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Andy De George
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* add toc/bread changes * add xaml-services * add ported wpf content * add xaml-services info to docfx * Fix links 1 * Fix links 2 * Fix links 3 * Fix links 4 * Fix links 5 * Fix links 6 * Fix links 7 * Fix xaml-services link * Fix snippets 1 * Fix snippets 2 * adjust breadcrumb and toc * remove preserve-view from breadcrumbs * add empty view to toc item xaml * update include * remove contextual toc for vs article :(
49 lines
2.4 KiB
Markdown
49 lines
2.4 KiB
Markdown
---
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title: "x:Null Markup Extension"
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ms.date: "03/30/2017"
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f1_keywords:
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- "NullExtension"
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- "x:NullExtension"
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- "x:Null"
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- "Null"
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- "xNull"
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helpviewer_keywords:
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- "Null markup extension in XAML [XAML Services]"
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- "x:Null markup extension [XAML Services]"
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- "XAML [XAML Services], x:Null markup extension"
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ms.assetid: 2e3ccc21-4996-481d-91b5-3910d8b3bfa3
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---
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# x:Null Markup Extension
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Specifies `null` as a value for a XAML member.
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## XAML Attribute Usage
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```xaml
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<object property="{x:Null}" .../>
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```
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## Remarks
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The keyword for a null reference in C# and C++ is null. The Microsoft Visual Basic keyword for a null reference is `Nothing`, but you always use `{x:Null}` as the XAML usage regardless which code-behind language you associate with the XAML.
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The `x:Null` markup extension has no settable properties.
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A null usage is often associated with the XAML member exposure of a CLR <xref:System.Nullable%601> value.
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The `x:Null` markup extension, like all XAML markup extensions, uses the braces (`{,}`) for escaping the handling of attribute values to be other than literals or event-handler references. Attribute syntax is the syntax most frequently used with this markup extension. An object element syntax `<x:Null />` is technically possible, but is rarely used because the `x:Null` markup extension has no positional parameters or construction arguments.
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For information about markup extensions, see [Markup Extensions and WPF XAML](../framework/wpf/advanced/markup-extensions-and-wpf-xaml.md).
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In .NET XAML Services, the handling for this markup extension is defined by the <xref:System.Windows.Markup.NullExtension> class.
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## WPF Usage Notes
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Note that `null` is not necessarily the initial unset value for a reference-type dependency property. The initial default value can vary for each dependency property and can be based on property-specific metadata. Many dependency properties do not accept `null` as a value, either through markup or code because of their validation callback implementations. For more information about dependency properties, see [Dependency Properties Overview](../framework/wpf/advanced/dependency-properties-overview.md).
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## See also
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- <xref:System.Windows.DependencyProperty.UnsetValue>
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- [XAML Overview (WPF)](../net/wpf/fundamentals/xaml.md)
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- [Markup Extensions and WPF XAML](../framework/wpf/advanced/markup-extensions-and-wpf-xaml.md)
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