WPF Binding of a Control inside a UserControl - wpf

Question

I have a ComboBox inside a UserControl. I use the UserControl in a Window with a DataGrid: the UserControl DataContext is the DataGrid SelectedItem. The ComboBox inside the UserControl is bound to the "ID" field of the SelectedItem through the SelectedValue Property. To achieve that I programmed a DependencyProperty in the UserControl like this
Public Shared SelectedValueProperty As DependencyProperty = _
DependencyProperty.Register("SelectedValue", GetType(Object), GetType(ucEditCombo))
Public Property SelectedValue() As Object
Get
Return CType(GetValue(SelectedValueProperty), Object)
End Get
Set(ByVal value As Object)
SetCurrentValue(SelectedValueProperty, value)
End Set
End Property
and bound the ComboBox SelectedValue Property to the UserControl SelectedValue Property in XAML:
<ComboBox SelectedValuePath="{Binding ElementName=EditCombo,Path=SelectedValuePath}"
DisplayMemberPath="{Binding ElementName=EditCombo,Path=DisplayMemberPath}"
ItemsSource="{Binding ElementName=EditCombo,Path=ItemsSource}"
SelectedValue="{Binding ElementName=EditCombo,Path=SelectedValue,Mode=TwoWay}"
Visibility="{Binding ElementName=EditCombo,Path=ComboVisibility}"
Name="cmb"/>
(EditCombo is the x:Name I give to the UserControl in XAML).
Running the application I not the following:
If I change the DataGrid selection, then the ComboBox displayed value changes correctly.
As soon as I change the ComboBox selected value directly on the control, the value in the DataGrid doesn't get updated AND the ComboBox isn't bound to the DataGrid SelectedItem any longer.

Answer 1

The problem regarding the update of the original DataGrid SelectedItem is not apparent, although I believe it is to do with the bindings on the ComboBox. I would need to see the Window xaml and the UserControl's code behind to be sure.
I attempted to reproduce the problem with an example, but I was unable to. So to speed up your movement forward, I have posted the example below.
Assumption: The UserControl is attempting to modify properties of the object bound to the SelectedValue DependencyProperty, not attempting to replace the object. If the assumption is incorrect, update the question with the extra code that is relevant to the problem and I will attempt to update my answer.
Firstly, I have a model that I use when creating code for StackOverflow questions called ItemModel.
Public Class ItemModel
Public Property Id as Guid
Public Property Text as String
End Class
and on the Window, I have created an ObservableCollection(Of ItemModel) property called Items. It contains 3 items with each item having its Text property set to one of the values "One", "Two" or "Three". The Id of each item is set to a new guid.
So, my main window xaml looks like
<Window x:Class="MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:this="clr-namespace:StackOverflow._20798974"
DataContext="{Binding RelativeSource={RelativeSource Self}}"
Title="MainWindow" Height="350" Width="525">
<DockPanel>
<StackPanel Orientation="Horizontal" DockPanel.Dock="Bottom">
<this:ucEditCombo SelectedValue="{Binding ElementName=MyGrid, Path=SelectedItem}" />
</StackPanel>
<DataGrid x:Name="MyGrid" ItemsSource="{Binding Path=Items}" AutoGenerateColumns="False" >
<DataGrid.Columns>
<DataGridTextColumn Header="Id" Binding="{Binding Id}"/>
<DataGridTextColumn Header="Text" Binding="{Binding Text}" Width="*"/>
</DataGrid.Columns>
</DataGrid>
</DockPanel>
</Window>
The code behind of the ucEditCombo UserControl contains the DependencyProperty you have defined in the question above and an ObservableCollection(Of String) which is populated with the values "One", "Two", "Three" and "Four".
The xaml for the user control is
<UserControl x:Class="ucEditCombo" 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" xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:this="clr-namespace:StackOverflow._20798974"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type this:ucEditCombo}}}">
<ComboBox ItemsSource="{Binding Path=Items}" SelectedValue="{Binding Path=SelectedValue.Text}" />
</Grid>
</UserControl>
The above example allows me to selected a DataGrid item and have the Text property of that item immediately appear in the ComboBox. It also allows the selection of a different value in the ComboBox and have that value updated in the DataGrid.
Points to note.
Setting the DataContext of the Control in the Controls constructor will break the SelectedItem binding on the Window. To get around this, I have set the DataContext on the first content element of the UserControl (a Grid element).
The DataContext is set to the UserControl itself, simplifying some of the bindings on the Combobox.
As we are modifying a property of a DependencyProperty, a lot of the WPF binding subsystem notifications are taken care of for us.
The default binding mode for SelectedValue and SelectedItem is TwoWay.
I hope this helps.