Exposing inner Control properties for binding in WPF

Question

[Edit]: I figured out how to do this on my own. I posted my solution in the hope that it will save someone else a few days of Googling. If you are a WPF guru, please look at my solution and let me know if there is a better / more elegant / more efficient way to do this. In particular, I am interested in knowing what I don't know... how is this solution going to screw me down the road? The problem really boils down to exposing inner control properties.

Problem: I am creating some code to auto-generate a data-bound GUI in WPF for an XML file. I have an xsd file that can help me determine the node types, etc. Simple Key/Value elements are easy.

When I parse this element:

<Key>value</Key> 

I can create a new 'KeyValueControl' and set the DataContext to this element. The KeyValueControl is defined as a UserControl and just has some simple bindings on it. It works great for any simple XElement.

The XAML inside this control looks like this:

<Label Content={Binding Path=Name} />  <TextBox Text={Binding Path=Value} /> 

The result is a line that has a label with the element name and a text box with the value that I can edit.

Now, there are times where I need to display lookup values instead of the actual value. I would like to create a 'KeyValueComboBox' similar to the above KeyValueControl but be able to specify (based on information in the file) the ItemsSource, DisplayMemberPath, and ValueMemberPath. The 'DisplayMemberPath' and 'ValueMemberPath' bindings would be the same as the KeyValueControl.

I don't know if a standard user control can handle this, or if I need to inherit from Selector.

The XAML in the control would look something like this:

<Label Content={Binding Path=Name} />  <ComboBox SelectedValue={Binding Path=Value}           ItemsSource={Binding [BOUND TO THE ItemsSource PROPERTY OF THIS CUSTOM CONTROL]           DisplayMemberPath={Binding [BOUND TO THE DisplayMemberPath OF THIS CUSTOM CONTROL]           SelectedValuePath={Binding [BOUND TO THE SelectedValuePath OF THIS CUSTOM CONTROL]/> 

In my code, I would then do something like this (assuming that this node is a 'Thing' and needs to display a list of Things so the user can select the ID:

var myBoundComboBox = new KeyValueComboBox(); myBoundComboBox.ItemsSource = getThingsList(); myBoundComboBox.DisplayMemberPath = "ThingName"; myBoundComboBox.ValueMemberPath = "ThingID" myBoundComboBox.DataContext = thisXElement; ... myStackPanel.Children.Add(myBoundComboBox) 

So my questions are:

1) Should I inherit my KeyValueComboBox from Control or Selector?

2) If I should inherit from Control, how do I expose the inner Combo Box's ItemsSource, DisplayMemberPath, and ValueMemberPath for binding?

3) If I need to inherit from Selector, can someone provide a small example of how I might get started with that? Again, I'm new to WPF so a nice, simple example would really help if that's the road I need to take.

Answer 1

I ended up figuring how how to do this on my own. I'm posting the answer here so that others can see a solution that works, and maybe a WPF guru will come by and show me a better/more elegant way to do this.

So, the answer ended up being #2. Exposing the inner properties turns out to be the right answer. Setting it up is actually pretty easy.. once you know how to do it. There aren't many complete examples of this (that I could find), so hopefully this one will help someone else that runs into this problem.

ComboBoxWithLabel.xaml.cs

The important thing in this file is the use of DependencyProperties. Note that all we're doing right now is just exposing the properties (LabelContent and ItemsSource). The XAML will take care of wiring the internal control's properties to these external properties.

namespace BoundComboBoxExample {     /// <summary>     /// Interaction logic for ComboBoxWithLabel.xaml     /// </summary>     public partial class ComboBoxWithLabel : UserControl     {         // Declare ItemsSource and Register as an Owner of ComboBox.ItemsSource         // the ComboBoxWithLabel.xaml will bind the ComboBox.ItemsSource to this         // property         public IEnumerable ItemsSource         {             get { return (IEnumerable)GetValue(ItemsSourceProperty); }             set { SetValue(ItemsSourceProperty, value); }         }          public static readonly DependencyProperty ItemsSourceProperty =           ComboBox.ItemsSourceProperty.AddOwner(typeof(ComboBoxWithLabel));          // Declare a new LabelContent property that can be bound as well         // The ComboBoxWithLable.xaml will bind the Label's content to this         public string LabelContent         {             get { return (string)GetValue(LabelContentProperty); }             set { SetValue(LabelContentProperty, value); }         }          public static readonly DependencyProperty LabelContentProperty =           DependencyProperty.Register("LabelContent", typeof(string), typeof(ComboBoxWithLabel));                public ComboBoxWithLabel()         {             InitializeComponent();         }     } } 

ComboBoxWithLabel.xaml

The XAML is pretty straightforward, with the exception of the bindings on the Label and the ComboBox ItemsSource. I found that the easiest way to get these bindings right is to declare the properties in the .cs file (as above) and then use the VS2010 designer to setup the binding source from the properties pane. Essentially, this is the only way I know of to bind an inner control's properties to the base control. If there's a better way to do it, please let me know.

<UserControl x:Class="BoundComboBoxExample.ComboBoxWithLabel"              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"               mc:Ignorable="d"               d:DesignHeight="28" d:DesignWidth="453" xmlns:my="clr-namespace:BoundComboBoxExample">     <Grid>         <DockPanel LastChildFill="True">             <!-- This will bind the Content property on the label to the 'LabelContent'                   property on this control-->             <Label Content="{Binding Path=LabelContent,                               RelativeSource={RelativeSource FindAncestor,                                               AncestorType=my:ComboBoxWithLabel,                                               AncestorLevel=1}}"                     Width="100"                     HorizontalAlignment="Left"/>             <!-- This will bind the ItemsSource of the ComboBox to this                   control's ItemsSource property -->             <ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor,                                      AncestorType=my:ComboBoxWithLabel,                                      AncestorLevel=1},                                      Path=ItemsSource}"></ComboBox>             <!-- you can do the same thing with SelectedValuePath,                   DisplayMemberPath, etc, but this illustrates the technique -->         </DockPanel>                  </Grid> </UserControl> 

MainWindow.xaml

The XAML to use this is not interesting at all.. which is exactly what I wanted. You can set the ItemsSource and the LabelContent via all the standard WPF techniques.

<Window x:Class="BoundComboBoxExample.MainWindow"         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"         Title="MainWindow" Height="86" Width="464" xmlns:my="clr-namespace:BoundComboBoxExample"         Loaded="Window_Loaded">     <Window.Resources>         <ObjectDataProvider x:Key="LookupValues" />     </Window.Resources>     <Grid>         <my:ComboBoxWithLabel LabelContent="Foo"                               ItemsSource="{Binding Source={StaticResource LookupValues}}"                               HorizontalAlignment="Left"                                Margin="12,12,0,0"                                x:Name="comboBoxWithLabel1"                                VerticalAlignment="Top"                                Height="23"                                Width="418" />     </Grid> </Window> 

For Completeness Sake, here is the MainWindow.xaml.cs

/// <summary> /// Interaction logic for MainWindow.xaml /// </summary> public partial class MainWindow : Window {     public MainWindow()     {         InitializeComponent();     }      private void Window_Loaded(object sender, RoutedEventArgs e)     {         ((ObjectDataProvider)FindResource("LookupValues")).ObjectInstance =             (from i in Enumerable.Range(0, 5)              select string.Format("Bar {0}", i)).ToArray();      } }