ItemsControl with only custom subelements - wpf

Question

I would like to build a custom component that layouts its childs in either a StackPanel or a Grid (with variable row count, which makes me consider the StackPanel instead). The items are custom elements/objects that just hold some configuration, based on which a few controls are created to display them (some labels and text boxes).
Ideally, the component should be used somehow like this (where SpecializedCustomPanelItem is a subtype of CustomPanelItem):
<CustomPanel>
<CustomPanelItem Param1="value A" Param2="value B">Text</CustomPanelItem>
<CustomPanelItem Param1="value C">Other text</CustomPanelItem>
<SpecializedCustomPanelItem>More text</SpecializedCustomPanelItem>
<!-- The number of items is variable -->
</CustomPanel>
I’ve read on the ItemsControl for a while now, and it fits my needs rather well. I would create simply types for the items, and make data templates for them available from inside the ItemsControl. Then they should already render fine.
However I would like to require the items inside that ItemsControl to be of a specific type (i.e. CustomPanelItem or a subtype). I actually thought that the ItemsControl would allow this, just like you within a ComboBox or a MenuItem, but it turns out that it actually allows any subtype, and if necessary wraps them in a item container.
So I have been thinking if an ItemsControl is actually what I am looking for, as I do not want any “fancy” things like selection or scrolling which most of those controls implement. I actually only want to build a simple interface to a common pattern in the application that auto generates those components and layouts them in a Grid/StackPanel.
Should I still be using the ItemsControl or rather build some more custom component?

Answer 1

In this case you don't really need a custom component. Changing the ItemsPanel type to whatever type you need + multiple templates for the Items should do the trick.
However to answer the question in the heading: If you want to force an items control to only accept a certain type of items, you will have to create
a. A CustomItemsControl
b. A CustomItemsControlItem
Then for the CustomItemsControl you should declare the attribute
[StyleTypedProperty(Property = "ItemContainerStyle", StyleTargetType = typeof(CustomItemsControlItem))]
Then you also will need to
protected override DependencyObject GetContainerForItemOverride()
{
return new CustomItemsControlItem();
// You can throw an exception here
}
protected override bool IsItemItsOwnContainerOverride(object item)
{
return item is CustomItemsControlItem;
}
If memory serves this should force the ItemsControl to not allow other types to be added as children and should throw exceptions. You could then do some magic inside CustomItemsControlItem by defining some DependencyProperties which you can then set when adding the items in XAML.
But yet if you have multiple types in your ViewModel that you want to display correctly, the correct way is still to provide multiple templates for the CustomItemsControlItem targetting your ViewModel types.
Hope this helps.

Answer 2

This sounds perfect for an ItemsControl
You can set it's ItemsPanelTemplate to define the kind of panel which will hold your items, and set the ItemContainerTemplate to define how to draw each item.
If items should be drawn differently based on what type they are, I'd suggest using implicit DataTemplates instead of setting the ItemContainerTemplate
<Window.Resources>
<DataTemplate DataType="{x:Type my:BasePanelItem}">
<my:CustomPanelItem Param1="{Binding Param1}" Param2="{Binding Param2}" Content="{Binding SomeValue}" />
</DataTemplate>
<DataTemplate DataType="{x:Type my:SpecializedPanelItem}">
<my:SpecializedCustomPanelItem Content="{Binding SomeValue}" />
</DataTemplate>
</Window.Resources>
<ItemsControl ItemsSource="{Binding MyItems}">
<!-- ItemsPanelTemplate -->
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<my:CustomPanel />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
</ItemsControl>
You mentioned that you wanted to perhaps use a dynamically created Grid instead of a StackPanel as well. If you do, you might be interested in some GridHelpers I have posted on my blog. This would allow you to bind the number of Columns/Rows on the Grid in the ItemsPanelTemplate
<ItemsControl ItemsSource="{Binding MyCollection}">
<!-- ItemsPanelTemplate -->
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Grid local:GridHelpers.RowCount="{Binding RowCount}"
local:GridHelpers.ColumnCount="{Binding ColumnCount}" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<!-- ItemContainerStyle -->
<ItemsControl.ItemContainerStyle>
<Style>
<Setter Property="Grid.Column" Value="{Binding ColumnIndex}" />
<Setter Property="Grid.Row" Value="{Binding RowIndex}" />
</Style>
</ItemsControl.ItemContainerStyle>
</ItemsControl>