Develop Reference

Unusual disabling behavior with Command buttons

I'm sorry for this being so wordy, but I want to make the situation perfectly clear. Please, if you are a WPF pro, take a look.
There are two CollectionViewSource bound to ItemsControls that use UserControl Views to display either StackPanels or custom Buttons. There is one for each side shown in the screenshot below. The problem I'm encountering is that when the parent collection property is set, all of the buttons in the DataTemplate view are disabled. Even the 2 buttons higher up are having the same problem even though they worked before my recent edits.
If I click on the form, or press any key, the buttons enable. As soon as the property is reset to a newly edited and sorted collection, they disable again. Rinse and repeat. This is what it looks like. The first frame is how it starts (gray using StackPanel), the 2nd is what it looks like when the RFS button is clicked, and the 3rd frame is what happens when I click anywhere or press a key.
I've been going in circles trying out things. The only thing that seems to work is a code-behind workaround that sets focus to one thing and then back. However, that would not be good for the user if they are trying to use one of the other dashboard items.
Since the WPF for all of this is very massive, I'll try to include just the relevant parts. These are the ItemsControls on the TabItemControl (UserControl).
<!-- BID (SELL) DEPTH -->
<ItemsControl Grid.Row="0" Grid.Column="0" x:Name="bidDepthList" ItemsSource="{Binding Source={StaticResource BidDepthCollection}}"
Visibility="{Binding Path=IsMultilegEnabled, Converter={StaticResource CollapsedConverter}}">
<ItemsControl.ItemTemplate>
<DataTemplate DataType="{x:Type viewmodels:DepthLevelViewModel}">
<v:DepthLevelRowView x:Name="BidDepthLevelRowViewControl" DataContext="{Binding}" HorizontalAlignment="Center" Margin="1,0,1,3" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
<!-- ASK (BUY) DEPTH -->
<ItemsControl Grid.Row="0" Grid.Column="1" x:Name="askDepthList" ItemsSource="{Binding Source={StaticResource AskDepthCollection}}"
Visibility="{Binding Path=IsMultilegEnabled, Converter={StaticResource CollapsedConverter}}">
<ItemsControl.ItemTemplate>
<DataTemplate DataType="{x:Type viewmodels:DepthLevelViewModel}">
<v:DepthLevelRowView x:Name="AskDepthLevelRowViewControl" DataContext="{Binding}" HorizontalAlignment="Center" Margin="1,0,1,3" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
The view being used has 4 "controls" inside of a grid. Only one of them is displayed at a time depending on the state (RFS OFF/RFS ON) and which side they are on (Sell/Buy). The others are collapsed. As you can see, this is fine.
The only common factor between them is that they have their Command set, as do most of the controls at the top that are disabling/enabling correctly. The fact that the buttons enable correctly if any mouse or keyboard action is taken tells me that the CanExecute handler is working, just not immediately. The other controls started working after I made these changes, but then the big buttons started misbehaving like the depth buttons have been doing.
I've tried using CommandManager.InvalidateRequerySuggested(); after altering the collections, but that didn't help.
NOTE: This is also happening even for something as simple as this:
<Button x:Name="TestBuyButton" Command="{x:Static ptcommands:OrderCommands.Buy}" CommandTarget="{Binding RelativeSource={RelativeSource Self}}" Content="Test Buy" />
I know the booleans are set correctly because I added test CheckBoxes to display the current value with IsChecked. And, they all enable, including the extremely basic Button, as soon as any input action is taken.
Is there something else I'm missing or a different approach I can take?
EDIT: I ended up using the Dispatcher to invoke my display update routine from the event thread over to the UI thread. The boolean values get set, but WPF still didn't requery the Command. BUT.. the CommandManager.InvalidateRequerySuggested() call then worked! The only thing I don't like about that is it sounds like a broadcast invalidation. I don't want all commands to be requeried. I just want the Buy and Sell commands to requery. I've tried all sorts of weirdness trying to get just those to work, but nothing has worked so far other than the global.
EDIT 2: It appears as though the InvalidateRequerySuggested is the way to go.
From MSDN CommandManager.InvalidateRequerySuggested:
The CommandManager only pays attention to certain conditions in determining when the command target has changed, such as change in keyboard focus. In situations where the CommandManager does not sufficiently determine a change in conditions that cause a command to not be able to execute, InvalidateRequerySuggested can be called to force the CommandManager to raise the RequerySuggested event.
This would explain why the focus changes and keypresses would cause the buttons to enable. The page also shows putting the call in a timer. Because of this, I assume it is not as resource intensive as I thought. So, I guess it's my permanent solution.

I had to make a few changes to get the buttons to enable without focus changes.
The boolean values were being set within a method called from an event thread. I had tried calling CommandManager.InvalidateRequerySuggested along with it, but nothing happened. Eventually I thought it might have something to do with the thread. This is what ended up resolving it.
In the parent form that the tab manager class (containing the event handlers and other logic) has access to, I added an invoke method:
Public Sub InvokeOnUiThread(ByRef uiAction As Action, Optional ByRef doAsync As Boolean = False)
Dim dispatchObject As Dispatcher = orderTicketView.Dispatcher
If (dispatchObject Is Nothing OrElse dispatchObject.CheckAccess()) Then
uiAction()
Else
If doAsync Then
dispatchObject.BeginInvoke(uiAction, DispatcherPriority.Normal)
Else
dispatchObject.Invoke(uiAction, DispatcherPriority.Normal)
End If
End If
End Sub
In the tab manager event handler, I changed it to call the update routine through the invoker:
formOrderTicketView.InvokeOnUiThread(New Action(AddressOf UpdateButtons))
At the bottom of the UpdateButtons method, I added a call to the CommandManager if a change has been made that would require a requery:
CommandManager.InvalidateRequerySuggested()
I do not know what type of performance hit this would have, but apparently WPF executes it in its own way when focus changes and such. Calling it directly is the advised way to force it.
From MSDN CommandManager.InvalidateRequerySuggested:
The CommandManager only pays attention to certain conditions in determining when the command target has changed, such as change in keyboard focus. In situations where the CommandManager does not sufficiently determine a change in conditions that cause a command to not be able to execute, InvalidateRequerySuggested can be called to force the CommandManager to raise the RequerySuggested event.
Since it is working now, I am taking this as the resolution.

The command that's bound to the button...is there a CanExecute delegate with it? If so, you have to raise CanExecuteChanged when that delegate should be re-evaluated. Also, make sure the implementation of CanExecute isn't broken and incorrectly returning false;

Accessing a component of a column in a DataGrid

I'm a Java developer by profession & was given some tasks in .NET as a pilot project.
It's a small invoicing application which needs to be developed with WPF & EntityFramework.
One of my tasks consist of showing a list of invoices in a window and upon clicking "edit" for any invoice, I should show the details of that invoice along with the invoice items that are assigned to that invoice.
Following is my XAML code fragment of showing invoice items.
<DataGrid x:Name="ProductGrid" AutoGenerateColumns="False" HorizontalAlignment="Stretch"
HorizontalContentAlignment="Stretch" ColumnWidth="*" Height="464" VerticalAlignment="Top" Margin="444,16,10,0" CanUserAddRows="false">
<DataGrid.Columns>
<DataGridTemplateColumn Width="55" Header="Selected">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<CheckBox Margin="2,0,2,0" HorizontalAlignment="Center" VerticalAlignment="Center"
Checked="Product_Selected" Unchecked="Product_Deselected" IsChecked="{Binding Path=selected}"/>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
<DataGridTemplateColumn Width="60" Header="Quantity">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<xctk:IntegerUpDown x:Name="UPDOWN" Increment="1" Minimum="0" HorizontalAlignment="Center" ValueChanged="Quantity_Changed"
VerticalAlignment="Center" Width="50" Value="{Binding productQuantity, Mode=TwoWay}"/>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
<DataGridTextColumn Header="Product Name" Width="250" Binding="{Binding Path=productName}"/>
<DataGridTextColumn Header="Weight" Binding="{Binding Path=productWeight}"/>
<DataGridTextColumn Header="Size" Binding="{Binding Path=productSize}"/>
<DataGridTextColumn Header="Sale price" Binding="{Binding Path=productSalePrice}"/>
</DataGrid.Columns>
</DataGrid>
Now, what I need to achieve is that when I select a checkbox, the code behind should automatically increase the value of the IntegerUpDown component to 1. Also if I deselect a checkbox, the code behind should automatically reset the value of the IntegerUpDown component to 0.
Following is my code fragment for Product_Selected event.
private void Product_Selected(object sender, RoutedEventArgs e)
{
var currRow = ProductGrid.CurrentItem; // Current row
InvoiceItemsDTO sel = (InvoiceItemsDTO)currRow; // Current row DTO OBJECT
if (sel != null)
{
if (sel.productQuantity == 0) // The user is trying to assign a new item to the invoice
{
int currentRowIndex = ProductGrid.Items.IndexOf(currRow); // Current row index
DataGridRow currentRow = ProductGrid.ItemContainerGenerator.ContainerFromIndex(currentRowIndex) as DataGridRow;
IntegerUpDown prop = ProductGrid.Columns[1].GetCellContent(currentRow) as IntegerUpDown; // Here I get a NULL for "prop"..!! :(
prop.Value = 1; // Automatically increase the value of IntegerUpDown from zero to one
}
}
}
To do this, I need to access the IntegerUpDown component of the selected row. Unfortunately, I have no idea of doing that.
I hope that some of you .NET geniuses may be able to help me in this matter.
Thanks very much in advance.
Reagrds,
Asela.

Okay, it's been some time since I answered any questions here, but yours is definitely worth some attention.
First of all, regarding this:
I'm a Java developer by profession
Forget java.
Most (if not all) of the (rather cumbersome and excessively verbose) patterns and paradigms you might be used to in java are of little or no use at all in C# and WPF.
This is because, in constrast to java, C# is a modern, professional-level language with many language features that provide ease of development and greatly reduce boilerplate.
In addition to that, WPF is an advanced, professional-level UI framework with tons of advanced features (most notably Data Binding and Data Templating) that allow you to create a high-level abstraction and completely separate your code and application logic from the UI components, achieving maximum flexibility without introducing nasty constructs or unnecessary coupling.
Such an abstraction is achieved by implementing a pattern called MVVM. This pattern is rather ubiquitous in most (if not all) modern UI technologies, both Web and Non-Web, except in the java world, which instead seems to believe (unsurprisingly) it's still 1990.
So, instead of trying to hammer concepts from legacy technologies and make them somehow fit into WPF, I suggest you take the time to understand, and embrace The WPF Mentality.
Now, I see several flaws in your code, both in terms of the code itself and in terms of the philosophy / approach you're using to write it.
First of all, the presence of things like Height="464" Margin="444,16,10,0" or the like in XAML indicate that you used the Visual Studio designer to build such UI. This is useful as a learning exercise, but it is highly discouraged for production code, for the reasons stated here.
I suggest you take the time to properly learn XAML and also look at this tutorial to understand how the WPF layout system works, and how to write resolution-independent, auto-adjustable WPF UIs rather than fixed-size, fixed-position layouts that don't properly adjust even when resizing the containing Window.
Again, the typical mistake developers make in WPF when coming from whatever other technologies is in how they approach things, rather than how they code them. Let's analyze your code:
var currRow = ProductGrid.CurrentItem; // Current row
InvoiceItemsDTO sel = (InvoiceItemsDTO)currRow; // Current row DTO OBJECT
if (sel != null)
{
//...
}
This code (aside from the fact that it could be shortened) is, at first glance, just fine. You're retrieving the underlying data object rather than trying to mess with the UI elements. This IS the correct approach in WPF. You need to operate on your data items and NOT the UI.
Let's rewrite it into a more C#-like way:
var row = ProductGrid.CurrentItem as InvoiceItemsDTO;
if (row != null)
{
//...
}
Note: The above code shows an example of how C# language level features (in this case, the as operator) help in reducing boilerplate (we now have 2 lines of code instead of 3) by allowing beautiful code that otherwise requires a bunch of horrible hacks in inferior technologies such as java.
Okay, So far so good, but then you slip away from this data-centric thinking into trying to manipulate the UI for some reason.
Think about it: you're trying to "update the Value property of the IntegerUpDown which corresponds to the currently selected row".
But, your XAML shows that the Value property of the IntegerUpDown is actually bound via Two-Way DataBinding to a property called productQuantity in the underlying data item.
So, basically, your code results in something like this:
get Data Item -> get UI item -> update UI item -> DataBinding updates Data Item.
See? you're creating a completely unnecessary indirection. Instead, simply operate on your data item rather than the UI, and let Two-Way databinding take care of the rest. That's the WPF mentality.
var row = ProductGrid.CurrentItem as InvoiceItemsDTO;
if (row != null)
{
row.productQuantity++;
}
See how much easier life is when you're dealing with modern technology?
But it doesn't even end there.
Your XAML also shows that the CheckBox you're dealing with has it's IsChecked property bound to a property called selected in the underlying Data item:
<CheckBox [...] IsChecked="{Binding Path=selected}"/>
This means that your InvoiceItemsDTO class has a public bool selected {...} property, right? So, instead of dealing with events at the UI level, (again), why don't you simply put the logic where it really belongs, and get rid of the UI dependencies, effectively making your code more testable, much cleaner, and simply beautiful?
public class InvoiceItemsDTO
{
private bool _selected;
public bool Selected
{
get { return _selected; }
set
{
_selected = value;
//This is where your code should be.
if (value)
ProductQuantity++;
else
ProductQuantity--;
}
}
}
As an aside, notice the use of proper casing. camelCasing is horrible, and thus reserved for private members only in C#. Not public ones.
See? simple, clean, testable, and beautiful, and it just works.
But, How does it work?
1 - When the Checkbox is clicked by the user, the IsChecked value is updated.
2 - WPF's DataBinding updates the value of the InvoiceItemsDTO.Selected property to true.
3 - Your code adds +1 to the ProductQuantity property.
4 - WPF's DataBinding reflects the ProductQuantity change in the UI, provided you have properly Implemented INotifyPropertyChange.
The same workflow occurs when un-checking the Checkbox, but with false value.
This removes the need for event handlers, casting, code behind, and other cumbersome approaches that require useless boilerplate and introduce unnecessary, undesired coupling.
Bottom line: C# Rocks. WPF Rocks. java is legacy.
Let me know if you need further help.

How can I prevent flickering when binding a boolean to the visibility of a control

I have a boolean property in my ViewModel, named lets say IsNotSupported that is used to show some warning information if a sensor is not supported. Therefore I use a BooleanToVisibilityConverter, that is added in the ressources:
<phone:PhoneApplicationPage.Resources>
<local:BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
</phone:PhoneApplicationPage.Resources>
and bind it to the stackpanel containing the warning:
<StackPanel x:Name="NotSupportedWarning" Visibility="{Binding IsNotSupported,
Converter={StaticResource BooleanToVisibilityConverter}}">
That works all quite well, but when loading the page, and the sensor is supported, the warning appears for just a fraction of a second and disappears afterwards. I know that this flickering is caused by the binding not having happened yet and therefore defaulting to visible.
That flicker it is annoying as hell... It should rather default to collapsed and be made visible only after it is clear that the warning should be shown. Also, this would avoid a second layouting pass after the binding and could therefore have positive performance impacts.
I had this problem over and over, and found nothing about it in the internet until I found this SO question, that is closely related, but is not found if searched for windows phone instead of silverlight. Both the problem and the solution might seem simple, but I really bugged me quite a long time, so I thought it might be a good idea to write a Q&A-style question about it to help others that are facing the same issue.

The solution is simple after you have seen it. You can control the default value of the binding (if the binding didnt happen yet) with FallbackValue. Your stackpanel XAML would look like:
<StackPanel x:Name="NotSupportedWarning" Visibility="{Binding IsNotSupported,
FallbackValue=Collapsed,
Converter={StaticResource BooleanToVisibilityConverter}}">
This way you get rid of the flicker and it does not have to be relayouted after the binding, if the warning stays hidden.

you can bind directly to a Visibility type of property instead of boolean and keep that property to collapsed by default plus you can implement INotifyPropertyChanged

WPF Combobox binding: can't change selection

After wasting hours on this, following on the heels of my Last Problem, I'm starting to feel that Framework 4 is a master of subtle evil, or my PC is haunted.
I have three comboboxes and a textbox on a WPF form, and I have an out-of-the-box Subsonic 3 ActiveRecord DAL.
When I load this "edit record" form, the comboboxes fill correctly, they select the correct items, and the textbox has the correct text. I can change the TextBox text and save the record just fine, but the comboboxes CANNOT BE CHANGED. The lists drop down and highlight, but when you click on an item, the item selected stays the same.
Here's my XAML:
<StackPanel Orientation="Horizontal" Margin="10,10,0,0">
<TextBlock Width="80">Asset</TextBlock>
<ComboBox Name="cboAsset" Width="180"
DisplayMemberPath="AssetName"
SelectedValuePath="AssetID"
SelectedValue="{Binding AssetID}" ></ComboBox>
</StackPanel>
<StackPanel Orientation="Horizontal" Margin="10,10,0,0">
<TextBlock Width="80">Status</TextBlock>
<ComboBox Name="cboStatus" Width="180"
DisplayMemberPath="JobStatusDesc" SelectedValuePath="JobStatusID"
SelectedValue="{Binding JobStatusID}" ></ComboBox>
</StackPanel>
<StackPanel Orientation="Horizontal" Margin="10,10,0,0">
<TextBlock Width="80">Category</TextBlock>
<ComboBox Name="cboCategories" Width="180"
DisplayMemberPath="CategoryName"
SelectedValuePath="JobCategoryID"
SelectedValue="{Binding JobCategoryID}" ></ComboBox>
</StackPanel>
<StackPanel Orientation="Horizontal" Margin="10,10,0,0">
<TextBlock Width="80">Reason</TextBlock>
<TextBox Name="txtReason" Width="380" Text="{Binding Reason}"/>
</StackPanel>
Here are the relevant snips of my code (intJobID is passed in):
SvcMgrDAL.Job oJob;
IQueryable<SvcMgrDAL.JobCategory> oCategories = SvcMgrDAL.JobCategory.All().OrderBy(x => x.CategoryName);
IQueryable<SvcMgrDAL.Asset> oAssets = SvcMgrDAL.Asset.All().OrderBy(x => x.AssetName);
IQueryable<SvcMgrDAL.JobStatus> oStatus = SvcMgrDAL.JobStatus.All();
cboCategories.ItemsSource = oCategories;
cboStatus.ItemsSource = oStatus;
cboAsset.ItemsSource = oAssets;
this.JobID = intJobID;
oJob = SvcMgrDAL.Job.SingleOrDefault(x => x.JobID == intJobID);
this.DataContext = oJob;
Things I've tried:
Explicitly setting IsReadOnly="false" and IsSynchronizedWithCurrentItem="True"
Changing the combobox ItemSources from IQueryables to Lists.
Building my own Job object (plain vanilla entity class using INotifyPropertyChanged).
Every binding mode for the comboboxes.
ItemsSource="{Binding}"
The Subsonic DAL doesn't implement INotifyPropertyChanged, but I don't see as it'd need to for simple binding like this. I just want to be able to pick something from the dropdown and save it.
Comparing it with my last problem (link at the top of this message), I seem to have something really wierd with data sources going on. Maybe it's a Subsonic thing?
EDIT: For some reason the set accessor is hit only on the AssetID property and only the first time. WPF is now heading for WTF :)
EDIT 2: You gotta be kidding me- I've removed the binding (ie it only has a displaymemberpath, a valuememberpath and an itemssouce) and it's STILL doing it! It accepts your first selection, and then won't change.

WPF Combo Boxes will not change the selected item if the currently selected item and the item that was just selected are considered equal by the object.Equals() method called on the newly selected object (i.e newlyslected.Equals(previoslySelected) ).
Overriding the Equals method on the class your binding the combobox items, should resolve the issue your are seeing.

I've narrowed it down to the Subsonic objects used as ComboBoxItems.
If you create a new class that uses exactly the same code as the relevant parts of the Subsonic one, it works.
If you use POCOs/datatables for the combos and Subsonic for the record being edited, it works.
But if you use Subsonic for both, it doesn't.
I had hoped to extend the subsonic objects and not have to code a full-blown BLL tier. Looks like I'm faced with doing that or throwing out Subsonic for the DAL. I might post a more specific question for the Subsonic folks.
Many thanks to all who contributed.

Old topic but I had the same problem and difficulty finding solution. This might help someone else.
Clue is above in WPF not detecting a different item has been seleted by user. (Symptom - event ComboBox_SelectionChanged only fires on first selection)
My scenario - lookup combo populated from IList built from a DISTINCT query. In this case the result of using NHibernate ICriteria.SetResultTransformer which only returns SOME fields, importantly NOT including the unique entity ID.
Solution - loop thru' IList after retrieval and give each entity a unique ID. WPF sees them as individuals and behaves appropriately.
Its only a value lookup - its the value content I was after.
The 'temporary' entities are never persisted. In this case it was a better approach than messing with overriding the object's Equals method for the sake of a simple GUI issue. An alternative would be to just copy or tranform the list into a format where WPF uses the value field to determine 'difference'...

Sounds like the field is somehow readonly, or that your change isn't being persisted. After the binding sets the new value, it will re-read the property to ensure that it was actually changed. If your property returns the old value, then it'll be re-selected in the combo box, giving the appearance that the value never changed.
I don't know that DAL, but can you step through the property setter code? You might also have an issue with type conversion.
EDIT reading your comment about the red rectangle -- it sounds as though your property (or something to do with the binding) is raising an exception. Unless, of course, you're using data validation in your UI. You might turn 'Break on all exceptions' in the debugger's settings, assuming you're using Visual Studio.
EDIT 2 You should check the VS Output pane for any error messages related to binding. You can also read this blog post which gives more info on debugging bindings.

It's hard to tell from a small sample of your code but try commenting out the line:
//this.DataContext = oJob;
and see if this helps.
Setting the DataContext and ItemsSource might be causing a conflict.

Did you write any global style for your combo box which may have a bug or something missing? Or are you using pure default styles for your combobox? Try removing any default styles applied.
Are you wiring up any events? If your code hooks up for event like PreviewMouseLeftButtonUp and marks event as handled then probably combobox may ignore and wont select anything.

Memory leak on CollectionView.View.Refresh

I have defined my binding thus:
<TreeView
ItemsSource="{Binding UsersView.View}"
ItemTemplate="{StaticResource MyDataTemplate}"
/>
The CollectionViewSource is defined thus:
private ObservableCollection<UserData> users;
public CollectionViewSource UsersView{get;set;}
UsersView=new CollectionViewSource{Source=users};
UsersView.SortDescriptions.Add(
new SortDescription("IsLoggedOn",ListSortDirection.Descending);
UsersView.SortDescriptions.Add(
new SortDescription("Username",ListSortDirection.Ascending);
So far, so good, this works as expected: The view shows first the users that are logged on in alphabetical order, then the ones that are not.
However, the IsLoggedIn property of the UserData is updated every few seconds by a backgroundworker thread and then the code calls:
UsersView.View.Refresh();
on the UI thread.
Again this works as expected: users that log on are moved from the bottom of the view to the top and vice versa. However: Every time I call the Refresh method on the view the application hoards 3,5MB of extra memory, which is only released after application shutdown (or after an OutOfMemoryException...)
I did some research and below is a list of fixes that did NOT work:
The UserData class implements INotifyPropertyChanged
Changing the underlying users collection does not make any difference at all: any IENumerable<UserData> as a source for the CollectionViewSource causes the problem.
-Changing the ColletionViewSource to a List<UserData> (and refreshing the binding) or inheriting from ObservableCollection to get access to the underlying Items collection to sort that in place does not work.
I am out of ideas! Help?
EDIT:
I found it:
The Resource MyDataTemplate contains a Label that is bound to a UserData object to show one of its properties, the UserData objects being handed down by the TreeView's ItemsSource. The Label has a ContextMenu defined thus:
<ContextMenu Background="Transparent" Width="325" Opacity=".8" HasDropShadow="True">
<PrivateMessengerUI:MyUserData IsReadOnly="True" >
<PrivateMessengerUI:MyUserData.DataContext>
<Binding Path="."/>
</PrivateMessengerUI:MyUserData.DataContext>
</PrivateMessengerUI:MyUserData>
</ContextMenu>
The MyUserData object is a UserControl that shows All properties of the UserData object. In this way the user first only sees one piece of data of a user and on a right click sees all of it.
When I remove the MyUserData UserControl from the DataTemplate the memory leak disappears! How can I still implement the behaviour as specified above?

Step one in troubleshooting memory leaks is to find the definitive source. This is not always obvious and can occasionally defy your intuition. Based on your explanation, if you remove your user control, the issue disappears, but when you put it back, you start leaking again. This very likely points to a memory leak within that control (though not necessarily). Perhaps your control fits into one of the many types of WPF memory leaks, or you have a more classic problem of subscribing to an event but not properly unwiring it when no longer needed (the weak event pattern is useful here).
Your best bet is to grab a tool like .NET Memory Profiler or ANTS Memory Profiler (both are excellent and have free trials). Use one of these tools find the objects that are hanging around after they should be gone. These tools provide help in tracing the chain of objects that are hanging onto your object. There are many good articles on memory profiling on their sites, here on SO and on the wide open web.

You could try 2 things:
First of all the DropShadow has some problems, as it hold some references that should be garbadge collected.. see this article for more information:
http://blog.ramondeklein.nl/index.php/2009/02/20/memory-leak-with-wpf-resources-in-rare-cases/
You can try to set th HasDropShadow to false, and see what happens with your memory.
Secondly, if we have multiple ContextMenu objects of the same menu, you might want to create a ContextMenu resource in your Resources, and reference it with the StaticResource extension like so:
<ContextMenu Background="Transparent" Width="325" Opacity=".8" x:Key="MyAwesomeContextMenu">
<PrivateMessengerUI:MyUserData IsReadOnly="True" >
<PrivateMessengerUI:MyUserData.DataContext>
<Binding Path="."/>
</PrivateMessengerUI:MyUserData.DataContext>
</PrivateMessengerUI:MyUserData>
</ContextMenu>
And use it where you defined your context menu:
ContextMenu="{StaticResource MyAwesomeContextMenu}"
Hope this helps a bit!