DataTemplate vs ItemContainerTemplate

Question

What is ItemContainerTemplate used for? It is derived from DataTemplate, but I don't see any difference between them except the ItemContainerTemplateKey property. When should I use one and when the other?

Answer 1

The ItemContainerTemplate describes the world around your Item. For example in a ListBox the selection rectangle around your ListBoxItem. The DataTemplate describes how you ListBoxItem apears and of which elements it consists.

Dr. WPF did a good example: http://drwpf.com/blog/category/item-containers/

Answer 2

The only difference between DataTemplate and ItemContainerTemplate is the way the resource dictionary key is automatically provided (assuming it is not set explicitly). Namely, DataTemplate is decorated with [DictionaryKeyProperty("DataTemplateKey")] attribute, and the DataTemplateKey is basically defined as:

public object DataTemplateKey
{
    get { return (DataType != null) ? new DataTemplateKey(DataType) : null; 
}

See DataTemplate source for reference.

ItemContainerTemplate derives from DataTemplate, but is decorated with [DictionaryKeyProperty("ItemContainerTemplateKey")] attribute (which in practice replaces the inherited one), and ItemContainerTemplateKey property is defined as follows:

public object ItemContainerTemplateKey
{
    get { return (DataType != null) ? new ItemContainerTemplateKey(DataType) : null; }
}

See ItemContainerTemplate source for reference.

The difference seems small - DataTemplate returns an instance of DataTemplateKey and ItemContainerTemplate returns an instance of ItemContainerTemplateKey (both derive from TemplateKey). So basically these two are equivalent¹:

<ItemContainerTemplate DataType="{x:Type sys:String}" />
<DataTemplate x:Key="{ItemContainerTemplateKey {x:Type sys:String}}" />

and so are these:

<ItemContainerTemplate x:Key="{DataTemplateKey {x:Type sys:String}}" />
<DataTemplate DataType="{x:Type sys:String}" />

The main practical difference between these two is that DataTemplate with default key is treated as an implicit template², whereas ItemContainerTemplate is not. In fact, you need to manually reference it, e.g.:

<ListBox ItemTemplate="{StaticResource {ItemContainerTemplate {x:Type sys:String}}}" />

I'm not sure about the intentions behind creating ItemContainerTemplate class. I guess it gives you a clearer overview of the code, where you know that such a template is specifically intended to be used in an ItemsControl (or a derived control). Also, I guess it would prove to be pretty simple to write a strongly reusable DataTemplateSelector that would take advantage of this class.

---

¹They're not equivalent in the sense that created objects are of different types, but functionally they're equivalent.

²Implicit templates are applied to all objects of corresponding type within the scope, unless a template is set explicitly.

Answer 3

You can put an ItemContainerTemplate in a ResourceDictionary, and it will automatically use the DataType as its key.

That's the only difference.