this appears to be a behavior that only occurs like that in Silverlight. If you try this example on WPF, the Changed fires after the bound property is updated. So you can just access your AppliedFilters property in the FilterChanged method and it will reflect the actual current situation. On Silverlight though, not so much. Even worse, this behavior didn’t even appear to be consistent to me. I did encounter situations in which the event fired after the property has been updated (resulting in the expected output).
A way to get around this is to clean up your component logic. If you look at it, you are mixing two different concepts: Event-driven UI logic, and clear data binding. Of course, doing it “properly” has multiple effects you likely cannot just ensure in an existing project, but you can at least try to get in the right direction here which should then also solve this issue.
So your logic right now uses data binding to provide the data for the view, and to reflect changes of the displayed items. But you are using events on the item level to perform additional logic depending on the former changes. As we have seen, the order of execution appears not be guaranteed across platforms, so it’s best to avoid having to rely on it.
In this case, you should have your data be the source of truth and make changes in the data tell you when applied filters change. You’re already halfway there by having an ObservableCollection and items that implement INotifyPropertyChanged. Unfortunately, an observable collection will only notify you about changes to the collection but not to changes to the contained items. But there are multiple solutions to expand the collection to also look at the items inside the collection.
This related question covers exactly that topic and there are multiple ideas on how to expand the observable collection for exactly that behavior. In my case, I have used the FullyObservableCollection implementation by Bob Sammers.
All you have to do for that is to change your ObservableCollection<FilterValue> into a FullyObservableCollection<FilterValue> and subscribe to the ItemPropertyChanged event:
AvailableFilters = new FullyObservableCollection<FilterValue>();
AvailableFilters.ItemPropertyChanged += AvailableFilters_ItemPropertyChanged;
In that event handler, you will then correctly see the proper behavior.

DataGrid Items not being updated on ObservableCollection change

I've got a DataGrid , where values shown in the columns aren't always being updated correctly.
Here's the definition:
<uic:DataGridControlEx Grid.Row="1"
ReadOnly="True"
Name="m_dgErgaenzungsfelder"
NavigationBehavior ="RowOnly"
SelectionMode="Extended"
AutoCreateColumns="False"
ItemsSource="{Binding Path=ErgaenzungsfelderEntities}"
SelectionChanged="OnDGSelectionChanged" >
<uic:DataGridControlEx.View>
<xc:TableView ColumnStretchMode="Last"
AllowColumnChooser="False"
VerticalGridLineThickness="0"
UseDefaultHeadersFooters="False"
ShowRowSelectorPane="False">
<xc:TableView.FixedHeaders>
<DataTemplate>
<xc:ColumnManagerRow/>
</DataTemplate>
</xc:TableView.FixedHeaders>
<xc:TableView.Theme>
<xc:Office2007SilverTheme />
</xc:TableView.Theme>
</xc:TableView>
</uic:DataGridControlEx.View>
<uic:DataGridControlEx.Columns>
<xc:Column Title="{LocText FGG1:ErgaenzungsfelderResources:ErgaenzungsfelderViewColumnName}"
FieldName="Name" />
<xc:Column Title="{LocText FGG1:ErgaenzungsfelderResources:ErgaenzungsfelderViewColumnType}"
FieldName="ErgaenzungsfeldType" >
<xc:Column.CellContentTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=., Converter={x:Static converters:ErgaenzungsfeldTypeTotextConverter.Instance}}" />
</DataTemplate>
</xc:Column.CellContentTemplate>
</xc:Column>
<xc:Column Title="{LocText FGG1:ErgaenzungsfelderResources:ErgaenzungsfelderViewColumnAuthor}"
FieldName="Author" />
<xc:Column Title="{LocText FGG1:ErgaenzungsfelderResources:ErgaenzungsfelderViewColumnCreationDate}"
FieldName="CreationDate" />
</uic:DataGridControlEx.Columns>
</uic:DataGridControlEx>
DataGridControlEx exentds the Xceed DataGridControl but doesn't influence binding.
And the code behind with the definition of the ObservableCollection the grid binds to, the constructor initializing the collection early and the method updating the items:
public ObservableCollection<ErgaenzungsfeldEntity> ErgaenzungsfelderEntities { get; private set; }
public ErgaenzungsfelderView() {
ErgaenzungsfelderEntities = new ObservableCollection<ErgaenzungsfeldEntity>();
InitializeComponent();
}
public void ShowErgaenzungsfelder(List<ErgaenzungsfeldEntity> entities) {
ErgaenzungsfelderEntities.Clear();
entities.ForEach(e => ErgaenzungsfelderEntities.Add(e));
//m_dgErgaenzungsfelder.GetBindingExpression(ItemsControl.ItemsSourceProperty).UpdateSource();
}
ErgaenzungsfeldEntity implements INotifyPropertyChanged and does notify property changes for every change e.g.:
public string Name {
get { return m_name; }
set {
m_name = value;
NotifyPropertyChanged("Name");
}
}
When updating a bound item through the GUI, the changes are being reflected correctly all the time. Through the GUI, items aren't being reloaded using the above mentioned ShowErgaenzungsfelder, but the bound item is being passed as a reference.
Issue:
Our service layer can notify events requiring to reload the elements. This will call ShowErgaenzungsfelder. When doing this, added entities will show up in the grid, removed entities will be removed. BUT, modified entities won't reflect the changes for the fields Name and ErgaenzungsfeldType (which are the only properties which can change).
E.g. changing the column sorting will trigger an update of the grid and display the correct values.
For the `ItemsSource, I've tried changing all these properties without success:
ItemsSource="{Binding Path=ErgaenzungsfelderEntities, UpdateSourceTrigger=Explicit, NotifyOnTargetUpdated=True, NotifyOnSourceUpdated=True, Mode=OneWay}"
With
UpdateSourceTrigger=Explicit and un-commented m_dgErgaenzungsfelder.GetBindingExpression(ItemsControl.ItemsSourceProperty).UpdateSource(); in ShowErgaenzungsfelder
UpdateSourceTrigger=PropertyChanged
UpdateSourceTrigger=Default
I'd be grateful for any input.

Some psychic debugging here.
The Events raised by the Service Layer are not running on the GUI Thread, and hence the ShowErgaenzungsfelder function is also not on the GUI Thread, nor are the events raised by changing the collection. WPF will receive these events on the non-GUI Thread and then attempt to update the GUI, but fail as it's doing so not on the GUI Thread and throw an error. WPF's behaviour when it generates an exception is to abort the operation and hide the exception, hence you see nothing. (In Visual Studio, you may see these exceptions in the Output panel; there's an option to show them there.)
To test this, you need to despatch the updates to the GUI Thread. You can do this as follows:
public ObservableCollection<ErgaenzungsfeldEntity> ErgaenzungsfelderEntities { get; private set; }
public ErgaenzungsfelderView() {
ErgaenzungsfelderEntities = new ObservableCollection<ErgaenzungsfeldEntity>();
InitializeComponent();
// This will be called on the GUI thread
this.guiContext = SynchronizationContext.Current;
}
private readonly SynchronizationContext guiContext;
public void ShowErgaenzungsfelder(List<ErgaenzungsfeldEntity> entities) {
this.guiContext.Send(this.ShowErgaenzungsfelderOnGuiThread, entities);
}
private void ShowErgaenzungsfelderOnGuiThread(object state) {
List<ErgaenzungsfeldEntity> entities = state as List<ErgaenzungsfeldEntity>;
ErgaenzungsfelderEntities.Clear();
entities.ForEach(e => ErgaenzungsfelderEntities.Add(e));
}

Bound TextBox does not update

I have a ComboBox bound to an ObservableCollection of objects (with several properties). The Combo Box accurately displays the desired property of all objects and I can select any item from the Combo as expected.
<ComboBox Height="23" Name="comboBox1" Width="120" Margin="5" ItemsSource="{Binding Issues}" DisplayMemberPath="Issue" SelectedValuePath="Issue" SelectedValue="{Binding Path=Issues}" IsEditable="False" SelectionChanged="comboBox1_SelectionChanged" LostFocus="comboBox1_LostFocus" KeyUp="comboBox1_KeyUp" Loaded="comboBox1_Loaded" DropDownClosed="comboBox1_DropDownClosed" IsSynchronizedWithCurrentItem="True" />
I have a series of text boxes which are supposed to display other properties of the selected object. This works fine too.
<TextBox Height="23" Name="textBox5" Width="59" IsReadOnly="True" Text="{Binding Issues/LastSale, StringFormat={}{0:N4}}" />
<TextBox Height="23" Name="textBox9" Width="90" IsReadOnly="True" Text="{Binding Path=Issues/LastUpdate, Converter={StaticResource TimeConverter}}" />
BUT... The properties of ObservableCollection are updated in the Code-Behind on a regular basis and I make a change to the OC by either adding or removing a dummy object in it every time the properties are updated. (I found this simpler than other solutions).
BUT...the data in the TextBoxes DO NOT change! :-( If I select a different object from the ComboBox I get updated info, but it does not change when the OC is changed.
The OC is composed of a bunch of these Objects:
public class IssuesItems
{
public String Issue { get; set; }
public Double LastSale { get; set; }
public DateTime LastUpdate { get; set; }
...
}
The OC is defined as:
public ObservableCollection<IssuesItems> Issues { get; set; }
and instantiated:
this.Issues = new ObservableCollection<IssuesItems>();
What am I doing wrong? Everything I read says that when the LastSale and LastUpdate properties are changed in the OC (and I do something to force an update of the OC) the data in the text boxes ought to change.

ObservableCollection implements INotifyCollectionChanged which allows GUI to refresh when any item is added or deleted from collection (you need not to worry about doing it manually).
But like i mentioned this is restricted to only addition/deletion of items from collection but if you want GUI to refresh when any underlying property gets changed, your underlying source class must implement INotifyPropertyChanged to give notification to GUI that property has changed so refresh yourself.
IssuesItems should implement INPC interface in your case.
Refer to this - How to implement INotifyPropertyChanged on class.
public class IssuesItems : INotifyPropertyChanged
{
private string issue;
public string Issue
{
get { return issue; }
set
{
if(issue != value)
{
issue= value;
// Call OnPropertyChanged whenever the property is updated
OnPropertyChanged("Issue");
}
}
}
// Declare the event
public event PropertyChangedEventHandler PropertyChanged;
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
}
Implement other properties just like Issue as mentioned above.

How to group checkboxes in treeview wpf mvvm when selection range is [0,1]

I have made a tree View in wpf Using MVVM .
it is working fine but here is one problem that leaf node contains some checkboxes and user have only two options either to select one or none .
So here how i can restricted user to select maximum only one cold drink.
I did one trick but it didn't work that when i have already selected a drink and then i select another one than i set the last selected value in the observable collection to false but it doesn't affect on view and selected check boxes remains selected although in collection only one option's value is true.
I cant use radio button instedof checkbox becasue user can select none of the options and i cant give an additional option for none of the above.
If any one have any solution so please let me know I'll be very thankful.
updated question:
i think i didn't define my problem in a proper way so i am giving my code snipperts here hope by this i'll get the solution o f my problem...
My View Model Class
namespace TestViewModels
{
public class ViewModel :ViewModelBase
{
private ObservableCollection<AvailableProducts> _MyTreeViewProperty
public ObservableCollection<AvailableProducts> MyTreeViewProperty
{
get { return _MyTreeViewProperty
set { _MyTreeViewProperty value;
RaisePropertyChanged("MyTreeViewProperty");}
}
}
public class AvailableProducts
{
private string _BrandName;
public string BrandName
{
get { return _BrandName
set { _BrandName = value; }
}
private bool _IsExpanded;
public bool IsExpanded
{
get
{
return _IsExpanded;
}
set
{
_IsExpanded = value;
}
}
private ObservableCollection<ProductTypes> _MyProductTypes
public ObservableCollection<ProductTypes> MyProductTypes
{
get { return _MyProductTypes}
set { _MyProductTypes= value; }
}
}
public class ProductTypes
{
private string _ProductTypeName;
public string ProductTypeName
{
get { return _ProductTypeName;
set { _ProductTypeNamevalue; }
}
private ObservableCollection<ProductSubTypes> _ProdSubTypes;
public ObservableCollection<ProductSubTypes> ProdSubTypes
{
get { return _ProdSubTypes;}
set { _ProdSubTypes;= value; }
}
}
public class ProductSubTypes
{
private string _ProductSubTypeName;
public string ProductSubTypeName
{
get { return _ProductSubTypeName;
set { _ProductSubTypeName;}
}
private int _ParentID;
public int ParentID
{
get { return _ParentID;}
set { _ParentID;= value; }
}
private bool _IsAssigned;
public bool IsAssigned
{
get { return _IsAssigned; }
set
{
_IsAssigned = value;
if _ParentID;!= 0)
{
//updating data in database
//Calling and setting new collection value in property
//issue : updated collection sets in setter of MyTreeViewProperty but before calling getter
// it comes to IsAssigned getter so view doesnt get updated collection of MyTreeViewProperty
}
RaisePropertyChanged("IsAssigned");
}
}
}
}
View
<Page x:Class="ShiftManagerViews.Pages.ProductTreeSelection
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
DataContext="{Binding ProductsTree, Source={StaticResource Locator}}"
mc:Ignorable="d" Width="870" Height="665"
>
<TreeView Margin="10,10,0,13" ItemsSource="{Binding MyTreeViewProperty, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left"
VerticalAlignment="Top" Width="800" Height="Auto" MinHeight="400" MaxHeight="800">
<TreeView.ItemContainerStyle>
<Style TargetType="{x:Type TreeViewItem}">
<Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
</Style>
</TreeView.ItemContainerStyle>
<TreeView.Resources>
<HierarchicalDataTemplate DataType="{x:Type local:AvailableProducts}"
ItemsSource="{Binding MyProductTypes}">
<WrapPanel>
<Image Width="20" Height="20" Source="/ShiftManagerViews;component/Images/12.bmp"/>
<Label Content="{Binding BrandName}" FontSize="14"/>
</WrapPanel>
</HierarchicalDataTemplate>
<HierarchicalDataTemplate DataType="{x:Type local:ProductTypes}"
ItemsSource="{Binding ProdSubTypes}">
<WrapPanel>
<Image Width="18" Height="15" Source="/ShiftManagerViews;component/Images/12.bmp"/>
<Label Content="{Binding ProductTypeName}" FontSize="13"/>
</WrapPanel>
</HierarchicalDataTemplate>
<!-- the template for showing the Leaf node's properties-->
<DataTemplate DataType="{x:Type local:ProductSubTypes}">
<StackPanel>
<CheckBox IsChecked="{Binding IsAssigned, Mode=TwoWay}" Content="{Binding ProductSubTypeName}" Height="25">
</CheckBox>
</StackPanel>
</DataTemplate>
</TreeView.Resources>
</TreeView>

What about using a ListBox to display sub-items instead of a TreeView? You can style that so the items contain a CheckBox to show IsSelected instead of highlighting the item.

I'd suggest your user interface is wrong. If the user can only pick one then it would be better to swap these for radio buttons and add a "None of the above" option. That'll then give you the behaviour you want for free and your UI will be more intuitive.
EDIT: Since you say you can't add a "None" option and want to use a checkbox (even though I strongly disagree on checkboxes where a radio button is more appropriate - a common UI error)...
The technical problem you are probably facing is that an ObservableCollection only raises notification events if the collection itself changes. i.e. Only if items are added or removed. It does not raised events when items within the collection change, therefore the changing the status of the checkbox in the code will not raise the event for the UI binding to act on.
One solution to this to write a custom class that extends ObservableCollection that does provide this behaviour
From MSDN:
If you need to know if someone has changed a property of one of the
items within the collection, you'll need to ensure that the items in
the collection implement the INotifyPropertyChanged interface, and
you'll need to manually attach property changed event handlers for
those objects. No matter how you change properties of objects within
the collection, the collection's PropertyChanged event will not fire.
As a matter of fact, the ObservableCollection's PropertyChanged event
handler is protected—you can't even react to it unless you inherit
from the class and expose it yourself. You could, of course, handle
the PropertyChanged event for each item within the collection from
your inherited collection

I upvoted Rachel's answer, it is a common way in WPF to databind sets of radio buttons or check boxes. If you still want to go the tree view way, below code works. All view related code is in the view, so below code follows MVVM principles. If you are a MVVM purist you can put the code behind and a TreeView control in a user control if you do not want any code behind.
XAML:
<TreeView ItemsSource="{Binding Path=Drinks}">
<TreeView.ItemTemplate>
<DataTemplate>
<CheckBox Content="{Binding .}" Checked="OnCheckBoxChecked" Unchecked="OnCheckBoxUnchecked" Loaded="OnCheckBoxLoaded" />
</DataTemplate>
</TreeView.ItemTemplate>
</TreeView>
Code behind + VM:
public partial class Window1
{
public Window1()
{
InitializeComponent();
DataContext = new VM();
}
private void OnCheckBoxChecked(object sender, System.Windows.RoutedEventArgs e)
{
foreach (CheckBox checkBox in _checkBoxes.Where(cb => cb != sender))
{
checkBox.IsChecked = false;
}
(DataContext as VM).CurrentDrink = (sender as CheckBox).Content.ToString();
}
private void OnCheckBoxUnchecked(object sender, System.Windows.RoutedEventArgs e)
{
(DataContext as VM).CurrentDrink = null;
}
private void OnCheckBoxLoaded(object sender, System.Windows.RoutedEventArgs e)
{
_checkBoxes.Add(sender as CheckBox);
}
private List<CheckBox> _checkBoxes = new List<CheckBox>();
}
public class VM
{
public List<string> Drinks
{
get
{
return new List<string>() { "Coffee", "Tea", "Juice" };
}
}
public string CurrentDrink { get; set; }
}

I did one trick but it didn't work that when i have already selected a
drink and then i select another one than i set the last selected value
in the observable collection to false but it doesn't affect on view
and selected check boxes remains selected although in collection only
one option's value is true.
Make sure that your child objects (AvailableProducts
and SubProductTypes) also implement INotifyPropertyChanged, this will make sure that the UI receives changes when modify the object.
Once all of you objects update the UI properly you will be able to layer in, and test, whatever custom business logic you need.
So if you have a product type that can only have one sub chosen, you could add a property on ProductType called OnlyAllowOneChild. Whenever, a child object raises a IsAssigned changed event, the parent can set false all other children. This of course requires you to have the parent either register for the children's PropertyChangedEvent, or got grab an EventAggregator (MVVMLight Messenger, or PRISM EvenAggregator) and create a messaging system.

Finally i am succeeded to solve my problem.
on Is Assigned property i am updating my database values and calling a method in view using MVVM Light messaging and passing currently selected leaf's parent id in it as a parameter...
Added a property in class Product Types to expand the parent node of the last selected leaf..
In view's method i am refreshing data context's source and passing currently selected leaf's parent id tO the VM to set its Is Expanded property value to true...
By this my view is working perfectly as same as i want...
If any body have solution better than this than I'll be happy to know.

WPF Data Binding with multiple controls

In WPF, I'm trying to bind multiple controls, but the second control isn't changing when the first control is changed.
I have two classes: a Task class, and a Log class, which is stored as a collection in the Task class. The list boxes below are bound to the Tasks, and the inner Logs for the selected Task.
The problem is that the list boxes are populated fine at first load, but if I select a different task, I'd expect the Logs to be update to the collection for the new Task, but it doesn't change from those from the originally selected task on first load. What am I missing?
In the designer:
<ListBox x:Name="listBoxTasks" ItemsSource="{Binding}" DisplayMemberPath="Key"
Grid.Row="0" Grid.Column="0" Grid.RowSpan="2">
</ListBox>
<ListBox x:Name="listBoxLogs"
ItemsSource="{Binding Logs}" DisplayMemberPath="EntryDate"
Grid.Row="1" Grid.Column="1">
</ListBox>
In the code behind:
public MainWindow()
{
InitializeComponent();
IMongoCollection<Task> tasks = DataManager.GetData();
this.DataContext = tasks.AsQueryable();
}
The Task class:
public class Task : BusinessBase<Task>
{
public ObjectId _Id { get; set; }
public string Key { get; set; }
public string Description { get; set; }
public string Summary { get; set; }
public string Details { get; set; }
public IEnumerable<Log> Logs { get; set; }
public IEnumerable<Link> Links { get; set; }
public IEnumerable<String> RelatedKeys { get; set; }
public IEnumerable<TaskItem> Items { get; set; }
}

Your Task class need to implement INotifyPropertyChanged interface so that as soon as there is any change in the underlying data it can tell WPF UI that something has changed now update/refresh your controls agains

Your task class need to implement INotifyPropertyChanged
http://msdn.microsoft.com/en-us/library/ms743695.aspx

You have to bind your first ListBox SelectedItem to object of Task model and add event handler for SelectionChanged. inside the this event you have to populate your logs by selected Task model also you have to implement INotifyPropertyChanged in your class.

It looks to me like the second binding should not work at all, as the DataContext is an enumerable of Tasks and the enumerable itself has no property called Logs. You could try working with IsSynchronizedWithCurrentItem and a binding to the current item:
<ListBox x:Name="listBoxTasks" ItemsSource="{Binding}" DisplayMemberPath="Key"
Grid.Row="0" Grid.Column="0" Grid.RowSpan="2"
IsSynchronizedWithCurrentItem="True"> <!-- Set this -->
</ListBox>
<ListBox x:Name="listBoxLogs" DisplayMemberPath="EntryDate"
Grid.Row="1" Grid.Column="1"
ItemsSource="{Binding /Logs}"> <!-- Note the slash which indicates a binding to the current item -->
</ListBox>
You could also bind to the SelectedItem of the other ListBox but this introduces a redundant dependency between the controls. Also note that if you change any property in your data-objects you need to implement the interface mentioned by the other answerers, INotifyPropertyChanged.

I have it all working now. I implemented INotifyPropertyChanged, although that didn't solve the problem.
I am now using the MVVM pattern. This helped...the NoRM library I was using didn't have a SelectionChanged event. I created a View Model and was able to convert those Models to ObservableCollections. Now I'm just setting the Logs control DataContext on selection changed for the Task class.