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ASP.NET Core Blazor CSS isolation

  • Artikkeli

Note

This isn't the latest version of this article. For the current release, see the .NET 9 version of this article.

Warning

This version of ASP.NET Core is no longer supported. For more information, see the .NET and .NET Core Support Policy. For the current release, see the .NET 9 version of this article.

Important

This information relates to a pre-release product that may be substantially modified before it's commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

For the current release, see the .NET 9 version of this article.

By Dave Brock

This article explains how CSS isolation scopes CSS to Razor components, which can simplify CSS and avoid collisions with other components or libraries.

Isolate CSS styles to individual pages, views, and components to reduce or avoid:

  • Dependencies on global styles that can be challenging to maintain.
  • Style conflicts in nested content.

Enable CSS isolation

To define component-specific styles, create a .razor.css file matching the name of the .razor file for the component in the same folder. The .razor.css file is a scoped CSS file.

For an Example component in an Example.razor file, create a file alongside the component named Example.razor.css. The Example.razor.css file must reside in the same folder as the Example component (Example.razor). The "Example" base name of the file is not case-sensitive.

Example.razor:

@page "/example"

<h1>Scoped CSS Example</h1>

Example.razor.css:

h1 { 
    color: brown;
    font-family: Tahoma, Geneva, Verdana, sans-serif;
}

The styles defined in Example.razor.css are only applied to the rendered output of the Example component. CSS isolation is applied to HTML elements in the matching Razor file. Any h1 CSS declarations defined elsewhere in the app don't conflict with the Example component's styles.

Note

In order to guarantee style isolation when bundling occurs, importing CSS in Razor code blocks isn't supported.

CSS isolation bundling

CSS isolation occurs at build time. Blazor rewrites CSS selectors to match markup rendered by the component. The rewritten CSS styles are bundled and produced as a static asset. The stylesheet is referenced inside the <head> tag (location of <head> content). The following <link> element is added to an app created from the Blazor project templates:

Blazor Web Apps:

<link href="@Assets["{ASSEMBLY NAME}.styles.css"]" rel="stylesheet">

Standalone Blazor WebAssembly apps:

<link href="{ASSEMBLY NAME}.styles.css" rel="stylesheet">

<link href="{ASSEMBLY NAME}.styles.css" rel="stylesheet">

The {ASSEMBLY NAME} placeholder is the project's assembly name.

The following example is from a hosted Blazor WebAssembly Client app. The app's assembly name is BlazorSample.Client, and the <link> is added by the Blazor WebAssembly project template when the project is created with the Hosted option (-ho|--hosted option using the .NET CLI or ASP.NET Core Hosted checkbox using Visual Studio):

<link href="BlazorSample.Client.styles.css" rel="stylesheet">

Within the bundled file, each component is associated with a scope identifier. For each styled component, an HTML attribute is appended with the format b-{STRING}, where the {STRING} placeholder is a ten-character string generated by the framework. The identifier is unique for each app. In the rendered Counter component, Blazor appends a scope identifier to the h1 element:

<h1 b-3xxtam6d07>

The {ASSEMBLY NAME}.styles.css file uses the scope identifier to group a style declaration with its component. The following example provides the style for the preceding <h1> element:

/* /Components/Pages/Counter.razor.rz.scp.css */
h1[b-3xxtam6d07] {
    color: brown;
}

At build time, a project bundle is created with the convention obj/{CONFIGURATION}/{TARGET FRAMEWORK}/scopedcss/projectbundle/{ASSEMBLY NAME}.bundle.scp.css, where the placeholders are:

  • {CONFIGURATION}: The app's build configuration (for example, Debug, Release).
  • {TARGET FRAMEWORK}: The target framework (for example, net6.0).
  • {ASSEMBLY NAME}: The app's assembly name (for example, BlazorSample).

Child component support

CSS isolation only applies to the component you associate with the format {COMPONENT NAME}.razor.css, where the {COMPONENT NAME} placeholder is usually the component name. To apply changes to a child component, use the ::deep pseudo-element to any descendant elements in the parent component's .razor.css file. The ::deep pseudo-element selects elements that are descendants of an element's generated scope identifier.

The following example shows a parent component called Parent with a child component called Child.

Parent.razor:

@page "/parent"

<div>
    <h1>Parent component</h1>

    <Child />
</div>

Child.razor:

<h1>Child Component</h1>

Update the h1 declaration in Parent.razor.css with the ::deep pseudo-element to signify the h1 style declaration must apply to the parent component and its children.

Parent.razor.css:

::deep h1 { 
    color: red;
}

The h1 style now applies to the Parent and Child components without the need to create a separate scoped CSS file for the child component.

The ::deep pseudo-element only works with descendant elements. The following markup applies the h1 styles to components as expected. The parent component's scope identifier is applied to the div element, so the browser knows to inherit styles from the parent component.

Parent.razor:

<div>
    <h1>Parent</h1>

    <Child />
</div>

However, excluding the div element removes the descendant relationship. In the following example, the style is not applied to the child component.

Parent.razor:

<h1>Parent</h1>

<Child />

The ::deep pseudo-element affects where the scope attribute is applied to the rule. When you define a CSS rule in a scoped CSS file, the scope is applied to the right most element. For example: div > a is transformed to div > a[b-{STRING}], where the {STRING} placeholder is a ten-character string generated by the framework (for example, b-3xxtam6d07). If you instead want the rule to apply to a different selector, the ::deep pseudo-element allows you do so. For example, div ::deep > a is transformed to div[b-{STRING}] > a (for example, div[b-3xxtam6d07] > a).

The ability to attach the ::deep pseudo-element to any HTML element allows you to create scoped CSS styles that affect elements rendered by other components when you can determine the structure of the rendered HTML tags. For a component that renders an hyperlink tag (<a>) inside another component, ensure the component is wrapped in a div (or any other element) and use the rule ::deep > a to create a style that's only applied to that component when the parent component renders.

Important

Scoped CSS only applies to HTML elements and not to Razor components or Tag Helpers, including elements with a Tag Helper applied, such as <input asp-for="..." />.

CSS preprocessor support

CSS preprocessors are useful for improving CSS development by utilizing features such as variables, nesting, modules, mixins, and inheritance. While CSS isolation doesn't natively support CSS preprocessors such as Sass or Less, integrating CSS preprocessors is seamless as long as preprocessor compilation occurs before Blazor rewrites the CSS selectors during the build process. Using Visual Studio for example, configure existing preprocessor compilation as a Before Build task in the Visual Studio Task Runner Explorer.

Many third-party NuGet packages, such as AspNetCore.SassCompiler, can compile SASS/SCSS files at the beginning of the build process before CSS isolation occurs.

CSS isolation configuration

CSS isolation is designed to work out-of-the-box but provides configuration for some advanced scenarios, such as when there are dependencies on existing tools or workflows.

Customize scope identifier format

Scope identifiers use the format b-{STRING}, where the {STRING} placeholder is a ten-character string generated by the framework. To customize the scope identifier format, update the project file to a desired pattern:

<ItemGroup>
  <None Update="Components/Pages/Example.razor.css" CssScope="custom-scope-identifier" />
</ItemGroup>

In the preceding example, the CSS generated for Example.razor.css changes its scope identifier from b-{STRING} to custom-scope-identifier.

Use scope identifiers to achieve inheritance with scoped CSS files. In the following project file example, a BaseComponent.razor.css file contains common styles across components. A DerivedComponent.razor.css file inherits these styles.

<ItemGroup>
  <None Update="Components/Pages/BaseComponent.razor.css" CssScope="custom-scope-identifier" />
  <None Update="Components/Pages/DerivedComponent.razor.css" CssScope="custom-scope-identifier" />
</ItemGroup>

Use the wildcard (*) operator to share scope identifiers across multiple files:

<ItemGroup>
  <None Update="Components/Pages/*.razor.css" CssScope="custom-scope-identifier" />
</ItemGroup>

Scope identifiers use the format b-{STRING}, where the {STRING} placeholder is a ten-character string generated by the framework. To customize the scope identifier format, update the project file to a desired pattern:

<ItemGroup>
  <None Update="Pages/Example.razor.css" CssScope="custom-scope-identifier" />
</ItemGroup>

In the preceding example, the CSS generated for Example.razor.css changes its scope identifier from b-{STRING} to custom-scope-identifier.

Use scope identifiers to achieve inheritance with scoped CSS files. In the following project file example, a BaseComponent.razor.css file contains common styles across components. A DerivedComponent.razor.css file inherits these styles.

<ItemGroup>
  <None Update="Pages/BaseComponent.razor.css" CssScope="custom-scope-identifier" />
  <None Update="Pages/DerivedComponent.razor.css" CssScope="custom-scope-identifier" />
</ItemGroup>

Use the wildcard (*) operator to share scope identifiers across multiple files:

<ItemGroup>
  <None Update="Pages/*.razor.css" CssScope="custom-scope-identifier" />
</ItemGroup>

Change base path for static web assets

The scoped.styles.css file is generated at the root of the app. In the project file, use the <StaticWebAssetBasePath> property to change the default path. The following example places the scoped.styles.css file, and the rest of the app's assets, at the _content path:

<PropertyGroup>
  <StaticWebAssetBasePath>_content/$(PackageId)</StaticWebAssetBasePath>
</PropertyGroup>

Disable automatic bundling

To opt out of how Blazor publishes and loads scoped files at runtime, use the DisableScopedCssBundling property. When using this property, it means other tools or processes are responsible for taking the isolated CSS files from the obj directory and publishing and loading them at runtime:

<PropertyGroup>
  <DisableScopedCssBundling>true</DisableScopedCssBundling>
</PropertyGroup>

Disable CSS isolation

Disable CSS isolation for a project by setting the <ScopedCssEnabled> property to false in the app's project file:

<ScopedCssEnabled>false</ScopedCssEnabled>

Razor class library (RCL) support

Isolated styles for components in a NuGet package or Razor class library (RCL) are automatically bundled:

  • The app uses CSS imports to reference the RCL's bundled styles. For a class library named ClassLib and a Blazor app with a BlazorSample.styles.css stylesheet, the RCL's stylesheet is imported at the top of the app's stylesheet:

    @import '_content/ClassLib/ClassLib.bundle.scp.css';

  • The RCL's bundled styles aren't published as a static web asset of the app that consumes the styles.

For more information on RCLs, see the following articles: