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What do OnPlatform and OnIdiom do? What is x:Static and when do you use it? How do you create a custom markup extension? What is the difference between StaticResource and DynamicResource? How do compiled XAML (XamlC) and x:DataType improve XAML?
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.NET MAUI › XAML & Markup Extensions

What are markup extensions in XAML?

Beginner

Answer

Markup extensions provide values to XAML attributes at parse time using {} syntax, e.g., {Binding}, {StaticResource}, {DynamicResource}, {x:Static}, {OnPlatform}, {OnIdiom}. They let you reference resources, bind data, or compute values declaratively instead of hardcoding.
<Label Text="{Binding Title}"
       TextColor="{StaticResource Primary}"
       FontSize="{OnPlatform iOS=17, Android=16}" />
Real-world example One label pulls its text from a binding, its color from a resource, and a platform-specific font size—all via markup extensions.

Common follow-ups: Name a few built-in extensions. | What syntax do they use?

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All 6 XAML & Markup Extensions questions