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x:Type Markup Extension

  • Стаття

Supplies the CLR Type object that is the underlying type for a specified XAML type.

XAML Attribute Usage

<object property="{x:Type prefix:typeNameValue}" .../>

XAML Object Element Usage

<x:Type TypeName="prefix:typeNameValue"/>

XAML Values

Value Description
prefix Optional. A prefix that maps a non-default XAML namespace. Specifying a prefix is frequently not necessary. See Remarks.
typeNameValue Required. A type name resolvable to the current default XAML namespace; or the specified mapped prefix if prefix is supplied.

Remarks

The x:Type markup extension has a similar function to the typeof() operator in C# or the GetType operator in Microsoft Visual Basic.

The x:Type markup extension supplies a from-string conversion behavior for properties that take the type Type. The input is a XAML type. The relationship between the input XAML type and the output CLR Type is that the output Type is the UnderlyingType of the input XamlType, after looking up the necessary XamlType based on XAML schema context and the IXamlTypeResolver service the context provides.

In .NET XAML Services, the handling for this markup extension is defined by the TypeExtension class.

In specific framework implementations, some properties that take Type as a value can accept the name of the type directly (the string value of the type Name). However, implementing this behavior is a complex scenario. For examples, see the "WPF Usage Notes" section that follows.

Attribute syntax is the most common syntax used with this markup extension. The string token provided after the x:Type identifier string is assigned as the TypeName value of the underlying TypeExtension extension class. Under the default XAML schema context for .NET XAML Services, which is based on CLR types, the value of this attribute is either the Name of the desired type, or contains that Name preceded by a prefix for a non-default XAML namespace mapping.

The x:Type markup extension can be used in object element syntax. In this case, specifying the value of the TypeName property is required to properly initialize the extension.

The x:Type markup extension can also be used as a verbose attribute; however this use is not typical: <object property="{x:Type TypeName=typeNameValue}" .../>

WPF Usage Notes

Default XAML Namespace and Type Mapping

The default XAML namespace for WPF programming contains most of the XAML types you need for typical XAML scenarios; therefore, you can often avoid prefixes when referencing XAML type values. You might need to map a prefix if you are referencing a type from a custom assembly or for types that exist in a WPF assembly but are from a CLR namespace that was not mapped to the default XAML namespace. For more information about prefixes, XAML namespaces, and mapping CLR namespaces, see XAML Namespaces and Namespace Mapping for WPF XAML.

Type Properties That Support Typename-as-String

WPF supports techniques that enable specifying the value of some properties of type Type without requiring an x:Type markup extension usage. Instead, you can specify the value as a string that names the type. Examples of this are ControlTemplate.TargetType and Style.TargetType. Support for this behavior is not provided through either type converters or markup extensions. Instead, this is a deferral behavior implemented through FrameworkElementFactory.

Silverlight supports a similar convention. In fact, Silverlight does not currently support {x:Type} in its XAML language support, and does not accept {x:Type} usages outside of a few circumstances that are intended to support WPF-Silverlight XAML migration. Therefore, the typename-as-string behavior is built-in to all Silverlight native property evaluation where a Type is the value.

XAML 2009

XAML 2009 provides additional support for generic types and modifies the feature behavior of x:TypeArguments and x:Type to provide this support.

  • x:TypeArguments and the associated object element for a generic object instantiation can be on elements other than the root. For more information, see the "XAML 2009" section of x:TypeArguments Directive.

  • XAML 2009 supports a syntax for specifying a generic type's constraint in markup. This can be used by x:TypeArguments, by x:Type, or by the two features in combination.

  • WPF XAML implementation when processing XAML 2009 for load also adds this capability to the implicit type conversion behavior for certain framework properties that use type Type.

In WPF, you can use XAML 2009 features but only for loose XAML (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.

See also