Question

all day long I am sitting and trying to find out why binding to AvalonEdits Document property isn’t working. AvalonEdit is an advanced WPF text editor – part of the SharpDevelop project.(it’s going to be used in SharpDevelop v4 Mirador).

So when I set up a simple project – one TextEditor (that’s the AvalonEdits real name in the library) and made a simple class that has one property – Document and it returns a dummy object with some static text the binding is working perfectly.

However in real life solution I’m binding a collection of SomeEditor objects to TabControl.
TabControl has DataTemplate for SomeEditor and there’s the TextEditor object.

<TabControl Grid.Column="1" x:Name="tabControlFiles" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" >
 <TabControl.Resources>
  <DataTemplate DataType="{x:Type m:SomeEditor}">
   <a:TextEditor 
   Document="{Binding Path=Document, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource NoopConverter}, IsAsync=True}" 
   x:Name="avalonEdit"></a:TextEditor> 
  </DataTemplate>  
 </TabControl.Resources>

 <TabControl.ItemContainerStyle>
  <Style BasedOn="{StaticResource TabItemStyle}" TargetType="{x:Type TabItem}">
   <Setter Property="IsSelected" Value="{Binding IsSelected}"></Setter>
  </Style>
 </TabControl.ItemContainerStyle>
</TabControl>

This doesn’t work. What I’ve investigated so far:

• DataContext of TextEditor is set to the proper instance of SomeEditor
• TextEditors Document property is set to some other instance than SomeEditor.Document property
• when I set breakpoint to no-op converter that is attached to that binding it shows me the correct value for Document (the converter is used!)

• I also dug through the VisualTree to obtain reference to TextEditor and called GetBindingExpression(TextEditor.DocumentProperty) and this did return nothing

• WPF produces the following information:

  System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=Document; DataItem=’SomeEditor’ (HashCode=26280264); target element is ‘TextEditor’ (Name=’avalonEdit’); target property is ‘Document’ (type ‘TextDocument’)

• SomeEditor instance that is bound to already has a created and cached copy of Document before the binding occurs. The getter is never called.

Anyone can tell me what might be wrong? Why BindingExpression isn’t set ? Why property getter is never called?

//edit: new tests and new results

I’ve read some more and set the binding in code behind. When I do that it works.
How come setting this in XAML doesn’t work and doing the same thing in code does?

//edit2: The code also fails when called immediately after adding the object to the observable collection that is used as higher level DataSource.(that’s not long after the xaml binding should fire). That makes me think this is timing issue. Anyone can tell something about it ?

//edit3: The binding code:

private List<T> GetObjectOfTypeInVisualTree<T>(DependencyObject dpob) where T : DependencyObject
{
    int count = VisualTreeHelper.GetChildrenCount(dpob);
    List<T> returnlist = new List<T>();

    for (int i = 0; i < count; i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(dpob, i);
        T childAsT = child as T;
        if (childAsT != null)
        {
            returnlist.Add(childAsT);
        }
        List<T> lst = GetObjectOfTypeInVisualTree<T>(child);
        if (lst != null)
        {
            returnlist.AddRange(lst);
        }
    }
    if (returnlist.Count > 0)
    {
        return returnlist;
    }
    return null;
}

private void RebindMenuItem_Click(object sender, RoutedEventArgs e)
{
    foreach (XHTMLStudioPrototypeFileEditor ed in CurrentProject.OpenedFiles)
    {

        List<ContentPresenter> cps = GetObjectOfTypeInVisualTree<ContentPresenter>(tabControlFiles);
        if (cps != null)
        {
            foreach (ContentPresenter cp in cps)
            {

                foreach (DataTemplate dt in tabControlFiles.Resources.Values)
                {
                    try
                    {
                        object o = dt.FindName("avalonEdit", cp);
                        TextEditor ted = (TextEditor)o;

                        bool isDataBound = BindingOperations.IsDataBound(ted, TextEditor.DocumentProperty);
                        if (!isDataBound)
                        {
                            BindingOperations.SetBinding(ted, TextEditor.DocumentProperty, new Binding("Document"));
                        }
                        Console.WriteLine(isDataBound);
                    }
                    catch (Exception)
                    {


                    }
                }
            }
        }
    }
}

Answer 1

Here are six more things to try:

Search your carefully application for any place at all where you directly assign to the Document property of a TextEditor. It looks like some code, somewhere is doing an avalonEdit.Document = ... which would overwrite the binding. I would search your entire app for the match-case whole-word strings “Document” and “DocumentProperty” and give each occurence a moment’s thought to see if it could be setting this property.

Set a breakpoint in TextEditor.OnDocumentChanged to see if the document is being properly bound and then changed back later. Check call stacks with “Just My Code” disabled and showing external code.

Try setting breakpoints in the NoopConverter.Convert, SomeEditor.get_Document, and TextEditor.OnDocumentChanged to figure out the precise sequence of operations. Also note when the Binding error message is shown.

Temporarily modify TextEditor’s constructor to store a reference to every instance in a public static List field so you can determine which TextEditors have ever been created, then write code that looks through them displaying their GetHashCode() and their BindingOperations.GetBindingExpression(editor, DocumentProperty) results. Make sure you take out the public static field when you’re done!

Take the “Path=” out of your XAML that constructs the Binding so it will better match the C# version. (I once had a problem where the XAML interpreted the path different than the Binding constructor because of the ITypeDescriptorContext passed to PropertyConverter.) The exact equivalent to the C# code you posted is Document="{Binding Document}".

Create a custom trace listener and set a breakpoint in it to get the call stack when the binding error is produced, search up the stack frames to find the objects involved and give them debugger object ids (right-click, Make Object ID), then investigate the actual values of properties to make sure they are as expected.

Enjoy!