silverlight - binding to two objects

Question

I want to create a user control, which can be bound to some data given from outside the control (requirement A), and some XAML properties must be bound to properties of the control itself (requirement B).

Let's say that I have a data class named StudentData and a control named StudentControl. I'm using this control inside a DataGrid. I put the StudentControl in the grid with a DataGridTemplateColumn. I somehow bind the StudentData in the current cell to the control. This is requirement A. This StudentControl wants to specify if some of the controls inside it are editable or not. The best way to do this is exposing a property, like StudentControl.AreSomeControlsEditable. Then I can bind the IsEnabled property of those controls to this property. This is requirement B.

Here's my first idea. I bind the current StudentData to a DP of StudentControl, then, inside the StudentControl, I change the data context to the control itself:

<UserControl DataContext="{Binding RelativeSource={RelativeSource Self}}">
  <TextBox
    Content="{Binding Path=ExposedStudentData.Field1}" *reqA*
    IsEnabled="{Binding Path=OtherProperty1}" /> *reqB*
</UserControl>

This way, I figured, the StudentControl gets the StudentData from outside, exposes this same StudentData and the other new properties (so the data and other properties are in one place now, not two), and then I can bind to these exposed properties in XAML (reqA+reqB are fulfilled at the same time). Except this doesn't work because of this bug. Basically, if I set the DataContext of the control to itself, then it gets set before the outer binding is applied. So if my binding path inside the cell is X, and then the data context changes, the binding engine will look for X in the new, self data context, and not the outer, grid data context.

Here's my second idea. I bind the current StudentData to the DataContext or a DP of the StudentControl. Then, to access other exposed properties of the StudentControl, I give a name to the UserControl and use ElementName binding:

<UserControl x:Name="self">
  <TextBox
    Content="{Binding Path=Field1}" *reqA*
    IsEnabled="{Binding ElementName=self, Path=OtherProperty1" /> *reqB*
</UserControl>

This way, I figured, the current data context is the StudentData, and it's not changed, and I can bind to that with a simple path (reqA), and I can bind to the other exposed properties with the ElementName stuff (reqB). It works in basic scenarios, but not in a DataGrid, because of this bug. I'm guessing the problem arises when there are multiple controls with the same name in the visual tree.

I'm really starting to hate Silverlight, I started using it a month ago, and I already reported 9 bugs. Whenever I try to achive something other than a simple hello world application or something that Microsoft and everyone else seems to be using Silverlight for, I encounter a new bug. So what now, how would you bind to a data class given from outside the control and some other properties exposed by the control at the same time? Without, of course, setting up the bindings from code (which I'm doing now, but it's a nightmare with ListBoxes and DataTemplates) or not using binding at all.

AbdouMoumen AbdouMoumen · Accepted Answer · 2011-05-03T10:57:55

I think your problem is with the DataContext and how it is inherited, as well as namescopes.

For the first, you should know that if a control doesn't specify its own DataContext, it will inherit that of its parent, and for ItemsControl derived controls, each Item will have its DataContext set to one of the data items in the ItemsSource collection.

For your second problem, if you're inside a DataTemplate, you're in a different namescope than outside of it. You can't access controls "by name" outside of a DataTemplate. What you can do here (for Silverlight 4 and below) is to traverse the Visual Tree yourself and look for the control you want. This, however, is going to become much easier and less painful with some new features in SL5, specifically the "FindAncestor" feature (which already exist in WPF).

Here's an article on MSDN about Namescopes in Silverlight. And here's another one about DataBinding which mentions how DataContext is inherited.

To achieve what you're looking for, I think this blog post should help you. It shows you how to implement "RelativeSource Binding with FindAncestor"-like behavior in Silverlight.

Hope this helps :)

silverlight - binding to two objects

2 Answers