[This article is for Windows 8.x and Windows Phone 8.x developers writing Windows Runtime apps. If you’re developing for Windows 10, see the latest documentation]
Provides a unique identifier for markup elements. For Windows Runtime XAML, this unique identifier is used by XAML localization processes and tools, such as using resources from a .resw resource file.
XAML attribute usage
A string that uniquely identifies a XAML element in an app, and becomes part of the resource path in a resource file. See Remarks.
Use x:Uid to identify an object element in your XAML. Typically this object element is an instance of a control class or other element that is displayed in a UI. The relationship between the string you use in x:Uid and the strings you use in a resources file is that the resource file strings are the x:Uid followed by a dot (.) and then by the name of a specific property of the element that's being localized. Consider this example:
<Button x:Uid="GoButton" Content="Go"/>
To specify content to replace the display text Go, you must specify a new resource that comes from a resource file. Your resource file should contain an entry for the resource named "GoButton.Content". Content in this case is a specific property that's inherited by the Button class. You might also provide localized values for other properties of this button, for example you could provide a resource-based value for "GoButton.FlowDirection". For more info on how to use x:Uid and resource files together, see Quickstart: Translating UI resources.
The validity of which strings can be used for an x:Uid value is controlled in a practical sense by which strings are legal as an identifier in a resource file and a resource path.
x:Uid is discrete from x:Name both because of the stated XAML localization scenario, and so that identifiers that are used for localization have no dependencies on the programming model implications of x:Name. Also, x:Name is governed by the XAML namescope concept, whereas uniqueness for x:Uid is controlled by the package resource index (PRI) system. For more info, see Resource Management System.
Windows Runtime XAML has somewhat different rules for x:Uid uniqueness than previous XAML-utilizing technologies used. For Windows Runtime XAML it is legal for the same x:Uid ID value to exist as a directive on multiple XAML elements. However, each such element must then share the same resolution logic when resolving the resources in a resource file. Also, all XAML files in a project share a single resource scope for purposes of x:Uid resolution, there is no concept of x:Uid scopes being aligned to individual XAML files.
In some cases you'll be using a resource path rather than built-in functionality of the package resource index (PRI) system. Any string used as an x:Uid value defines a resource path that begins with ms-resource:///Resources/ and includes the x:Uid string. The path is completed by the names of the properties you specify in a resources file or are otherwise targeting.
Don't put x:Uid on property elements, that isn't allowed in Windows Runtime XAML.