ASP.NET Core Blazor app base path

This article explains the app base path in ASP.NET Core Blazor apps, including configuration guidance.

The app base path is the app's root URL path. Successful routing in Blazor apps requires framework configuration for any root URL path that isn't at the default app base path /.

Consider the following ASP.NET Core app and Blazor sub-app:

• The ASP.NET Core app is named MyApp:
  • The app physically resides at d:/MyApp.
  • Requests are received at https://www.contoso.com/{MYAPP RESOURCE}.
• A Blazor app named CoolApp is a sub-app of MyApp:
  • The sub-app physically resides at d:/MyApp/CoolApp.
  • Requests are received at https://www.contoso.com/CoolApp/{COOLAPP RESOURCE}.

Without specifying additional configuration for CoolApp, the sub-app in this scenario has no knowledge of where it resides on the server. For example, the app can't construct correct relative URLs to its resources without knowing that it resides at the relative URL path /CoolApp/. This scenario also applies in various hosting and reverse proxy scenarios when an app isn't hosted at a root URL path.

Background

An anchor tag's destination (href) can be composed with either of two endpoints:

• Absolute locations that include a scheme (defaults to the page's scheme if omitted), host, port, and path or just a forward slash (/) followed by the path.

  Examples: https://example.com/a/b/c or /a/b/c

• Relative locations that contain just a path and do not start with a forward slash (/). These are resolved relative to the current document URL or the <base> tag's value, if specified.

  Example: a/b/c

The presence of a trailing slash (/) in a configured app base path is significant to compute the base path for URLs of the app. For example, https://example.com/a has a base path of https://example.com/, while https://example.com/a/ with a trailing slash has a base path of https://example.com/a.

For the sources of links that pertain to Blazor in ASP.NET Core apps:

If you're rendering a Blazor app from different documents (for example, /Admin/B/C/ and /Admin/D/E/), you must take the app base path into account, or the base path is different when the app renders in each document and the resources are fetched from the wrong URLs.

There are two approaches to deal with the challenge of resolving relative links correctly:

• Map the resources dynamically using the document they were rendered on as the root.
• Set a consistent base path for the document and map the resources under that base path.

The first option is more complicated and isn't the most typical approach, as it makes navigation different for each document. Consider the following example for rendering a page /Something/Else:

• Rendered under /Admin/B/C/, the page is rendered with a path of /Admin/B/C/Something/Else.
• Rendered under /Admin/D/E/, the page is rendered at the same path of /Admin/B/C/Something/Else.

Under the first approach, routing offers IDynamicEndpointMetadata and MatcherPolicy, which in combination can be the basis for implementing a completely dynamic solution that determines at runtime about how requests are routed.

For the second option, which is the usual approach taken, the app sets the base path in the document and maps the server endpoints to paths under the base. The following guidance adopts this approach.

Server-side Blazor

Map the SignalR hub of a server-side Blazor app by passing the path to MapBlazorHub in the Program file:

app.MapBlazorHub("base/path");

The benefit of using MapBlazorHub is that you can map patterns, such as "{tenant}" and not just concrete paths.

You can also map the SignalR hub when the app is in a virtual folder with a branched middleware pipeline. In the following example, requests to /base/path/ are handled by Blazor's SignalR hub:

app.Map("/base/path/", subapp => {
    subapp.UsePathBase("/base/path/");
    subapp.UseRouting();
    subapp.UseEndpoints(endpoints => endpoints.MapBlazorHub());
});

Configure the <base> tag, per the guidance in the Configure the app base path section.

Standalone Blazor WebAssembly

In a standalone Blazor WebAssembly app, only the <base> tag is configured, per the guidance in the Configure the app base path section.

Configure the app base path

To provide configuration for the Blazor app's base path of https://www.contoso.com/CoolApp/, set the app base path (<base>), which is also called the relative root path.

By configuring the app base path, a component that isn't in the root directory can construct URLs relative to the app's root path. Components at different levels of the directory structure can build links to other resources at locations throughout the app. The app base path is also used to intercept selected hyperlinks where the href target of the link is within the app base path URI space. The Router component handles the internal navigation.

Place the <base> tag in <head> markup (location of <head> content) before any elements with attribute values that are URLs, such as the href attributes of <link> elements.

• In a server-side Blazor app, use either of the following approaches:

  • Option 1: Use the <base> tag to set the app's base path (location of <head> content):

    <base href="/CoolApp/">
    

    The trailing slash is required.

  • Option 2: Call UsePathBase first in the app's request processing pipeline (Program.cs) immediately after the WebApplicationBuilder is built (builder.Build()) to configure the base path for any following middleware that interacts with the request path:

    app.UsePathBase("/CoolApp");
    

    Calling UsePathBase is recommended when you also wish to run the Blazor Server app locally. For example, supply the launch URL in Properties/launchSettings.json:

    "launchUrl": "https://localhost:{PORT}/CoolApp",
    

    The {PORT} placeholder in the preceding example is the port that matches the secure port in the applicationUrl configuration path. The following example shows the full launch profile for an app at port 7279:

    "BlazorSample": {
      "commandName": "Project",
      "dotnetRunMessages": true,
      "launchBrowser": true,
      "applicationUrl": "https://localhost:7279;http://localhost:5279",
      "launchUrl": "https://localhost:7279/CoolApp",
      "environmentVariables": {
        "ASPNETCORE_ENVIRONMENT": "Development"
    }
    

    For more information on the launchSettings.json file, see Use multiple environments in ASP.NET Core. For additional information on Blazor app base paths and hosting, see <base href="/" /> or base-tag alternative for Blazor MVC integration (dotnet/aspnetcore #43191).

• Standalone Blazor WebAssembly (wwwroot/index.html):

  <base href="/CoolApp/">
  

  The trailing slash is required.

Don't prefix links throughout the app with a forward slash. Either avoid the use of a path segment separator or use dot-slash (./) relative path notation:

• ❌ Incorrect: <a href="/account">
• ✔️ Correct: <a href="account">
• ✔️ Correct: <a href="./account">

In Blazor WebAssembly web API requests with the HttpClient service, confirm that JSON helpers (HttpClientJsonExtensions) don't prefix URLs with a forward slash (/):

• ❌ Incorrect: var rsp = await client.GetFromJsonAsync("/api/Account");
• ✔️ Correct: var rsp = await client.GetFromJsonAsync("api/Account");

Don't prefix Navigation Manager relative links with a forward slash. Either avoid the use of a path segment separator or use dot-slash (./) relative path notation (Navigation is an injected NavigationManager):

• ❌ Incorrect: Navigation.NavigateTo("/other");
• ✔️ Correct: Navigation.NavigateTo("other");
• ✔️ Correct: Navigation.NavigateTo("./other");

In typical configurations for Azure/IIS hosting, additional configuration usually isn't required. In some non-IIS hosting and reverse proxy hosting scenarios, additional Static File Middleware configuration might be required:

• To serve static files correctly (for example, app.UseStaticFiles("/CoolApp");).
• To serve the Blazor script (_framework/blazor.*.js). For more information, see ASP.NET Core Blazor static files.

For a Blazor WebAssembly app with a non-root relative URL path (for example, <base href="/CoolApp/">), the app fails to find its resources when run locally. To overcome this problem during local development and testing, you can supply a path base argument that matches the href value of the <base> tag at runtime. Don't include a trailing slash. To pass the path base argument when running the app locally, execute the dotnet watch (or dotnet run) command from the app's directory with the --pathbase option:

dotnet watch --pathbase=/{RELATIVE URL PATH (no trailing slash)}

For a Blazor WebAssembly app with a relative URL path of /CoolApp/ (<base href="/CoolApp/">), the command is:

dotnet watch --pathbase=/CoolApp

If you prefer to configure the app's launch profile to specify the pathbase automatically instead of manually with dotnet watch (or dotnet run), set the commandLineArgs property in Properties/launchSettings.json. The following also configures the launch URL (launchUrl):

"commandLineArgs": "--pathbase=/{RELATIVE URL PATH (no trailing slash)}",
"launchUrl": "{RELATIVE URL PATH (no trailing slash)}",

Using CoolApp as the example:

"commandLineArgs": "--pathbase=/CoolApp",
"launchUrl": "CoolApp",

Using either dotnet watch (or dotnet run) with the --pathbase option or a launch profile configuration that sets the base path, the Blazor WebAssembly app responds locally at http://localhost:port/CoolApp.

For more information on the launchSettings.json file, see Use multiple environments in ASP.NET Core. For additional information on Blazor app base paths and hosting, see <base href="/" /> or base-tag alternative for Blazor MVC integration (dotnet/aspnetcore #43191).

Obtain the app base path from configuration

The following guidance explains how to obtain the path for the <base> tag from an app settings file for different environments.

Add the app settings file to the app. The following example is for the Staging environment (appsettings.Staging.json):

{
  "AppBasePath": "staging/"
}

In a server-side Blazor app, load the base path from configuration in <head> content:

@inject IConfiguration Config

...

<head>
    ...
    <base href="/@(Config.GetValue<string>("AppBasePath"))" />
    ...
</head>

app.UsePathBase($"/{app.Configuration.GetValue<string>("AppBasePath")}");

If there's no configuration value to load, for example in non-staging environments, the preceding href resolves to the root path /.

The examples in this section focus on supplying the app base path from app settings, but the approach of reading the path from IConfiguration is valid for any configuration provider. For more information, see the following resources:

• ASP.NET Core Blazor configuration
• Configuration in ASP.NET Core