Access parent DataContext from DataTemplate

Question

I have a ListBox which binds to a child collection on a ViewModel  The listbox items are styled in a datatemplate based on a property on the parent ViewModel    lt Style x Key  curveSpeedNonConstantParameterCell  gt      lt Style Triggers gt         lt DataTrigger Binding   Binding Path DataContext CurveSpeedMustBeSpecified             ElementName someParentElementWithReferenceToRootDataContext              Value  True  gt             lt Setter Property  Control Visibility  Value  Hidden  gt  lt  Setter gt         lt  DataTrigger gt      lt  Style Triggers gt   lt  Style gt    I get the following output error   System Windows Data Error  39   BindingExpression path error     CurveSpeedMustBeSpecified  property not found on      object    BindingListCollectionView   HashCode 20467555      BindingExpression Path DataContext CurveSpeedMustBeSpecified    DataItem  Grid   Name  nonConstantCurveParametersGrid     target element is  TextBox   Name        target property is  NoTarget   type  Object     So if I change the the binding expression to  Path DataContext CurrentItem CurveSpeedMustBeSpecified  it works  but only as long as the datacontext of the parent user control is a BindingListCollectionView  This is not acceptable because the rest of the user control binds to the properties of the CurrentItem on the BindingList automatically    How can I specify the binding expression inside the style so that it works regardless of the parent data context being a collection view or a single item

User · Answer

RelativeSource vs  ElementName These two approaches can achieve the same result  RelativeSource Binding  quot  Binding Path DataContext MyBindingProperty             RelativeSource  RelativeSource AncestorType  x Type Window    quot   This method looks for a control of a type Window  in this example  in the visual tree and when it finds it you basically can access it s DataContext using the Path DataContext      The Pros about this method is that you don t need to be tied to a name and it s kind of dynamic  however  changes made to your visual tree can affect this method and possibly break it  ElementName Binding  quot  Binding Path DataContext MyBindingProperty  ElementName MyMainWindow   This method referes to a solid static Name so as long as your scope can see it  you re fine You should be sticking to your naming convention not to break this method of course The approach is qute simple and all you need is to specify a Name  quot     quot  for your Window UserControl  Although all three types  RelativeSource  Source  ElementName  are capable of doing the same thing  but according to the following MSDN article  each one better be used in their own area of specialty  How to  Specify the Binding Source Find the brief description of each plus a link to a more details one in the table on the bottom of the page

User · Answer

I had problems with the relative source in Silverlight  After searching and reading I did not find a suitable solution without using some additional Binding library  But  here is another approach for gaining access to the parent DataContext by directly referencing an element of which you know the data context  It uses Binding ElementName and works quite well  as long as you respect your own naming and don t have heavy reuse of templates styles across components    lt ItemsControl x Name  level1Lister  ItemsSource  Binding MyLevel1List  gt     lt ItemsControl ItemTemplate gt       lt DataTemplate gt         lt Button Content  Binding MyLevel2Property                Command  Binding ElementName level1Lister                         Path DataContext MyLevel1Command                CommandParameter  Binding MyLevel2Property  gt         lt  Button gt       lt DataTemplate gt     lt ItemsControl ItemTemplate gt   lt  ItemsControl gt    This also works if you put the button into Style Template    lt Border Resources gt     lt Style x Key  buttonStyle  TargetType  Button  gt       lt Setter Property  Template  gt         lt Setter Value gt           lt ControlTemplate TargetType  Button  gt             lt Button Command  Binding ElementName level1Lister                                     Path DataContext MyLevel1Command                    CommandParameter  Binding MyLevel2Property  gt                  lt ContentPresenter  gt             lt  Button gt           lt  ControlTemplate gt         lt  Setter Value gt       lt  Setter gt     lt  Style gt   lt  Border Resources gt    lt ItemsControl x Name  level1Lister  ItemsSource  Binding MyLevel1List  gt     lt ItemsControl ItemTemplate gt       lt DataTemplate gt         lt Button Content   Binding MyLevel2Property                  Style   StaticResource buttonStyle    gt       lt DataTemplate gt     lt ItemsControl ItemTemplate gt   lt  ItemsControl gt    At first I thought that the x Names of parent elements are not accessible from within a templated item  but since I found no better solution  I just tried  and it works fine

User · Answer

the issue is that a DataTemplate isn t part of an element its applied to it   this means if you bind to the template you re binding to something that has no context   however if you put a element inside the template then when that element is applied to the parent it gains a context and the binding then works   so this will not work   lt DataTemplate  gt       lt DataTemplate Resources gt           lt CollectionViewSource x Key  projects  Source   Binding Projects    gt    but this works perfectly   lt DataTemplate  gt       lt GroupBox Header  Projects  gt           lt GroupBox Resources gt               lt CollectionViewSource x Key  projects  Source   Binding Projects    gt    because after the datatemplate is applied the groupbox is placed in the parent and will have access to its Context  so all you have to do is remove the style from the template and move it into an element in the template  note that the context for a itemscontrol is the item not the control ie ComboBoxItem for ComboBox not the ComboBox itself in which case you should use the controls ItemContainerStyle instead

User · Answer

Yes  you can solve it using the ElementName Something as suggested by Juve   BUT   If a child element  on which you use this kind of binding  is a user control which uses the same element name as you specify in the parent control  then the binding goes to the wrong object    I know this post is not a solution but I thought everyone who uses the ElementName in the binding should know this  since it s a possible runtime bug    lt UserControl x Class  MyNiceControl               x Name  TheSameName  gt     the content      lt  UserControl gt    lt UserControl x Class  AnotherUserControl  gt           lt ListView x Name  TheSameName  gt               lt ListView ItemTemplate gt                   lt DataTemplate gt                       lt MyNiceControl Width   Binding DataContext Width  ElementName TheSameName     gt                   lt  DataTemplate gt               lt  ListView ItemTemplate gt           lt  ListView gt   lt  UserControl gt

User · Answer

I was searching how to do something similar in WPF and I got this solution    lt ItemsControl ItemsSource   Binding MyItems Mode OneWay   gt   lt ItemsControl ItemsPanel gt       lt ItemsPanelTemplate gt           lt StackPanel Orientation  Vertical    gt       lt  ItemsPanelTemplate gt   lt  ItemsControl ItemsPanel gt   lt ItemsControl ItemTemplate gt       lt DataTemplate gt           lt RadioButton              Content   Binding                Command   Binding Path DataContext CustomCommand                           RelativeSource  RelativeSource Mode FindAncestor                                AncestorType  x Type ItemsControl                  CommandParameter   Binding     gt       lt  DataTemplate gt   lt  ItemsControl ItemTemplate gt      I hope this works for somebody else  I have a data context which is set automatically to the ItemsControls  and this data context has two properties  MyItems -which is a collection-  and one command  CustomCommand   Because of the ItemTemplate is using a DataTemplate  the DataContext of upper levels is not directly accessible  Then the workaround to get the DC of the parent is use a relative path and filter by ItemsControl type

User · Answer

You can use RelativeSource to find the parent element  like this -  Binding   Binding Path DataContext CurveSpeedMustBeSpecified   RelativeSource  RelativeSource AncestorType  x Type local YourParentElementType       See this SO question for more details about RelativeSource

[wpf] Access parent DataContext from DataTemplate

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