Prerender and integrate ASP.NET Core Razor components

This article explains Razor component integration scenarios for Blazor apps, including prerendering of Razor components on the server.

Important

Framework changes across ASP.NET Core releases led to different sets of instructions in this article. Before using this article's guidance, confirm that the document version selector at the top of this article matches the version of ASP.NET Core that you intend to use for your app.

Razor components can be integrated into Razor Pages and MVC apps in a hosted Blazor WebAssembly solution. When the page or view is rendered, components can be prerendered at the same time.

Prerendering can improve Search Engine Optimization (SEO) by rendering content for the initial HTTP response that search engines can use to calculate page rank.

Solution configuration

Prerendering configuration

To set up prerendering for a hosted Blazor WebAssembly app:

1. Host the Blazor WebAssembly app in an ASP.NET Core app. A standalone Blazor WebAssembly app can be added to an ASP.NET Core solution, or you can use a hosted Blazor WebAssembly app created from the Blazor WebAssembly project template with the hosted option:

   • Visual Studio: In the Additional information dialog, select the ASP.NET Core Hosted checkbox when creating the Blazor WebAssembly app. In this article's examples, the solution is named BlazorHosted.
   • Visual Studio Code/.NET CLI command shell: dotnet new blazorwasm -ho (use the -ho|--hosted option). Use the -o|--output {LOCATION} option to create a folder for the solution and set the solution's project namespaces. In this article's examples, the solution is named BlazorHosted (dotnet new blazorwasm -ho -o BlazorHosted).

   For the examples in this article, the hosted solution's name (assembly name) is BlazorHosted. The client project's namespace is BlazorHosted.Client, and the server project's namespace is BlazorHosted.Server.

2. Delete the wwwroot/index.html file from the Blazor WebAssembly Client project.

3. In the Client project, delete the following lines in Program.cs:

   - builder.RootComponents.Add<App>("#app");
   - builder.RootComponents.Add<HeadOutlet>("head::after");
   

4. Add _Host.cshtml file to the Server project's Pages folder. You can obtain the files from a project created from the Blazor Server template using Visual Studio or using the .NET CLI with the dotnet new blazorserver -o BlazorServer command in a command shell (the -o BlazorServer option creates a folder for the project). After placing the files into the Server project's Pages folder, make the following changes to the files.

   Make the following changes to the _Host.cshtml file:

   • Update the Pages namespace at the top of the file to match the namespace of the Server app's pages. The {APP NAMESPACE} placeholder in the following example represents the namespace of the donor app's pages that provided the _Host.cshtml file:

     Delete:

     - @namespace {APP NAMESPACE}.Pages
     

     Add:

     @namespace BlazorHosted.Server.Pages
     

   • Add an @using directive for the Client project at the top of the file:

     @using BlazorHosted.Client
     

   • Update the stylesheet links to point to the WebAssembly project's stylesheets. In the following example, the client project's namespace is BlazorHosted.Client. The {APP NAMESPACE} placeholder represents the namespace of the donor app that provided the _Host.cshtml file. Update the Component Tag Helper (<component> tag) for the HeadOutlet component to prerender the component.

     Delete:

     - <link href="css/site.css" rel="stylesheet" />
     - <link href="{APP NAMESPACE}.styles.css" rel="stylesheet" />
     - <component type="typeof(HeadOutlet)" render-mode="ServerPrerendered" />
     

     Add:

     <link href="css/app.css" rel="stylesheet" />
     <link href="BlazorHosted.Client.styles.css" rel="stylesheet" />
     <component type="typeof(HeadOutlet)" render-mode="WebAssemblyPrerendered" />
     

     Note

     Leave the <link> element that requests the Bootstrap stylesheet (css/bootstrap/bootstrap.min.css) in place.

   • Update the Blazor script source to use the client-side Blazor WebAssembly script:

     Delete:

     - <script src="_framework/blazor.server.js"></script>
     

     Add:

     <script src="_framework/blazor.webassembly.js"></script>
     

   • Update the render-mode of the Component Tag Helper to prerender the root App component with WebAssemblyPrerendered:

     Delete:

     - <component type="typeof(App)" render-mode="ServerPrerendered" />
     

     Add:

     <component type="typeof(App)" render-mode="WebAssemblyPrerendered" />
     

     Important

     Prerendering isn't supported for authentication endpoints (/authentication/ path segment). For more information, see ASP.NET Core Blazor WebAssembly additional security scenarios.

5. In the Program.cs file of the Server project, change the fallback endpoint from the index.html file to the _Host.cshtml page:

   Delete:

   - app.MapFallbackToFile("index.html");
   

   Add:

   app.MapFallbackToPage("/_Host");
   

6. If the Client and Server projects use one or more common services during prerendering, factor the service registrations into a method that can be called from both projects. For more information, see ASP.NET Core Blazor dependency injection.

7. Run the Server project. The hosted Blazor WebAssembly app is prerendered by the Server project for clients.

Configuration for embedding Razor components into pages and views

The following sections and examples for embedding Razor components from the Client Blazor WebAssembly app into pages and views of the server app require additional configuration.

The Server project must have the following files and folders.

Razor Pages:

• Pages/Shared/_Layout.cshtml
• Pages/Shared/_Layout.cshtml.css
• Pages/_ViewImports.cshtml
• Pages/_ViewStart.cshtml

MVC:

• Views/Shared/_Layout.cshtml
• Views/Shared/_Layout.cshtml.css
• Views/_ViewImports.cshtml
• Views/_ViewStart.cshtml

The preceding files can be obtained by generating an app from the ASP.NET Core project templates using:

• Visual Studio's new project creation tools.
• Opening a command shell and executing dotnet new webapp -o {PROJECT NAME} (Razor Pages) or dotnet new mvc -o {PROJECT NAME} (MVC). The option -o|--output with a value for the {PROJECT NAME} placeholder provides a name for the app and creates a folder for the app.

Update the namespaces in the imported _ViewImports.cshtml file to match those in use by the Server project receiving the files.

Pages/_ViewImports.cshtml (Razor Pages):

@using BlazorHosted.Server
@namespace BlazorHosted.Server.Pages
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

Views/_ViewImports.cshtml (MVC):

@using BlazorHosted.Server
@using BlazorHosted.Server.Models
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

Update the imported layout file, which is Pages/Shared/_Layout.cshtml for Razor Pages or Views/Shared/_Layout.cshtml for MVC.

First, delete the title and the stylesheet from the donor project, which is RPDonor.styles.css in the following example. The {PROJECT NAME} placeholder represents the donor project's app name.

- <title>@ViewData["Title"] - {PROJECT NAME}</title>
- <link rel="stylesheet" href="~/RPDonor.styles.css" asp-append-version="true" />

Include the Client project's styles in the layout file. In the following example, the Client project's namespace is BlazorHosted.Client. The <title> element can be updated at the same time.

Place the following lines in the <head> content of the layout file:

<title>@ViewData["Title"] - BlazorHosted</title>
<link href="css/app.css" rel="stylesheet" />
<link rel="stylesheet" href="BlazorHosted.Client.styles.css" asp-append-version="true" />
<component type="typeof(HeadOutlet)" render-mode="WebAssemblyPrerendered" />

The imported layout contains two Home (Index page) and Privacy navigation links. To make the Home links point to the hosted Blazor WebAssembly app, change the hyperlinks:

- <a class="navbar-brand" asp-area="" asp-page="/Index">{PROJECT NAME}</a>
+ <a class="navbar-brand" href="/">BlazorHosted</a>

- <a class="nav-link text-dark" asp-area="" asp-page="/Index">Home</a>
+ <a class="nav-link text-dark" href="/">Home</a>

In an MVC layout file:

- <a class="navbar-brand" asp-area="" asp-controller="Home" 
-     asp-action="Index">{PROJECT NAME}</a>
+ <a class="navbar-brand" href="/">BlazorHosted</a>

- <a class="nav-link text-dark" asp-area="" asp-controller="Home" 
-     asp-action="Index">Home</a>
+ <a class="nav-link text-dark" href="/">Home</a>

Update the <footer> element's app name. The following example uses the app name BlazorHosted:

- &copy; {DATE} - {DONOR NAME} - <a asp-area="" asp-page="/Privacy">Privacy</a>
+ &copy; {DATE} - BlazorHosted - <a asp-area="" asp-page="/Privacy">Privacy</a>

In the preceding example, the {DATE} placeholder represents the copyright date in an app generated from the Razor Pages or MVC project template.

To make the Privacy link lead to a privacy page (Razor Pages), add a privacy page to the Server project.

Pages/Privacy.cshtml in the Server project:

@page
@model PrivacyModel
@{
    ViewData["Title"] = "Privacy Policy";
}
<h1>@ViewData["Title"]</h1>

<p>Use this page to detail your site's privacy policy.</p>

For an MVC-based privacy view, create a privacy view in the Server project.

View/Home/Privacy.cshtml in the Server project:

@{
    ViewData["Title"] = "Privacy Policy";
}
<h1>@ViewData["Title"]</h1>

<p>Use this page to detail your site's privacy policy.</p>

In the Home controller of the MVC app, return the view.

Add the following code to Controllers/HomeController.cs:

public IActionResult Privacy()
{
    return View();
}

If you import files from a donor app, be sure to update any namespaces in the files to match that of the Server project (for example, BlazorHosted.Server).

Import static assets to the Server project from the donor project's wwwroot folder:

• wwwroot/css folder and contents
• wwwroot/js folder and contents
• wwwroot/lib folder and contents

If the donor project is created from an ASP.NET Core project template and the files aren't modified, you can copy the entire wwwroot folder from the donor project into the Server project and remove the favicon icon file.

Warning

Avoid placing the static asset into both the Client and Server wwwroot folders. If the same file is present in both folders, an exception is thrown because the static assets share the same web root path. Therefore, host a static asset in either of the wwwroot folders, not both.

After adopting the preceding configuration, embed Razor components into pages or views of the Server project. Use the guidance in the following sections of this article:

• Render components in a page or view with the Component Tag Helper
• Render components in a page or view with a CSS selector

Render components in a page or view with the Component Tag Helper

After configuring the solution, including the additional configuration, the Component Tag Helper supports two render modes for rendering a component from a Blazor WebAssembly app in a page or view:

• WebAssembly
• WebAssemblyPrerendered

In the following Razor Pages example, the Counter component is rendered in a page. To make the component interactive, the Blazor WebAssembly script is included in the page's render section. To avoid using the full namespace for the Counter component with the Component Tag Helper ({ASSEMBLY NAME}.Pages.Counter), add an @using directive for the client project's Pages namespace. In the following example, the Client project's namespace is BlazorHosted.Client.

In the Server project, Pages/RazorPagesCounter1.cshtml:

@page
@using BlazorHosted.Client.Pages

<component type="typeof(Counter)" render-mode="WebAssemblyPrerendered" />

@section Scripts {
    <script src="_framework/blazor.webassembly.js"></script>
}

Run the Server project. Navigate to the Razor page at /razorpagescounter1. The prerendered Counter component is embedded in the page.

RenderMode configures whether the component:

• Is prerendered into the page.
• Is rendered as static HTML on the page or if it includes the necessary information to bootstrap a Blazor app from the user agent.

For more information on the Component Tag Helper, including passing parameters and RenderMode configuration, see Component Tag Helper in ASP.NET Core.

Additional work might be required depending on the static resources that components use and how layout pages are organized in an app. Typically, scripts are added to a page or view's Scripts render section and stylesheets are added to the layout's <head> element content.

Set child content through a render fragment

The Component Tag Helper doesn't support receiving a RenderFragment delegate for child content (for example, param-ChildContent="..."). We recommend creating a Razor component (.razor) that references the component you want to render with the child content you want to pass and then invoke the Razor component from the page or view.

Ensure that top-level prerendered components aren't trimmed out on publish

If a Component Tag Helper directly references a component from a library that's subject to trimming on publish, the component might be trimmed out during publish because there are no references to it from client-side app code. As a result, the component isn't prerendered, leaving a blank spot in the output. If this occurs, instruct the trimmer to preserve the library component by adding a DynamicDependency attribute to any class in the client-side app. To preserve a component called SomeLibraryComponentToBePreserved, add the following to any component:

@using System.Diagnostics.CodeAnalysis
@attribute [DynamicDependency(DynamicallyAccessedMemberTypes.All, 
    typeof(SomeLibraryComponentToBePreserved))]

The preceding approach usually isn't required because the app usually prerenders its components (which are not trimmed), which in turn references components from libraries (causing them also not to be trimmed). Only use DynamicDependency explicitly for prerendering a library component directly when the library is subject to trimming.

Render components in a page or view with a CSS selector

After configuring the solution, including the additional configuration, add root components to the Client project of a hosted Blazor WebAssembly solution in the Program.cs file. In the following example, the Counter component is declared as a root component with a CSS selector that selects the element with the id that matches counter-component. In the following example, the Client project's namespace is BlazorHosted.Client.

In Program.cs file of the Client project, add the namespace for the project's Razor components to the top of the file:

using BlazorHosted.Client.Pages;

After the builder is established in Program.cs, add the Counter component as a root component:

builder.RootComponents.Add<Counter>("#counter-component");

In the following Razor Pages example, the Counter component is rendered in a page. To make the component interactive, the Blazor WebAssembly script is included in the page's render section.

In the Server project, Pages/RazorPagesCounter2.cshtml:

@page

<div id="counter-component">Loading...</div>

@section Scripts {
    <script src="_framework/blazor.webassembly.js"></script>
}

Run the Server project. Navigate to the Razor page at /razorpagescounter2. The prerendered Counter component is embedded in the page.

Additional work might be required depending on the static resources that components use and how layout pages are organized in an app. Typically, scripts are added to a page or view's Scripts render section and stylesheets are added to the layout's <head> element content.

Note

The preceding example throws a JSException if a Blazor WebAssembly app is prerendered and integrated into a Razor Pages or MVC app simultaneously with the use of a CSS selector. Navigating to one of the Client project's Razor components or navigating to a page or view of the Server with an embedded component throws one or more JSException.

This is normal behavior because prerendering and integrating a Blazor WebAssembly app with routable Razor components is incompatible with the use of CSS selectors.

If you've been working with the examples in the preceding sections and just wish to see the CSS selector work in your sample app, comment out the specification of the App root component of the Client project's Program.cs file:

- builder.RootComponents.Add<App>("#app");
+ //builder.RootComponents.Add<App>("#app");

Navigate to the page or view with the embedded Razor component that uses a CSS selector (for example, /razorpagescounter2 of the preceding example). The page or view loads with the embedded component, and the embedded component functions as expected.

Razor components can be integrated into Razor Pages and MVC apps. When the page or view is rendered, components can be prerendered at the same time.

Prerendering can improve Search Engine Optimization (SEO) by rendering content for the initial HTTP response that search engines can use to calculate page rank.

After configuring the project, use the guidance in the following sections depending on the project's requirements:

• For components that are directly routable from user requests. Follow this guidance when visitors should be able to make an HTTP request in their browser for a component with an @page directive.
  • Use routable components in a Razor Pages app
  • Use routable components in an MVC app
• For components that aren't directly routable from user requests, see the Render components from a page or view section. Follow this guidance when the app embeds components into existing pages and views with the Component Tag Helper.

Configuration

Use the following guidance to integrate Razor components into pages and views of an existing Razor Pages or MVC app.

1. Add an imports file to the root folder of the project with the following content. Change the {APP NAMESPACE} placeholder to the namespace of the project.

   _Imports.razor:

   @using System.Net.Http
   @using Microsoft.AspNetCore.Authorization
   @using Microsoft.AspNetCore.Components.Authorization
   @using Microsoft.AspNetCore.Components.Forms
   @using Microsoft.AspNetCore.Components.Routing
   @using Microsoft.AspNetCore.Components.Web
   @using Microsoft.AspNetCore.Components.Web.Virtualization
   @using Microsoft.JSInterop
   @using {APP NAMESPACE}
   

2. In the project's layout file (Pages/Shared/_Layout.cshtml in Razor Pages apps or Views/Shared/_Layout.cshtml in MVC apps):

   • Add the following <base> tag and HeadOutlet component Tag Helper to the <head> element:

     <base href="~/" />
     <component type="typeof(Microsoft.AspNetCore.Components.Web.HeadOutlet)" 
         render-mode="ServerPrerendered" />
     

     The href value (the app base path) in the preceding example assumes that the app resides at the root URL path (/). If the app is a sub-application, follow the guidance in the App base path section of the Host and deploy ASP.NET Core Blazor article.

     The HeadOutlet component is used to render head (<head>) content for page titles (PageTitle component) and other head elements (HeadContent component) set by Razor components. For more information, see Control head content in ASP.NET Core Blazor apps.

   • Add a <script> tag for the blazor.server.js script immediately before the Scripts render section (@await RenderSectionAsync(...)):

     <script src="_framework/blazor.server.js"></script>
     

     The framework adds the blazor.server.js script to the app. There's no need to manually add a blazor.server.js script file to the app.

   Note

   Typically, the layout loads via a _ViewStart.cshtml file.

3. Register the Blazor Server services in Program.cs where services are registered:

   builder.Services.AddServerSideBlazor();
   

4. Add the Blazor Hub endpoint to the endpoints of Program.cs where routes are mapped. Place the following line after the call to MapRazorPages (Razor Pages) or MapControllerRoute (MVC):

   app.MapBlazorHub();
   

5. Integrate components into any page or view. For example, add a Counter component to the project's Shared folder.

   Pages/Shared/Counter.razor (Razor Pages) or Views/Shared/Counter.razor (MVC):

   <h1>Counter</h1>
   
   <p>Current count: @currentCount</p>
   
   <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
   
   @code {
       private int currentCount = 0;
   
       private void IncrementCount()
       {
           currentCount++;
       }
   }
   

   Razor Pages:

   In the project's Index page of a Razor Pages app, add the Counter component's namespace and embed the component into the page. When the Index page loads, the Counter component is prerendered in the page. In the following example, replace the {APP NAMESPACE} placeholder with the project's namespace.

   Pages/Index.cshtml:

   @page
   @using {APP NAMESPACE}.Pages.Shared
   @model IndexModel
   @{
       ViewData["Title"] = "Home page";
   }
   
   <component type="typeof(Counter)" render-mode="ServerPrerendered" />
   

   MVC:

   In the project's Index view of an MVC app, add the Counter component's namespace and embed the component into the view. When the Index view loads, the Counter component is prerendered in the page. In the following example, replace the {APP NAMESPACE} placeholder with the project's namespace.

   Views/Home/Index.cshtml:

   @using {APP NAMESPACE}.Views.Shared
   @{
       ViewData["Title"] = "Home Page";
   }
   
   <component type="typeof(Counter)" render-mode="ServerPrerendered" />
   

For more information, see the Render components from a page or view section.

Use routable components in a Razor Pages app

This section pertains to adding components that are directly routable from user requests.

To support routable Razor components in Razor Pages apps:

1. Follow the guidance in the Configuration section.

2. Add an App component to the project root with the following content.

   App.razor:

   @using Microsoft.AspNetCore.Components.Routing
   
   <Router AppAssembly="typeof(App).Assembly">
       <Found Context="routeData">
           <RouteView RouteData="routeData" />
       </Found>
       <NotFound>
           <PageTitle>Not found</PageTitle>
           <p role="alert">Sorry, there's nothing at this address.</p>
       </NotFound>
   </Router>
   

3. Add a _Host page to the project with the following content. Replace the {APP NAMESPACE} placeholder with the app's namespace.

   Pages/_Host.cshtml:

   @page
   @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
   
   <component type="typeof(App)" render-mode="ServerPrerendered" />
   

   Note

   The preceding example assumes that the HeadOutlet component and Blazor script (_framework/blazor.server.js) are rendered by the app's layout. For more information, see the Configuration section.

   RenderMode configures whether the App component:

   • Is prerendered into the page.
   • Is rendered as static HTML on the page or if it includes the necessary information to bootstrap a Blazor app from the user agent.

   For more information on the Component Tag Helper, including passing parameters and RenderMode configuration, see Component Tag Helper in ASP.NET Core.

4. In the Program.cs endpoints, add a low-priority route for the _Host page as the last endpoint:

   app.MapFallbackToPage("/_Host");
   

5. Add routable components to the project. The following example is a RoutableCounter component based on the Counter component in the Blazor project templates.

   Pages/RoutableCounter.razor:

   @page "/routable-counter"
   
   <PageTitle>Routable Counter</PageTitle>
   
   <h1>Routable Counter</h1>
   
   <p>Current count: @currentCount</p>
   
   <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
   
   @code {
       private int currentCount = 0;
   
       private void IncrementCount()
       {
           currentCount++;
       }
   }
   

6. Run the project and navigate to the routable RoutableCounter component at /routable-counter.

For more information on namespaces, see the Component namespaces section.

Use routable components in an MVC app

This section pertains to adding components that are directly routable from user requests.

To support routable Razor components in MVC apps:

1. Follow the guidance in the Configuration section.

2. Add an App component to the project root with the following content.

   App.razor:

   @using Microsoft.AspNetCore.Components.Routing
   
   <Router AppAssembly="typeof(App).Assembly">
       <Found Context="routeData">
           <RouteView RouteData="routeData" />
       </Found>
       <NotFound>
           <PageTitle>Not found</PageTitle>
           <p role="alert">Sorry, there's nothing at this address.</p>
       </NotFound>
   </Router>
   

3. Add a _Host view to the project with the following content. Replace the {APP NAMESPACE} placeholder with the app's namespace.

   Views/Home/_Host.cshtml:

   @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
   
   <component type="typeof(App)" render-mode="ServerPrerendered" />
   

   Note

   The preceding example assumes that the HeadOutlet component and Blazor script (_framework/blazor.server.js) are rendered by the app's layout. For more information, see the Configuration section.

   RenderMode configures whether the App component:

   • Is prerendered into the page.
   • Is rendered as static HTML on the page or if it includes the necessary information to bootstrap a Blazor app from the user agent.

   For more information on the Component Tag Helper, including passing parameters and RenderMode configuration, see Component Tag Helper in ASP.NET Core.

4. Add an action to the Home controller.

   Controllers/HomeController.cs:

   public IActionResult Blazor()
   {
      return View("_Host");
   }
   

5. In the Program.cs endpoints, add a low-priority route for the controller action that returns the _Host view:

   app.MapFallbackToController("Blazor", "Home");
   

6. Create a Pages folder in the MVC app and add routable components. The following example is a RoutableCounter component based on the Counter component in the Blazor project templates.

   Pages/RoutableCounter.razor:

   @page "/routable-counter"
   
   <PageTitle>Routable Counter</PageTitle>
   
   <h1>Routable Counter</h1>
   
   <p>Current count: @currentCount</p>
   
   <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
   
   @code {
       private int currentCount = 0;
   
       private void IncrementCount()
       {
           currentCount++;
       }
   }
   

7. Run the project and navigate to the routable RoutableCounter component at /routable-counter.

For more information on namespaces, see the Component namespaces section.

Render components from a page or view

This section pertains to adding components to pages or views, where the components aren't directly routable from user requests.

To render a component from a page or view, use the Component Tag Helper.

Render stateful interactive components

Stateful interactive components can be added to a Razor page or view.

When the page or view renders:

• The component is prerendered with the page or view.
• The initial component state used for prerendering is lost.
• New component state is created when the SignalR connection is established.

The following Razor page renders a Counter component:

<h1>Razor Page</h1>

<component type="typeof(Counter)" render-mode="ServerPrerendered" 
    param-InitialValue="InitialValue" />

@functions {
    [BindProperty(SupportsGet=true)]
    public int InitialValue { get; set; }
}

For more information, see Component Tag Helper in ASP.NET Core.

Render noninteractive components

In the following Razor page, the Counter component is statically rendered with an initial value that's specified using a form. Since the component is statically rendered, the component isn't interactive:

<h1>Razor Page</h1>

<form>
    <input type="number" asp-for="InitialValue" />
    <button type="submit">Set initial value</button>
</form>

<component type="typeof(Counter)" render-mode="Static" 
    param-InitialValue="InitialValue" />

@functions {
    [BindProperty(SupportsGet=true)]
    public int InitialValue { get; set; }
}

For more information, see Component Tag Helper in ASP.NET Core.

Component namespaces

When using a custom folder to hold the project's Razor components, add the namespace representing the folder to either the page/view or to the _ViewImports.cshtml file. In the following example:

• Components are stored in the Components folder of the project.
• The {APP NAMESPACE} placeholder is the project's namespace. Components represents the name of the folder.

@using {APP NAMESPACE}.Components

The _ViewImports.cshtml file is located in the Pages folder of a Razor Pages app or the Views folder of an MVC app.

For more information, see ASP.NET Core Razor components.

Persist prerendered state

Without persisting prerendered state, state used during prerendering is lost and must be recreated when the app is fully loaded. If any state is setup asynchronously, the UI may flicker as the prerendered UI is replaced with temporary placeholders and then fully rendered again.

To persist state for prerendered components, use the Persist Component State Tag Helper (reference source). Add the Tag Helper's tag, <persist-component-state />, inside the closing </body> tag of the _Host page in an app that prerenders components.

Note

Documentation links to .NET reference source usually load the repository's default branch, which represents the current development for the next release of .NET. To select a tag for a specific release, use the Switch branches or tags dropdown list. For more information, see How to select a version tag of ASP.NET Core source code (dotnet/AspNetCore.Docs #26205).

In Pages/_Host.cshtml of Blazor apps that are either WebAssembly prerendered (WebAssemblyPrerendered) in a hosted Blazor WebAssembly app or ServerPrerendered in a Blazor Server app:

<body>
    ...

    <persist-component-state />
</body>

Decide what state to persist using the PersistentComponentState service. PersistentComponentState.RegisterOnPersisting registers a callback to persist the component state before the app is paused. The state is retrieved when the application resumes.

In the following example:

• The {TYPE} placeholder represents the type of data to persist (for example, WeatherForecast[]).
• The {TOKEN} placeholder is a state identifier string (for example, fetchdata).

@implements IDisposable
@inject PersistentComponentState ApplicationState

...

@code {
    private {TYPE} data;
    private PersistingComponentStateSubscription persistingSubscription;

    protected override async Task OnInitializedAsync()
    {
        persistingSubscription = 
            ApplicationState.RegisterOnPersisting(PersistData);

        if (!ApplicationState.TryTakeFromJson<{TYPE}>(
            "{TOKEN}", out var restored))
        {
            data = await ...;
        }
        else
        {
            data = restored!;
        }
    }

    private Task PersistData()
    {
        ApplicationState.PersistAsJson("{TOKEN}", data);

        return Task.CompletedTask;
    }

    void IDisposable.Dispose()
    {
        persistingSubscription.Dispose();
    }
}

The following example is an updated version of the FetchData component in a hosted Blazor WebAssembly app based on the Blazor project template. The WeatherForecastPreserveState component persists weather forecast state during prerendering and then retrieves the state to initialize the component. The Persist Component State Tag Helper persists the component state after all component invocations.

Pages/WeatherForecastPreserveState.razor:

@page "/weather-forecast-preserve-state"
@using BlazorSample.Shared
@implements IDisposable
@inject IWeatherForecastService WeatherForecastService
@inject PersistentComponentState ApplicationState

<PageTitle>Weather Forecast</PageTitle>

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from the server.</p>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <table class="table">
        <thead>
            <tr>
                <th>Date</th>
                <th>Temp. (C)</th>
                <th>Temp. (F)</th>
                <th>Summary</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var forecast in forecasts)
            {
                <tr>
                    <td>@forecast.Date.ToShortDateString()</td>
                    <td>@forecast.TemperatureC</td>
                    <td>@forecast.TemperatureF</td>
                    <td>@forecast.Summary</td>
                </tr>
            }
        </tbody>
    </table>
}

@code {
    private WeatherForecast[] forecasts = Array.Empty<WeatherForecast>();
    private PersistingComponentStateSubscription persistingSubscription;

    protected override async Task OnInitializedAsync()
    {
        persistingSubscription = 
            ApplicationState.RegisterOnPersisting(PersistForecasts);

        if (!ApplicationState.TryTakeFromJson<WeatherForecast[]>(
            "fetchdata", out var restored))
        {
            forecasts = 
                await WeatherForecastService.GetForecastAsync(DateOnly.FromDateTime(DateTime.Now));
        }
        else
        {
            forecasts = restored!;
        }
    }

    private Task PersistForecasts()
    {
        ApplicationState.PersistAsJson("fetchdata", forecasts);

        return Task.CompletedTask;
    }

    void IDisposable.Dispose()
    {
        persistingSubscription.Dispose();
    }
}

By initializing components with the same state used during prerendering, any expensive initialization steps are only executed once. The rendered UI also matches the prerendered UI, so no flicker occurs in the browser.

The persisted prerendered state is transferred to the client, where it's used to restore the component state. ASP.NET Core Data Protection ensures that the data is transferred securely in Blazor Server apps. For prerendering in a hosted Blazor WebAssembly app, the data is exposed to the browser and must not contain sensitive, private information.

Additional Blazor WebAssembly resources

• State management: Handle prerendering
• Prerendering support with assembly lazy loading
• Razor component lifecycle subjects that pertain to prerendering
  • Component initialization (OnInitialized{Async})
  • After component render (OnAfterRender{Async})
  • Stateful reconnection after prerendering: Although the content in the section focuses on Blazor Server and stateful SignalR reconnection, the scenario for prerendering in hosted Blazor WebAssembly apps (WebAssemblyPrerendered) involves similar conditions and approaches to prevent executing developer code twice. To preserve state during the execution of initialization code while prerendering, see Persist prerendered state section of this article.
  • Prerendering with JavaScript interop
• Authentication and authorization subjects that pertain to prerendering
  • General aspects
  • Prerendering with authentication in hosted Blazor WebAssembly apps
• Host and deploy: Blazor WebAssembly
• Handle errors: Prerendering
• OnNavigateAsync is executed twice when prerendering: Handle asynchronous navigation events with OnNavigateAsync

Prerendered state size and SignalR message size limit

A large prerendered state size may exceed the SignalR circuit message size limit, which results in the following:

• The SignalR circuit fails to initialize with an error on the client: Circuit host not initialized.
• The reconnection UI on the client appears when the circuit fails. Recovery isn't possible.

To resolve the problem, use either of the following approaches:

• Reduce the amount of data that you are putting into the prerendered state.
• Increase the SignalR message size limit. WARNING: Increasing the limit may increase the risk of Denial of Service (DoS) attacks.

Additional Blazor Server resources

• State management: Handle prerendering
• Razor component lifecycle subjects that pertain to prerendering
  • Component initialization (OnInitialized{Async})
  • After component render (OnAfterRender{Async})
  • Stateful reconnection after prerendering
  • Prerendering with JavaScript interop
• Authentication and authorization: General aspects
• Handle Errors: Prerendering
• Host and deploy: Blazor Server
• Threat mitigation: Cross-site scripting (XSS)
• OnNavigateAsync is executed twice when prerendering: Handle asynchronous navigation events with OnNavigateAsync