Collections and collection types for XAML

This article describes how to define properties of types that are intended to support a collection, and to support the XAML syntax for instantiating collection items as element children of a parent object element or property element.

XAML Collection Concepts

Conceptually, any relationship in XAML where there are multiple child items within the scope of a XAML object element or XAML property element must be implemented as a collection. That collection must be associated with a particular XAML property of the XAML type that is the parent in that relationship. The property must be a collection because a XAML processor expects to assign each item in markup to be a newly added item of the backing collection property.

At the XAML language level, the exact requirements of collection support are not fully defined. The concept that a collection can be either a list or a dictionary(but not both) is defined at the XAML language level, but which backing types represent either lists or dictionaries is not defined by the XAML language.

In .NET XAML Services, the concept of XAML collection support is more clearly defined in terms of .NET backing types. Specifically, the XAML support for collections is based on several .NET concepts and APIs that are used for lists and dictionaries in general .NET programming.

1. The IList interface indicates a list collection.

2. The IDictionary interface indicates a dictionary collection.

3. Array represents an array, and an array supports IList methods.

In each of these collection concepts, a .NET XAML Services XAML processor expects to call the Add method on a specific instance of the collection property's type. Or, in a serialization scenario, a XAML processor produces discrete XAML-type instances for each item found in the list, dictionary, or array based on each collection's specific concept of "Items". These items are: IList.Item[]; IDictionary.Item[]; the explicit Array.IList.Item[] for Array.

Generic Collections

Generic collections can be useful for general .NET programming, and can also be used for XAML collection properties. However, the generic interfaces IList<T> and IDictionary<TKey,TValue> are not identified by .NET XAML Services XAML processors as being equivalent to the non-generic IList or IDictionary. Rather than implementing the interfaces, a recommended approach for generic collection property types is to derive from the classes List<T> or Dictionary<TKey,TValue>. These classes implement the non-generic interfaces and thus include the expected support for XAML collections in the base implementation.

Read-Only Collections and Initialization Logic

In .NET programming, it is a common design pattern to make any property that holds a value of a collection as a read-only collection. This pattern permits the instance that owns the collection property to better control what happens to the collection.. Specifically, the pattern prevents accidental replacement of the entire pre-existing collection by setting the property. In this pattern, any access to the collection by callers should instead be made by calling methods or properties as supported by the collection type and/or the relevant collection interfaces such as IList.

Using this pattern implies that any class that exposes a read-only collection property must first initialize that property to hold an empty collection. Typically the initialization is performed as part of the construction behavior for the class. To be useful for XAML, it is important that such logic is always referenced by the parameterless constructor, because XAML generally calls the parameterless constructor prior to processing the properties (collection properties or otherwise).

XAML Type System Support and Collections

Beyond the basic mechanics of parsing XAML and populating or serializing collection properties, the XAML type system as implemented in .NET XAML Services includes several design features that pertain to collections in XAML.

1. IsCollection returns true if the XAML type is backed by a type that provides XAML collection support.

2. IsDictionary and IsArray can further identify which collection mode the XAML type supports. For custom XAML processors that are based on .NET XAML Services and the XAML type system but not based on existing XamlWriter implementations, knowing which collection mode is used might be necessary in order to know which method to invoke for collection processing.

3. Each of the previous property values is potentially influenced by overrides of LookupCollectionKind on a XAML type.