How to get {x:DataType} for a DataTemplate in C# for custom DataTemplateSelector

Question

I'm writing a custom DataTemplateSelector for a ComboBox control and I'll need to use it to display different DateTemplates for different kind of objects, in both the closed and open modes for the ComboBox.

Here's the DataTemplateSelector I came up with:

public class ComboBoxTypedDataTemplateSelector : DataTemplateSelector
{
    public IEnumerable<DataTemplate> SelectedTemplates { get; set; }

    public IEnumerable<DataTemplate> DropDownTemplates { get; set; }

    protected override DataTemplate SelectTemplateCore(object item, DependencyObject container)
    {
        IEnumerable<DataTemplate> source = container.FindParent<ComboBoxItem>() == null
            ? SelectedTemplates // Get the template for the closed mode
            : DropDownTemplates; // Get the template for the open UI mode
        Type type = item.GetType();
        return null; // Some LINQ to get the first DataTemplate in source with the {x:DataType} that equals type
    }
}

public sealed class DataTemplatesCollection : List<DataTemplate> { }

And here's how I'd use it in XAML:

<ComboBox>
    <mvvm:ComboBoxTypedDataTemplateSelector>
        <mvvm:ComboBoxTypedDataTemplateSelector.SelectedTemplates>
            <mvvm:DataTemplatesCollection>
                <DataTemplate x:DataType="models:SomeType">
                    <TextBlock Text="{x:Bind ...}"/>
                </DataTemplate>
                <DataTemplate x:DataType="models:SomeOtherType">
                    <TextBlock Text="{x:Bind ...}"/>
                </DataTemplate>
            </mvvm:DataTemplatesCollection>
        </mvvm:ComboBoxTypedDataTemplateSelector.SelectedTemplates>
        <mvvm:ComboBoxTypedDataTemplateSelector.DropDownTemplates>
            <mvvm:DataTemplatesCollection>
                <DataTemplate x:DataType="models:SomeType">
                    <TextBlock Text="{x:Bind ...}"/>
                </DataTemplate>
                <DataTemplate x:DataType="models:SomeOtherType">
                    <TextBlock Text="{x:Bind ...}"/>
                </DataTemplate>
            </mvvm:DataTemplatesCollection>
        </mvvm:ComboBoxTypedDataTemplateSelector.DropDownTemplates>
    </mvvm:ComboBoxTypedDataTemplateSelector>
</ComboBox>

Now, the only piece of the puzzle I'm missing, I can't figure out how to get that {x:DataType} property in C# (I know it's not actually a real property, but I hope there's a way to retrieve it via code). I need something like that to be able to get the right DataTemplate for each object, from the right templates group. Is there a way I can achieve that?

NOTE: I know I could just write a specific DataTemplateSelector that has the hardcoded names of the templates to return for each item type, and I can use that method as a fallback option. But, I was wondering if it was possible to write a more generic selector with this approach in order to make it more modular and be able to reuse it in the future.

Thanks for your help!

EDIT: following the suggestion by Vincent, I wrote an attached property to store a given Type in a DataTemplate:

public class DataTypeHelper
{
    public static Type GetAttachedDataType(DataTemplate element)
    {
        return (Type)element.GetValue(AttachedDataTypeProperty);
    }

    public static void SetAttachedDataType(DataTemplate element, Type value)
    {
        element.SetValue(AttachedDataTypeProperty, value);
    }

    public static readonly DependencyProperty AttachedDataTypeProperty =
        DependencyProperty.RegisterAttached("AttachedDataType", typeof(Type), typeof(DataTypeHelper), new PropertyMetadata(default(Type)));
}

And I've tried to use it like this:

...
 <DataTemplate x:DataType="someXlmns:SomeClass"
               mvvm:DataTypeHelper.AttachedDataType="someXlmns:SomeClass">
     ...
 </DataTemplate>

But I'm getting a XamlParseException at the line where I set the attached property to my type. I've tried to set that property to "Grid" (just as a test) and it doesn't crash, I don't understand why isn't it working with my custom type.

EDIT #2: looks like the x:Type markup extension is not available in UWP and I couldn't find another way (I don't think it's possible at all) to get a Type instance directly from XAML, so I had to just use the type name in XAML and then compare it to item.GetType().Name in the template selector.

The ability to assign a Type property directly from XAML would have been better as it'd also would have had syntax/spell-check in the XAML designer, but at least this approach works fine.

Answer 1

You cannot retrieve this value. This is just a hint for the compiler to allow it to generate the appropriate code for the binding.

You can either create a custom attached property to store what you need or use the Name property.

<local:Selector x:Key="selector" >
    <local:Selector.Template1>
        <DataTemplate x:DataType="local:Item" x:Name="template1" >
            <.../>
        </DataTemplate>
    </local:Selector.Template1>
</local:Selector>

Then in the selector implementation

Template1.GetValue(FrameworkElement.NameProperty);

Answer 2

Here's my 2 cents:

[ContentProperty(Name = nameof(Templates))]
public class TypedDataTemplateSelector : DataTemplateSelector
{
  public IList<TypedDataTemplate> Templates { get; } 
    = new ObservableCollection<TypedDataTemplate>();

  public TypedDataTemplateSelector()
  {
    var incc = (INotifyCollectionChanged)Templates;
    incc.CollectionChanged += (sender, e) =>
    {
      if (e?.NewItems.Cast<TypedDataTemplate>()
          .Any(tdt => tdt?.DataType == null || tdt?.Template == null) == true)
        throw new InvalidOperationException("All items must have all properties set.");
    };
  }

  protected override DataTemplate SelectTemplateCore(object item, 
      DependencyObject container)
  {
    if (item == null) return null;
    if (!Templates.Any()) throw new InvalidOperationException("No DataTemplates found.");

    var result =
      Templates.FirstOrDefault(t => t.DataType.IsAssignableFrom(item.GetType()));
    if (result == null)
      throw new ArgumentOutOfRangeException(
        $"Could not find a matching template for type '{item.GetType()}'.");

    return result.Template;
  }
}

[ContentProperty(Name = nameof(Template))]
public class TypedDataTemplate
{
  public Type DataType { get; set; }
  public DataTemplate Template { get; set; }
}

Usage:

<ContentControl Content="{Binding}">
  <ContentControl.ContentTemplateSelector>
    <v:TypedDataTemplateSelector>
      <v:TypedDataTemplate DataType="data:Person">
        <DataTemplate>
          <StackPanel>
            <TextBox Header="First name" Text="{Binding FirstName}" />
            <TextBox Header="Last name" Text="{Binding LastName}" />
          </StackPanel>
        </DataTemplate>
      </v:TypedDataTemplate>
      <v:TypedDataTemplate DataType="data:Company">
        <DataTemplate>
          <StackPanel>
            <TextBox Header="Company name" Text="{Binding CompanyName}" />
          </StackPanel>
        </DataTemplate>
      </v:TypedDataTemplate>
    </v:TypedDataTemplateSelector>
  </ContentControl.ContentTemplateSelector>
</ContentControl>