ASP.NET Core Blazor static files

Article
11/12/2024

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.

This article describes Blazor app configuration for serving static files.

Static asset delivery in server-side Blazor apps

Serving static assets is managed by either routing endpoint conventions or a middleware described in the following table.

Feature	API	.NET Version	Description
Map Static Assets routing endpoint conventions	MapStaticAssets	.NET 9 or later	Optimizes the delivery of static assets to clients.
Static Files Middleware	UseStaticFiles	All .NET versions	Serves static assets to clients without the optimizations of Map Static Assets but useful for some tasks that Map Static Assets isn't capable of managing.

Configure Map Static Assets by calling MapStaticAssets in the app's request processing pipeline, which performs the following:

Sets the ETag and Last-Modified headers.
Sets caching headers.
Uses Caching Middleware.
When possible, serves compressed static assets.
Works with a Content Delivery Network (CDN) (for example, Azure CDN) to serve the app's static assets closer to the user.
Fingerprinting assets to prevent reusing old versions of files.

Map Static Assets operates by combining build and publish processes to collect information about the static assets in the app. This information is utilized by the runtime library to efficiently serve the static assets to browsers.

Map Static Assets can replace UseStaticFiles in most situations. However, Map Static Assets is optimized for serving the assets from known locations in the app at build and publish time. If the app serves assets from other locations, such as disk or embedded resources, UseStaticFiles should be used.

Map Static Assets (MapStaticAssets) replaces calling UseBlazorFrameworkFiles in apps that serve Blazor WebAssembly framework files, and explicitly calling UseBlazorFrameworkFiles in a Blazor Web App isn't necessary because the API is automatically called when invoking AddInteractiveWebAssemblyComponents.

Map Static Assets provides the following benefits that aren't available when calling UseStaticFiles:

Build-time compression for all the assets in the app, including JavaScript (JS) and stylesheets but excluding image and font assets that are already compressed. Gzip (Content-Encoding: gz) compression is used during development. Gzip with Brotli (Content-Encoding: br) compression is used during publish.
Fingerprinting for all assets at build time with a Base64-encoded string of the SHA-256 hash of each file's content. This prevents reusing an old version of a file, even if the old file is cached. Fingerprinted assets are cached using the immutable directive, which results in the browser never requesting the asset again until it changes. For browsers that don't support the immutable directive, a max-age directive is added.
- Even if an asset isn't fingerprinted, content based ETags are generated for each static asset using the fingerprint hash of the file as the ETag value. This ensures that the browser only downloads a file if its content changes (or the file is being downloaded for the first time).
- Internally, Blazor maps physical assets to their fingerprints, which allows the app to:
  - Find automatically-generated Blazor assets, such as Razor component scoped CSS for Blazor's CSS isolation feature, and JS assets described by JS import maps.
  - Generate link tags in the <head> content of the page to preload assets.
During Visual Studio Hot Reload development testing:
- Integrity information is removed from the assets to avoid issues when a file is changed while the app is running.
- Static assets aren't cached to ensure that the browser always retrieves current content.

When Interactive WebAssembly or Interactive Auto render modes are enabled:

Blazor creates an endpoint to expose the resource collection as a JS module.
The URL is emitted to the body of the request as persisted component state when a WebAssembly component is rendered into the page.
During WebAssembly boot, Blazor retrieves the URL, imports the module, and calls a function to retrieve the asset collection and reconstruct it in memory. The URL is specific to the content and cached forever, so this overhead cost is only paid once per user until the app is updated.
The resource collection is also exposed at a human-readable URL (_framework/resource-collection.js), so JS has access to the resource collection for enhanced navigation or to implement features of other frameworks and third-party components.

Map Static Assets doesn't provide features for minification or other file transformations. Minification is usually handled by custom code or third-party tooling.

Static File Middleware (UseStaticFiles) is useful in the following situations that Map Static Assets (MapStaticAssets) can't handle:

Applying a path prefix to Blazor WebAssembly static asset files, which is covered in the Prefix for Blazor WebAssembly assets section.
Configuring file mappings of extensions to specific content types and setting static file options, which is covered in the File mappings and static file options section.

For more information, see Static files in ASP.NET Core.

Deliver assets with Map Static Assets routing endpoint conventions

This section applies to server-side Blazor apps.

Assets are delivered via the ComponentBase.Assets property, which resolves the fingerprinted URL for a given asset. In the following example, Bootstrap, the Blazor project template app stylesheet (app.css), and the CSS isolation stylesheet (based on an app's namespace of BlazorSample) are linked in a root component, typically the App component (Components/App.razor):

<link rel="stylesheet" href="@Assets["bootstrap/bootstrap.min.css"]" />
<link rel="stylesheet" href="@Assets["app.css"]" />
<link rel="stylesheet" href="@Assets["BlazorSample.styles.css"]" />

Import maps

This section applies to server-side Blazor apps.

The Import Map component (ImportMap) represents an import map element (<script type="importmap"></script>) that defines the import map for module scripts. The Import Map component is placed in <head> content of the root component, typically the App component (Components/App.razor).

<ImportMap />

If a custom ImportMapDefinition isn't assigned to an Import Map component, the import map is generated based on the app's assets.

The following examples demonstrate custom import map definitions and the import maps that they create.

Basic import map:

new ImportMapDefinition(
    new Dictionary<string, string>
    {
        { "jquery", "https://cdn.example.com/jquery.js" },
    },
    null,
    null);

The preceding code results in the following import map:

{
  "imports": {
    "jquery": "https://cdn.example.com/jquery.js"
  }
}

Scoped import map:

new ImportMapDefinition(
    null,
    new Dictionary<string, IReadOnlyDictionary<string, string>>
    {
        ["/scoped/"] = new Dictionary<string, string>
        {
            { "jquery", "https://cdn.example.com/jquery.js" },
        }
    },
    null);

The preceding code results in the following import map:

{
  "scopes": {
    "/scoped/": {
      "jquery": "https://cdn.example.com/jquery.js"
    }
  }
}

Import map with integrity:

new ImportMapDefinition(
    new Dictionary<string, string>
    {
        { "jquery", "https://cdn.example.com/jquery.js" },
    },
    null,
    new Dictionary<string, string>
    {
        { "https://cdn.example.com/jquery.js", "sha384-abc123" },
    });

The preceding code results in the following import map:

{
  "imports": {
    "jquery": "https://cdn.example.com/jquery.js"
  },
  "integrity": {
    "https://cdn.example.com/jquery.js": "sha384-abc123"
  }
}

Combine import map definitions (ImportMapDefinition) with ImportMapDefinition.Combine.

Import map created from a ResourceAssetCollection that maps static assets to their corresponding unique URLs:

ImportMapDefinition.FromResourceCollection(
    new ResourceAssetCollection(
    [
        new ResourceAsset(
            "jquery.fingerprint.js",
            [
                new ResourceAssetProperty("integrity", "sha384-abc123"),
                new ResourceAssetProperty("label", "jquery.js"),
            ])
    ]));

The preceding code results in the following import map:

{
  "imports": {
    "./jquery.js": "./jquery.fingerprint.js"
  },
  "integrity": {
    "jquery.fingerprint.js": "sha384-abc123"
  }
}

Configure Static File Middleware to serve static assets to clients by calling UseStaticFiles in the app's request processing pipeline. For more information, see Static files in ASP.NET Core.

In releases prior to .NET 8, Blazor framework static files, such as the Blazor script, are served via Static File Middleware. In .NET 8 or later, Blazor framework static files are mapped using endpoint routing, and Static File Middleware is no longer used.

Summary of static file `<link>` `href` formats

This section applies to all .NET releases and Blazor apps.

The following tables summarize static file <link> href formats by .NET release.

For the location of <head> content where static file links are placed, see ASP.NET Core Blazor project structure. Static asset links can also be supplied using <HeadContent> components in individual Razor components.

For the location of <head> content where static file links are placed, see ASP.NET Core Blazor project structure.

.NET 9 or later

App type	`href` value	Examples
Blazor Web App	`@Assets["{PATH}"]`	`<link rel="stylesheet" href="@Assets["app.css"]" />` `<link href="@Assets["_content/ComponentLib/styles.css"]" rel="stylesheet" />`
Blazor Server†	`@Assets["{PATH}"]`	`<link href="@Assets["css/site.css"]" rel="stylesheet" />` `<link href="@Assets["_content/ComponentLib/styles.css"]" rel="stylesheet" />`
Standalone Blazor WebAssembly	`{PATH}`	`<link rel="stylesheet" href="css/app.css" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`

.NET 8.x

App type	`href` value	Examples
Blazor Web App	`{PATH}`	`<link rel="stylesheet" href="app.css" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`
Blazor Server†	`{PATH}`	`<link href="css/site.css" rel="stylesheet" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`
Standalone Blazor WebAssembly	`{PATH}`	`<link rel="stylesheet" href="css/app.css" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`

.NET 7.x or earlier

App type	`href` value	Examples
Blazor Server†	`{PATH}`	`<link href="css/site.css" rel="stylesheet" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`
Hosted Blazor WebAssembly‡	`{PATH}`	`<link href="css/app.css" rel="stylesheet" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`
Blazor WebAssembly	`{PATH}`	`<link href="css/app.css" rel="stylesheet" />` `<link href="_content/ComponentLib/styles.css" rel="stylesheet" />`

†Blazor Server is supported in .NET 8 or later but is no longer a project template after .NET 7.
‡We recommend updating Hosted Blazor WebAssembly apps to Blazor Web Apps when adopting .NET 8 or later.

Static Web Asset Project Mode

This section applies to the .Client project of a Blazor Web App.

The required <StaticWebAssetProjectMode>Default</StaticWebAssetProjectMode> setting in the .Client project of a Blazor Web App reverts Blazor WebAssembly static asset behaviors back to the defaults, so that the project behaves as part of the hosted project. The Blazor WebAssembly SDK (Microsoft.NET.Sdk.BlazorWebAssembly) configures static web assets in a specific way to work in "standalone" mode with a server simply consuming the outputs from the library. This isn't appropriate for a Blazor Web App, where the WebAssembly portion of the app is a logical part of the host and must behave more like a library. For example, the project doesn't expose the styles bundle (for example, BlazorSample.Client.styles.css) and instead only provides the host with the project bundle, so that the host can include it in its own styles bundle.

Changing the value (Default) of <StaticWebAssetProjectMode> or removing the property from the .Client project isn't supported.

Static files in non-`Development` environments

This section applies to server-side static files.

When running an app locally, static web assets are only enabled in the Development environment. To enable static files for environments other than Development during local development and testing (for example, Staging), call UseStaticWebAssets on the WebApplicationBuilder in the Program file.

Warning

Call UseStaticWebAssets for the exact environment to prevent activating the feature in production, as it serves files from separate locations on disk other than from the project if called in a production environment. The example in this section checks for the Staging environment by calling IsStaging.

if (builder.Environment.IsStaging())
{
    builder.WebHost.UseStaticWebAssets();
}

Prefix for Blazor WebAssembly assets

This section applies to Blazor Web Apps.

Use the WebAssemblyComponentsEndpointOptions.PathPrefix endpoint option to set the path string that indicates the prefix for Blazor WebAssembly assets. The path must correspond to a referenced Blazor WebAssembly application project.

endpoints.MapRazorComponents<App>()
    .AddInteractiveWebAssemblyRenderMode(options => 
        options.PathPrefix = "{PATH PREFIX}");

In the preceding example, the {PATH PREFIX} placeholder is the path prefix and must start with a forward slash (/).

In the following example, the path prefix is set to /path-prefix:

endpoints.MapRazorComponents<App>()
    .AddInteractiveWebAssemblyRenderMode(options => 
        options.PathPrefix = "/path-prefix");

Static web asset base path

This section applies to standalone Blazor WebAssembly apps.

Publishing the app places the app's static assets, including Blazor framework files (_framework folder assets), at the root path (/) in published output. The <StaticWebAssetBasePath> property specified in the project file (.csproj) sets the base path to a non-root path:

<PropertyGroup>
  <StaticWebAssetBasePath>{PATH}</StaticWebAssetBasePath>
</PropertyGroup>

In the preceding example, the {PATH} placeholder is the path.

Without setting the <StaticWebAssetBasePath> property, a standalone app is published at /BlazorStandaloneSample/bin/Release/{TFM}/publish/wwwroot/.

In the preceding example, the {TFM} placeholder is the Target Framework Moniker (TFM) (for example, net6.0).

If the <StaticWebAssetBasePath> property in a standalone Blazor WebAssembly app sets the published static asset path to app1, the root path to the app in published output is /app1.

In the standalone Blazor WebAssembly app's project file (.csproj):

<PropertyGroup>
  <StaticWebAssetBasePath>app1</StaticWebAssetBasePath>
</PropertyGroup>

In published output, the path to the standalone Blazor WebAssembly app is /BlazorStandaloneSample/bin/Release/{TFM}/publish/wwwroot/app1/.

In the preceding example, the {TFM} placeholder is the Target Framework Moniker (TFM) (for example, net6.0).

This section applies to standalone Blazor WebAssembly apps and hosted Blazor WebAssembly solutions.

<PropertyGroup>
  <StaticWebAssetBasePath>{PATH}</StaticWebAssetBasePath>
</PropertyGroup>

In the preceding example, the {PATH} placeholder is the path.

Without setting the <StaticWebAssetBasePath> property, the client app of a hosted solution or a standalone app is published at the following paths:

In the Server project of a hosted Blazor WebAssembly solution: /BlazorHostedSample/Server/bin/Release/{TFM}/publish/wwwroot/
In a standalone Blazor WebAssembly app: /BlazorStandaloneSample/bin/Release/{TFM}/publish/wwwroot/

If the <StaticWebAssetBasePath> property in the Client project of a hosted Blazor WebAssembly app or in a standalone Blazor WebAssembly app sets the published static asset path to app1, the root path to the app in published output is /app1.

In the Client app's project file (.csproj) or the standalone Blazor WebAssembly app's project file (.csproj):

<PropertyGroup>
  <StaticWebAssetBasePath>app1</StaticWebAssetBasePath>
</PropertyGroup>

In published output:

Path to the client app in the Server project of a hosted Blazor WebAssembly solution: /BlazorHostedSample/Server/bin/Release/{TFM}/publish/wwwroot/app1/
Path to a standalone Blazor WebAssembly app: /BlazorStandaloneSample/bin/Release/{TFM}/publish/wwwroot/app1/

The <StaticWebAssetBasePath> property is most commonly used to control the paths to published static assets of multiple Blazor WebAssembly apps in a single hosted deployment. For more information, see Multiple hosted ASP.NET Core Blazor WebAssembly apps. The property is also effective in standalone Blazor WebAssembly apps.

In the preceding examples, the {TFM} placeholder is the Target Framework Moniker (TFM) (for example, net6.0).

File mappings and static file options

This section applies to server-side static files.

To create additional file mappings with a FileExtensionContentTypeProvider or configure other StaticFileOptions, use one of the following approaches. In the following examples, the {EXTENSION} placeholder is the file extension, and the {CONTENT TYPE} placeholder is the content type. The namespace for the following API is Microsoft.AspNetCore.StaticFiles.

Configure options through dependency injection (DI) in the Program file using StaticFileOptions:

var provider = new FileExtensionContentTypeProvider();
provider.Mappings["{EXTENSION}"] = "{CONTENT TYPE}";

builder.Services.Configure<StaticFileOptions>(options =>
{
    options.ContentTypeProvider = provider;
});

app.UseStaticFiles();

Pass the StaticFileOptions directly to UseStaticFiles in the Program file:

var provider = new FileExtensionContentTypeProvider();
provider.Mappings["{EXTENSION}"] = "{CONTENT TYPE}";

app.UseStaticFiles(new StaticFileOptions { ContentTypeProvider = provider });

Configure options through dependency injection (DI) in the Program file using StaticFileOptions:
```
using Microsoft.AspNetCore.StaticFiles;

...

var provider = new FileExtensionContentTypeProvider();
provider.Mappings["{EXTENSION}"] = "{CONTENT TYPE}";

builder.Services.Configure<StaticFileOptions>(options =>
{
    options.ContentTypeProvider = provider;
});
```
This approach configures the same file provider used to serve the Blazor script. Make sure that your custom configuration doesn't interfere with serving the Blazor script. For example, don't remove the mapping for JavaScript files by configuring the provider with provider.Mappings.Remove(".js").

Use two calls to UseStaticFiles in the Program file:

Configure the custom file provider in the first call with StaticFileOptions.
The second middleware serves the Blazor script, which uses the default static files configuration provided by the Blazor framework.

using Microsoft.AspNetCore.StaticFiles;

...

var provider = new FileExtensionContentTypeProvider();
provider.Mappings["{EXTENSION}"] = "{CONTENT TYPE}";

app.UseStaticFiles(new StaticFileOptions { ContentTypeProvider = provider });
app.UseStaticFiles();

You can avoid interfering with serving _framework/blazor.server.js by using MapWhen to execute a custom Static File Middleware:

app.MapWhen(ctx => !ctx.Request.Path
    .StartsWithSegments("/_framework/blazor.server.js"),
        subApp => subApp.UseStaticFiles(new StaticFileOptions() { ... }));

Serve files from multiple locations

The guidance in this section only applies to Blazor Web Apps.

To serve files from multiple locations with a CompositeFileProvider:

Add the namespace for Microsoft.Extensions.FileProviders to the top of the Program file of the server project.
In the server project's Program file before the call to UseStaticFiles:
- Create a PhysicalFileProvider with the path to the static assets.
- Create a CompositeFileProvider from the WebRootFileProvider and the PhysicalFileProvider. Assign the composite file provider back to the app's WebRootFileProvider.

Example:

Create a new folder in the server project named AdditionalStaticAssets. Place an image into the folder.

Add the following using statement to the top of the server project's Program file:

using Microsoft.Extensions.FileProviders;

In the server project's Program file before the call to UseStaticFiles, add the following code:

var secondaryProvider = new PhysicalFileProvider(
    Path.Combine(builder.Environment.ContentRootPath, "AdditionalStaticAssets"));
app.Environment.WebRootFileProvider = new CompositeFileProvider(
    app.Environment.WebRootFileProvider, secondaryProvider);

In the app's Home component (Home.razor) markup, reference the image with an <img> tag:

<img src="{IMAGE FILE NAME}" alt="{ALT TEXT}" />

In the preceding example:

The {IMAGE FILE NAME} placeholder is the image file name. There's no need to provide a path segment if the image file is at the root of the AdditionalStaticAssets folder.
The {ALT TEXT} placeholder is the image alternate text.

Run the app.

Share via

ASP.NET Core Blazor static files

Static asset delivery in server-side Blazor apps

Deliver assets with Map Static Assets routing endpoint conventions

Import maps

Summary of static file `<link>` `href` formats

.NET 9 or later

.NET 8.x

.NET 7.x or earlier

Static Web Asset Project Mode

Static files in non-`Development` environments

Prefix for Blazor WebAssembly assets

Static web asset base path

File mappings and static file options

Serve files from multiple locations

Additional resources

Additional resources

Share via

ASP.NET Core Blazor static files

Static asset delivery in server-side Blazor apps

Deliver assets with Map Static Assets routing endpoint conventions

Import maps

Summary of static file <link> href formats

.NET 9 or later

.NET 8.x

.NET 7.x or earlier

Static Web Asset Project Mode

Static files in non-Development environments

Prefix for Blazor WebAssembly assets

Static web asset base path

File mappings and static file options

Serve files from multiple locations

Additional resources

Additional resources

Summary of static file `<link>` `href` formats

Static files in non-`Development` environments