DataTemplates in WPF

Question

I have a general question about data templates in WPF. Let's say I have an abstract class called "Question," and various subclasses like "MathQuestion," "GeographyQuestion," etc. In some contexts, rendering the questions as a "Question" using the "Question" data template is good enough, but let's say that I have a list of random Question objects of varying subclasses that I want to display in-turn. I want to display them to the user using their specific data templates rather than their generic Question data template, but since I don't know that at design time, is there anyway to tell WPF, "hey, here's a list of Quesitons, but use reflection to figure out their specific types and use THAT data template?"

What I've thought of so far: I thought that in addition to having my question collection, I could create another collection of the specific types using reflection and somehow bind that to "blah," then I'd get the desired affect, but you can only bind to DependencyProperties in WPF, so I'm not sure what I'd bind to. I really don't like this idea, and my gut tells me there's a more elegant way to approach this problem.

I'm not looking for specific code here, just a general strategy to accomplish what I'm trying to do. Also, I'm using MVVM for the most part if that helps.

Thanks

Answer 1

I'm thinking something like this should work right out of the box:

<UserControl.Resources>
    <DataTemplate DataType="{x:Type vm:GenericQuestionViewModel}">
        <v:GenericQuestion/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type tvm:GeographyQuestionViewModel}">
        <tv:GeographyQuestion/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type tvm:BiologyQuestionViewModel}">
        <tv:BiologyQuestion/>
    </DataTemplate>
</UserControl.Resources>

<ContentControl Content="{Binding QuestionViewModel}">

---

Edit:

Yes, this definitely should work. Here's a more complete example:

Main View Model

public class MainWindowViewModel : ViewModelBase
{
    public ObservableCollection<QuestionViewModel> QuestionViewModels { get; set; }

    public MainWindowViewModel()
    {
        QuestionViewModels = new ObservableCollection<QuestionViewModel>
        {
            new GenericQuestionViewModel(),
            new GeographyQuestionViewModel(),
            new BiologyQuestionViewModel()
        };
    }
}

Question View Models

public abstract class QuestionViewModel : ViewModelBase
{
}

public class GenericQuestionViewModel : QuestionViewModel
{
}

public class GeographyQuestionViewModel : QuestionViewModel
{
}

public class BiologyQuestionViewModel : QuestionViewModel
{
}

Question User Controls

<UserControl x:Class="WpfApplication1.GenericQuestion" ...>
    <Grid>
        <TextBlock Text="Generic Question" />
    </Grid>
</UserControl>

<UserControl x:Class="WpfApplication1.GeographyQuestion" ...>
    <Grid>
        <TextBlock Text="Geography Question" />
    </Grid>
</UserControl>

<UserControl x:Class="WpfApplication1.BiologyQuestion" ...>
    <Grid>
        <TextBlock Text="Biology Question" />
    </Grid>
</UserControl>

Main Window

<Window x:Class="WpfApplication1.MainWindow" ...
        Title="MainWindow"
        Height="900"
        Width="525">
    <Window.DataContext>
        <local:MainWindowViewModel />
    </Window.DataContext>
    <Window.Resources>
        <DataTemplate DataType="{x:Type local:GenericQuestionViewModel}">
            <local:GenericQuestion />
        </DataTemplate>
        <DataTemplate DataType="{x:Type local:GeographyQuestionViewModel}">
            <local:GeographyQuestion />
        </DataTemplate>
        <DataTemplate DataType="{x:Type local:BiologyQuestionViewModel}">
            <local:BiologyQuestion />
        </DataTemplate>
    </Window.Resources>
    <ItemsControl ItemsSource="{Binding QuestionViewModels}">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <ContentControl Content="{Binding}" />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</Window>

Update

Kyle Tolle pointed out a nice simplification for setting ItemsControl.ItemTemplate. Here is the resulting code:

<ItemsControl ItemsSource="{Binding QuestionViewModels}"
              ItemTemplate="{Binding}" /> 

Answer 2

Generally, if you need to dynamically change the DataTemplate based on some non-static logic, you'd use a DataTemplateSelector. Another option it to use DataTriggers in your DataTemplate to modify the look appropriately.