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x:Key Directive

Uniquely identifies elements that are created and referenced in a XAML-defined dictionary. Adding an x:Key value to a XAML object element is the most common way to identify a resource in a resource dictionary, for example in a WPF ResourceDictionary.

<object x:Key="stringKeyValue".../>
-or-
<object x:Key="{markupExtensionUsage}".../>

<object.Resources>
  <object x:Key="stringKeyValue".../>
</object.Resources>
-or-
<object.Resources>
  <object x:Key="{markupExtensionUsage}".../>
</object.Resources>

stringKeyValue

A text string to use as a key. The text string must conform to the XamlName Grammar.

markupExtensionUsage

Within the markup extension delimiters {}, a markup extension usage that provides an object to use as a key. See Remarks.

x:Key supports the XAML resource dictionary concept. XAML as a language doesn't define a resource dictionary implementation, that is left to specific UI frameworks. To learn more about how XAML resource dictionaries are implemented in WPF, see XAML Resources.

In XAML 2006 and WPF, x:Key must be provided as an attribute. You can still use nonstring keys, but this requires a markup extension usage in order to provide the nonstring value in attribute form. If you are using XAML 2009, x:Key can be specified as an element, to explicitly support dictionaries keyed by object types other than strings without requiring a markup extension intermediate. See the "XAML 2009" section in this topic. The remainder of the Remarks section applies specifically to the XAML 2006 implementation.

The attribute value of x:Key can be any string defined in the XamlName Grammar or can be an object evaluated through a markup extension. See "WPF Usage Notes" for an example from WPF.

Child elements of a parent element that is an IDictionary implementation must typically include an x:Key attribute that specifies a unique key value within that dictionary. Frameworks might implement aliased key properties to substitute for x:Key on particular types; types that define such properties should be attributed with DictionaryKeyPropertyAttribute.

The code equivalent of specifying x:Key is the key that is used for the underlying IDictionary. For example, an x:Key that is applied in markup for a resource in WPF is equivalent to the value of the key parameter of ResourceDictionary.Add when you add the resource to a WPF ResourceDictionary in code.

WPF Usage Notes

Child objects of a parent object that is an IDictionary implementation, such as the WPF ResourceDictionary, must typically include an x:Key attribute, and the key value must be unique within that dictionary. There are two notable exceptions:

In the overall WPF XAML implementation and application model, key uniqueness is not checked by the XAML markup compiler. Instead, missing or nonunique x:Key values cause load-time XAML parser errors. However, Visual Studio handling of dictionaries for WPF can often note such errors in the design phase.

Note that in the syntax shown, the ResourceDictionary object is implicit in how the WPF XAML processor produces a collection to populate a Resources collection. A ResourceDictionary is not typically provided explicitly as an element in markup, although it can be in some cases if wanted for clarity (it would be a collection object element between the Resources property element and the items within that populate the dictionary). For information about why a collection object is almost always an implicit element in markup, see XAML Syntax In Detail.

In the WPF XAML implementation, the handling for resource dictionary keys is defined by the ResourceKey abstract class. However the WPF XAML processor produces different underlying extension types for keys based on their usages. For example, the key for a DataTemplate or any derived class is handled separately, and produces a distinct DataTemplateKey object.

Keys and names use different directives and language elements (x:Key versus x:Name) in the basic XAML definition. Keys and names are also used in different situations by the WPF definition and application of these concepts. For details, see WPF XAML Namescopes.

As stated previously, the value of a key can be supplied through a markup extension and can be other than a string value. An example WPF scenario is that the value of x:Key may be a ComponentResourceKey. Certain controls expose a style key of that type for a custom style resource that influences part of the appearance and behavior of that control without totally replacing the style. An example of such a key is ButtonStyleKey.

The WPF merged dictionary feature introduces additional considerations for key uniqueness and key lookup behavior. For more information, see Merged Resource Dictionaries.

XAML 2009 relaxes the restriction that x:Key always be provided in attribute form.

In WPF, you can use XAML 2009 features, but only for XAML that is not markup-compiled. Markup-compiled XAML for WPF and the BAML form of XAML do not currently support the XAML 2009 keywords and features.

Under XAML 2009, you can specify x:Key elements through the following usage:

<object>
  <x:Key>
keyObject
  </x:Key>
...
</object>

keyObject

Object element for the object that is used as the key for a given object in a specialized dictionary.

  • The container/parent for this kind of use is not shown here. object is expected to be a child of an object element that represents a specialized dictionary implementation. keyObject is expected to be an object instance (or a value of a value type) that is appropriate as the key for that particular specialized dictionary implementation.

  • WPF does not implement dictionaries that require this usage. Object keys is more a general feature of the XAML language, possibly useful for certain custom dictionary scenarios where creating the dictionary in XAML is desirable. For WPF features such as implicit styles that use non-string keys for resources, other techniques for establishing or specifying the keys exist, so using an object key is not necessary.

  • keyObject could also be a markup extension usage in object element form, rather than a direct object instance.

x:Key for Silverlight is documented separately. For more information, see XAML Namespace (x:) Language Features (Silverlight).

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