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49 lines
4.2 KiB
Markdown
49 lines
4.2 KiB
Markdown
---
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title: "Generics in XAML"
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ms.date: "03/30/2017"
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helpviewer_keywords:
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- "generics [XAML Services]"
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ms.assetid: 835bfed7-585c-4216-ae67-b674edab8b92
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---
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# Generics in XAML
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.NET XAML Services as implemented in <xref:System.Xaml?displayProperty=fullName> provides support for using generic CLR types. This support includes specifying the constraints of generics as a type argument and enforcing the constraint by calling the appropriate `Add` method for generic collection cases. This topic describes aspects of using and referencing generic types in XAML.
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## x:TypeArguments
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`x:TypeArguments` is a directive defined by the XAML language. When it is used as a member of a XAML type that is backed by a generic type, `x:TypeArguments` passes constraining type arguments of the generic to the backing constructor. For reference syntax that pertains to .NET XAML Services use of `x:TypeArguments`, which includes syntax examples, see [x:TypeArguments Directive](xtypearguments-directive.md).
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Because `x:TypeArguments` takes a string, and has type converter backing, it is typically declared in XAML usage as an attribute.
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In the XAML node stream, the information declared by `x:TypeArguments` can be obtained from <xref:System.Xaml.XamlType.TypeArguments%2A?displayProperty=nameWithType> at a `StartObject` position in the node stream. The return value of <xref:System.Xaml.XamlType.TypeArguments%2A?displayProperty=nameWithType> is a list of <xref:System.Xaml.XamlType> values. Determination of whether a XAML type represents a generic type can be made by calling <xref:System.Xaml.XamlType.IsGeneric%2A?displayProperty=nameWithType>.
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## Rules and Syntax Conventions for Generics in XAML
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In XAML, a generic type must always be represented as a constrained generic. An unconstrained generic is never present in the XAML type system or a XAML node stream and cannot be represented in XAML markup. A generic can be referenced within XAML attribute syntax for cases where it is a nested type constraint of a generic being referenced by `x:TypeArguments`, or for cases where `x:Type` supplies a CLR type reference for a generic type. Referencing generics is supported through the <xref:System.Xaml.Schema.XamlTypeTypeConverter> class defined by .NET XAML Services.
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The XAML attribute syntax form enabled by <xref:System.Xaml.Schema.XamlTypeTypeConverter> alters the typical MSIL / CLR syntax convention that uses angle brackets for types and constraints of generics, and instead substitutes parentheses for the constraint container. For an example, see [x:TypeArguments Directive](xtypearguments-directive.md).
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## Generics and XAML 2009 Features
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If you use XAML 2009 instead of mapping the CLR base types to obtain XAML types for common language primitives, you can use [XAML 2009 built-in types](types-for-primitives.md) as information items in `x:TypeArguments`. For example, you could declare the following (prefix mappings not shown, but `x` is the XAML language XAML namespace for XAML 2009):
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```xaml
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<my:BusinessObject x:TypeArguments="x:String,x:Int32"/>
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```
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## Generics Support in WPF
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For XAML 2006 usage when specifically targeting WPF, [x:Class](xclass-directive.md) must also be provided on the same element as `x:TypeArguments`, and that element must be the root element in a XAML document. The root element must map to a generic type with at least one type argument. An example is <xref:System.Windows.Navigation.PageFunction%601>.
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Possible workarounds to support generic usages include defining a custom markup extension that can return generic types, or providing a wrapping class definition that derives from a generic type but flattens the generic constraint in its own class definition.
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In WPF you can use XAML 2009 features together with `x:TypeArguments`, 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.
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Custom workflows in Windows Workflow Foundation for .NET Framework 3.5 do not support generic XAML usage.
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## See also
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- [x:TypeArguments Directive](xtypearguments-directive.md)
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- [x:Class Directive](xclass-directive.md)
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- [Built-in Types for Common XAML Language Primitives](types-for-primitives.md)
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