How do you conditionally bind data?

Question

How do I conditionally bind data to a combo box? I have a combo box that by default should display an ID. However, if the user checks a check box then the display should display both the ID and a NAME. For example, "OO1: Sam". I have the default ID displaying correctly. I'm just not sure how to get both the ID and the NAME to display based on the "IsChecked" state of the check box.

WPF and C# .Net 3.5

Answer 1

Here's one way, using a style on the ComboBox with triggers to set the ItemTemplate dynamically:

Edit: Changing the style into a resource. Note that this is still binding to the CheckBox directly using element binding - if you want it to be more flexible you could bind the CheckBox's IsChecked property to a property of your ViewModel and rely on that changing rather than IsChecked.

Let's move the style into the Resources section of our Window:

<Window.Resources>
    <Style x:Key="myStyle" TargetType="ComboBox">
        <Setter Property="ItemTemplate">
            <Setter.Value>
                <DataTemplate>
                    <StackPanel>
                        <TextBlock Text="{Binding Name}" />
                    </StackPanel>
                </DataTemplate>
            </Setter.Value>
        </Setter>

        <Style.Triggers>
            <DataTrigger Binding="{Binding IsChecked,ElementName=chk}" Value="True">
                <Setter Property="ItemTemplate">
                    <Setter.Value>
                        <DataTemplate>
                            <StackPanel Orientation="Horizontal">
                                <TextBlock Text="{Binding ID}" />
                                <TextBlock Text=": " />
                                <TextBlock Text="{Binding Name}" />
                            </StackPanel>
                        </DataTemplate>
                    </Setter.Value>
                </Setter>
            </DataTrigger>
        </Style.Triggers>
    </Style>
</Window.Resources>

Now we define the CheckBox and a couple of ComboBoxes that rely on it:

<CheckBox x:Name="chk" Content="Click Me" />

<ComboBox ItemsSource="{Binding}" Style="{StaticResource myStyle}" />

<ComboBox ItemsSource="{Binding}" Style="{StaticResource myStyle}" />