ASP.NET Core Blazor render modes

This article explains control of Razor component rendering in Blazor Web Apps, either at compile time or runtime.

Note

This guidance doesn't apply to standalone Blazor WebAssembly apps.

Render modes

Every component in a Blazor Web App adopts a render mode to determine the hosting model that it uses, where it's rendered, and whether or not it's interactive.

The following table shows the available render modes for rendering Razor components in a Blazor Web App. To apply a render mode to a component use the @rendermode directive on the component instance or on the component definition. Later in this article, examples are shown for each render mode scenario.

Name Description Render location Interactive
Static Server Static server-side rendering (static SSR) Server No
Interactive Server Interactive server-side rendering (interactive SSR) using Blazor Server. Server Yes
Interactive WebAssembly Client-side rendering (CSR) using Blazor WebAssembly†. Client Yes
Interactive Auto Interactive SSR using Blazor Server initially and then CSR on subsequent visits after the Blazor bundle is downloaded. Server, then client Yes

†Client-side rendering (CSR) is assumed to be interactive. "Interactive client-side rendering" and "interactive CSR" aren't used by the industry or in the Blazor documentation.

Prerendering is enabled by default for interactive components. Guidance on controlling prerendering is provided later in this article. For general industry terminology on client and server rendering concepts, see ASP.NET Core Blazor fundamentals.

The following examples demonstrate setting the component's render mode with a few basic Razor component features.

To test the render mode behaviors locally, you can place the following components in an app created from the Blazor Web App project template. When you create the app, select options from dropdown menus (Visual Studio) or apply the CLI options (.NET CLI) to enable both server-side and client-side interactivity. For guidance on how to create a Blazor Web App, see Tooling for ASP.NET Core Blazor.

Enable support for interactive render modes

A Blazor Web App must be configured to support interactive render modes. The following extensions are automatically applied to apps created from the Blazor Web App project template during app creation. Individual components are still required to declare their render mode per the Render modes section after the component services and endpoints are configured in the app's Program file.

Services for Razor components are added by calling AddRazorComponents.

Component builder extensions:

MapRazorComponents discovers available components and specifies the root component for the app (the first component loaded), which by default is the App component (App.razor).

Endpoint convention builder extensions:

Note

For orientation on the placement of the API in the following examples, inspect the Program file of an app generated from the Blazor Web App project template. For guidance on how to create a Blazor Web App, see Tooling for ASP.NET Core Blazor.

Example 1: The following Program file API adds services and configuration for enabling interactive SSR:

builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents();
app.MapRazorComponents<App>()
    .AddInteractiveServerRenderMode();

Example 2: The following Program file API adds services and configuration for enabling the Interactive WebAssembly render mode:

builder.Services.AddRazorComponents()
    .AddInteractiveWebAssemblyComponents();
app.MapRazorComponents<App>()
    .AddInteractiveWebAssemblyRenderMode();

Example 3: The following Program file API adds services and configuration for enabling the Interactive Server, Interactive WebAssembly, and Interactive Auto render modes:

builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents()
    .AddInteractiveWebAssemblyComponents();
app.MapRazorComponents<App>()
    .AddInteractiveServerRenderMode()
    .AddInteractiveWebAssemblyRenderMode();

Blazor uses the Blazor WebAssembly hosting model to download and execute components that use the Interactive WebAssembly render mode. A separate client project is required to set up Blazor WebAssembly hosting for these components. The client project contains the startup code for the Blazor WebAssembly host and sets up the .NET runtime for running in a browser. The Blazor Web App template adds this client project for you when you select the option to enable WebAssembly interactivity. Any components using the Interactive WebAssembly render mode should be built from the client project, so they get included in the downloaded app bundle.

Apply a render mode to a component instance

To apply a render mode to a component instance use the @rendermode Razor directive attribute where the component is used.

In the following example, interactive server-side rendering (interactive SSR) is applied to the Dialog component instance:

<Dialog @rendermode="InteractiveServer" />

Note

Blazor templates include a static using directive for RenderMode in the app's _Imports file (Components/_Imports.razor) for shorter @rendermode syntax:

@using static Microsoft.AspNetCore.Components.Web.RenderMode

Without the preceding directive, components must specify the static RenderMode class in @rendermode syntax:

<Dialog @rendermode="RenderMode.InteractiveServer" />

You can also reference custom render mode instances instantiated directly with custom configuration. For more information, see the Custom shorthand render modes section later in this article.

Apply a render mode to a component definition

To specify the render mode for a component as part of its definition, use the @rendermode Razor directive and the corresponding render mode attribute.

@page "..."
@rendermode InteractiveServer

Applying a render mode to a component definition is commonly used when applying a render mode to a specific page. Routable pages by default use the same render mode as the Router component that rendered the page.

Technically, @rendermode is both a Razor directive and a Razor directive attribute. The semantics are similar, but there are differences. The @rendermode directive is on the component definition, so the referenced render mode instance must be static. The @rendermode directive attribute can take any render mode instance.

Note

Component authors should avoid coupling a component's implementation to a specific render mode. Instead, component authors should typically design components to support any render mode or hosting model. A component's implementation should avoid assumptions on where it's running (server or client) and should degrade gracefully when rendered statically. Specifying the render mode in the component definition may be needed if the component isn't instantiated directly (such as with a routable page component) or to specify a render mode for all component instances.

Apply a render mode to the entire app

To set the render mode for the entire app, indicate the render mode at the highest-level interactive component in the app's component hierarchy that isn't a root component.

Note

Making a root component interactive, such as the App component, isn't supported. Therefore, the render mode for the entire app can't be set directly by the App component.

For apps based on the Blazor Web App project template, a render mode assigned to the entire app is typically specified where the Routes component is used in the App component (Components/App.razor):

<Routes @rendermode="InteractiveServer" />

The Router component propagates its render mode to the pages it routes.

You also typically must set the same interactive render mode on the HeadOutlet component, which is also found in the App component of a Blazor Web App generated from the project template:

<HeadOutlet @rendermode="InteractiveServer" />

For apps that adopt an interactive client-side (WebAssembly or Auto) rendering mode and enable the render mode for the entire app via the Routes component:

  • Place or move the layout and navigation files of the server app's Components/Layout folder into the .Client project's Layout folder. Create a Layout folder in the .Client project if it doesn't exist.
  • Place or move the components of the server app's Components/Pages folder into the .Client project's Pages folder. Create a Pages folder in the .Client project if it doesn't exist.
  • Place or move the Routes component of the server app's Components folder into the .Client project's root folder.

To enable global interactivity when creating a Blazor Web App:

  • Visual Studio: Set the Interactivity location dropdown list to Global.
  • .NET CLI: Use the -ai|--all-interactive option.

For more information, see Tooling for ASP.NET Core Blazor.

Apply a render mode programatically

Properties and fields can assign a render mode.

The second approach described in this section, setting the render mode by component instance, is especially useful when your app specification calls for either of the following scenarios:

  • You have an area (folder) of the app with components that must adopt static server-side rendering (static SSR) and only run on the server. The app controls the render mode globally by setting the render mode on the Routes component in the App component based on the path to the folder.
  • You have components around the app in various locations (not in a single folder) that must adopt static SSR and only run on the server. The app controls the render mode on a per-component basis by setting the render mode with the @rendermode directive in component instances. Reflection is used in the App component to set the render mode on the Routes component.

In both cases, the component that must adopt static SSR must also force a full-page reload.

The preceding two scenarios are covered with examples in the Fine control of render modes section later in this article. The following two subsections focus on basic approaches for setting the render mode.

Set the render mode by component definition

A component definition can define a render mode via a private field:

@rendermode renderModeForPage

...

@code {
    private static IComponentRenderMode renderModeForPage = InteractiveServer;
}

Set the render mode by component instance

The following example applies interactive server-side rendering (interactive SSR) to any request.

<Routes @rendermode="RenderModeForPage" />

...

@code {
    private IComponentRenderMode? RenderModeForPage => InteractiveServer;
}

Additional information on render mode propagation is provided in the Render mode propagation section later in this article. The Fine control of render modes section shows how to use the preceding approach to adopt static SSR in either specific areas of the app (folders) or for specific components spread around the app with per-component render mode assignments.

Prerendering

Prerendering is the process of initially rendering page content on the server without enabling event handlers for rendered controls. The server outputs the HTML UI of the page as soon as possible in response to the initial request, which makes the app feel more responsive to users. Prerendering can also improve Search Engine Optimization (SEO) by rendering content for the initial HTTP response that search engines use to calculate page rank.

Prerendering is enabled by default for interactive components.

To disable prerendering for a component instance, pass the prerender flag with a value of false to the render mode:

  • <... @rendermode="new InteractiveServerRenderMode(prerender: false)" />
  • <... @rendermode="new InteractiveWebAssemblyRenderMode(prerender: false)" />
  • <... @rendermode="new InteractiveAutoRenderMode(prerender: false)" />

To disable prerendering in a component definition:

  • @rendermode @(new InteractiveServerRenderMode(prerender: false))
  • @rendermode @(new InteractiveWebAssemblyRenderMode(prerender: false))
  • @rendermode @(new InteractiveAutoRenderMode(prerender: false))

To disable prerendering for the entire app, indicate the render mode at the highest-level interactive component in the app's component hierarchy that isn't a root component.

For apps based on the Blazor Web App project template, a render mode assigned to the entire app is specified where the Routes component is used in the App component (Components/App.razor). The following example sets the app's render mode to Interactive Server with prerendering disabled:

<Routes @rendermode="new InteractiveServerRenderMode(prerender: false)" />

Also, disable prerendering for the HeadOutlet component in the App component:

<HeadOutlet @rendermode="new InteractiveServerRenderMode(prerender: false)" />

Making a root component, such as the App component, interactive with the @rendermode directive at the top of the root component's definition file (.razor) isn't supported. Therefore, prerendering can't be disabled directly by the App component.

Static server-side rendering (static SSR)

By default, components use the static server-side rendering (static SSR). The component renders to the response stream and interactivity isn't enabled.

In the following example, there's no designation for the component's render mode, and the component inherits the default render mode from its parent. Therefore, the component is statically rendered on the server. The button isn't interactive and doesn't call the UpdateMessage method when selected. The value of message doesn't change, and the component isn't rerendered in response to UI events.

RenderMode1.razor:

@page "/render-mode-1"

<button @onclick="UpdateMessage">Click me</button> @message

@code {
    private string message = "Not clicked yet.";

    private void UpdateMessage()
    {
        message = "Somebody clicked me!";
    }
}

If using the preceding component locally in a Blazor Web App, place the component in the server project's Components/Pages folder. The server project is the solution's project with a name that doesn't end in .Client. When the app is running, navigate to /render-mode-1 in the browser's address bar.

Enhanced navigation with static SSR requires special attention when loading JavaScript. For more information, see ASP.NET Core Blazor JavaScript with static server-side rendering (static SSR).

Interactive server-side rendering (interactive SSR)

Interactive server-side rendering (interactive SSR) renders the component interactively from the server using Blazor Server. User interactions are handled over a real-time connection with the browser. The circuit connection is established when the Server component is rendered.

In the following example, the render mode is set interactive SSR by adding @rendermode InteractiveServer to the component definition. The button calls the UpdateMessage method when selected. The value of message changes, and the component is rerendered to update the message in the UI.

RenderMode2.razor:

@page "/render-mode-2"
@rendermode InteractiveServer

<button @onclick="UpdateMessage">Click me</button> @message

@code {
    private string message = "Not clicked yet.";

    private void UpdateMessage()
    {
        message = "Somebody clicked me!";
    }
}

If using the preceding component locally in a Blazor Web App, place the component in the server project's Components/Pages folder. The server project is the solution's project with a name that doesn't end in .Client. When the app is running, navigate to /render-mode-2 in the browser's address bar.

Client-side rendering (CSR)

Client-side rendering (CSR) renders the component interactively on the client using Blazor WebAssembly. The .NET runtime and app bundle are downloaded and cached when the WebAssembly component is initially rendered. Components using CSR must be built from a separate client project that sets up the Blazor WebAssembly host.

In the following example, the render mode is set to CSR with @rendermode InteractiveWebAssembly. The button calls the UpdateMessage method when selected. The value of message changes, and the component is rerendered to update the message in the UI.

RenderMode3.razor:

@page "/render-mode-3"
@rendermode InteractiveWebAssembly

<button @onclick="UpdateMessage">Click me</button> @message

@code {
    private string message = "Not clicked yet.";

    private void UpdateMessage()
    {
        message = "Somebody clicked me!";
    }
}

If using the preceding component locally in a Blazor Web App, place the component in the client project's Pages folder. The client project is the solution's project with a name that ends in .Client. When the app is running, navigate to /render-mode-3 in the browser's address bar.

Automatic (Auto) rendering

Automatic (Auto) rendering determines how to render the component at runtime. The component is initially rendered with interactive server-side rendering (interactive SSR) using the Blazor Server hosting model. The .NET runtime and app bundle are downloaded to the client in the background and cached so that they can be used on future visits.

The Auto render mode never dynamically changes the render mode of a component already on the page. The Auto render mode makes an initial decision about which type of interactivity to use for a component, then the component keeps that type of interactivity for as long as it's on the page. One factor in this initial decision is considering whether components already exist on the page with WebAssembly/Server interactivity. Auto mode prefers to select a render mode that matches the render mode of existing interactive components. The reason that the Auto mode prefers to use an existing interactivity mode is to avoid introducing a new interactive runtime that doesn't share state with the existing runtime.

Components using the Auto render mode must be built from a separate client project that sets up the Blazor WebAssembly host.

In the following example, the component is interactive throughout the process. The button calls the UpdateMessage method when selected. The value of message changes, and the component is rerendered to update the message in the UI. Initially, the component is rendered interactively from the server, but on subsequent visits it's rendered from the client after the .NET runtime and app bundle are downloaded and cached.

RenderMode4.razor:

@page "/render-mode-4"
@rendermode InteractiveAuto

<button @onclick="UpdateMessage">Click me</button> @message

@code {
    private string message = "Not clicked yet.";

    private void UpdateMessage()
    {
        message = "Somebody clicked me!";
    }
}

If using the preceding component locally in a Blazor Web App, place the component in the client project's Pages folder. The client project is the solution's project with a name that ends in .Client. When the app is running, navigate to /render-mode-4 in the browser's address bar.

Render mode propagation

Render modes propagate down the component hierarchy.

Rules for applying render modes:

  • The default render mode is Static.
  • The Interactive Server (InteractiveServer), Interactive WebAssembly (InteractiveWebAssembly), and Interactive Auto (InteractiveAuto) render modes can be used from a component, including using different render modes for sibling components.
  • You can't switch to a different interactive render mode in a child component. For example, a Server component can't be a child of a WebAssembly component.
  • Parameters passed to an interactive child component from a Static parent must be JSON serializable. This means that you can't pass render fragments or child content from a Static parent component to an interactive child component.

The following examples use a non-routable, non-page SharedMessage component. The render mode agnostic SharedMessage component doesn't apply a render mode with an @attribute directive. If you're testing these scenarios with a Blazor Web App, place the following component in the app's Components folder.

SharedMessage.razor:

<p>@Greeting</p>

<button @onclick="UpdateMessage">Click me</button> @message

<p>@ChildContent</p>

@code {
    private string message = "Not clicked yet.";

    [Parameter]
    public RenderFragment? ChildContent { get; set; }

    [Parameter]
    public string Greeting { get; set; } = "Hello!";

    private void UpdateMessage()
    {
        message = "Somebody clicked me!";
    }
}

Render mode inheritance

If the SharedMessage component is placed in a statically-rendered parent component, the SharedMessage component is also rendered statically and isn't interactive. The button doesn't call UpdateMessage, and the message isn't updated.

RenderMode5.razor:

@page "/render-mode-5"

<SharedMessage />

If the SharedMessage component is placed in a component that defines the render mode, it inherits the applied render mode.

In the following example, the SharedMessage component is interactive over a SignalR connection to the client. The button calls UpdateMessage, and the message is updated.

RenderMode6.razor:

@page "/render-mode-6"
@rendermode InteractiveServer

<SharedMessage />

Child components with different render modes

In the following example, both SharedMessage components are prerendered (by default) and appear when the page is displayed in the browser.

  • The first SharedMessage component with interactive server-side rendering (interactive SSR) is interactive after the SignalR circuit is established.
  • The second SharedMessage component with client-side rendering (CSR) is interactive after the Blazor app bundle is downloaded and the .NET runtime is active on the client.

RenderMode7.razor:

@page "/render-mode-7"

<SharedMessage @rendermode="InteractiveServer" />
<SharedMessage @rendermode="InteractiveWebAssembly" />

Child component with a serializable parameter

The following example demonstrates an interactive child component that takes a parameter. Parameters must be serializable.

RenderMode8.razor:

@page "/render-mode-8"

<SharedMessage @rendermode="InteractiveServer" Greeting="Welcome!" />

Non-serializable component parameters, such as child content or a render fragment, are not supported. In the following example, passing child content to the SharedMessage component results in a runtime error.

RenderMode9.razor:

@page "/render-mode-9"

<SharedMessage @rendermode="InteractiveServer">
    Child content
</SharedMessage>

Error:

System.InvalidOperationException: Cannot pass the parameter 'ChildContent' to component 'SharedMessage' with rendermode 'InteractiveServerRenderMode'. This is because the parameter is of the delegate type 'Microsoft.AspNetCore.Components.RenderFragment', which is arbitrary code and cannot be serialized.

To circumvent the preceding limitation, wrap the child component in another component that doesn't have the parameter. This is the approach taken in the Blazor Web App project template with the Routes component (Components/Routes.razor) to wrap the Router component.

WrapperComponent.razor:

<SharedMessage>
    Child content
</SharedMessage>

RenderMode10.razor:

@page "/render-mode-10"

<WrapperComponent @rendermode="InteractiveServer" />

In the preceding example:

  • The child content is passed to the SharedMessage component without generating a runtime error.
  • The SharedMessage component renders interactively on the server.

Child component with a different render mode than its parent

Don't try to apply a different interactive render mode to a child component than its parent's render mode.

The following component results in a runtime error when the component is rendered:

RenderMode11.razor:

@page "/render-mode-11"
@rendermode InteractiveServer

<SharedMessage @rendermode="InteractiveWebAssembly" />

Error:

Cannot create a component of type 'BlazorSample.Components.SharedMessage' because its render mode 'Microsoft.AspNetCore.Components.Web.InteractiveWebAssemblyRenderMode' is not supported by Interactive Server rendering.

Fine control of render modes

There are cases where the app's specification calls for components to adopt static server-side rendering (static SSR) and only run on the server, while the rest of the app uses an interactive render mode.

There are two approaches that can be taken for fine control of render modes, each of which is described in the following subsections:

  • Area (folder) of static SSR components: You have an area (folder) of the app with components that must adopt static SSR and share the same route path prefix. The app controls the render mode globally by setting the render mode on the Routes component in the App component based on the path to the folder.

  • Static SSR components spread out across the app: You have components spread around the app in various locations that must adopt static SSR and only run on the server. The static SSR-only components aren't in a single folder and don't share a common route path prefix. The app controls the render mode on a per-component basis by setting the render mode with the @rendermode directive in component instances. Reflection is used in the App component to set the render mode on the Routes component.

In both cases, the component that must adopt static SSR must also force a full-page reload.

The following examples use the HttpContext cascading parameter to determine if the page is statically-rendered. A null HttpContext indicates that the component is rendering interactively, which is useful as a signal in app code to trigger a full-page reload.

Area (folder) of static SSR components

The approach described in this subsection is used by the Blazor Web App project template with individual authentication and global interactivity.

An area (folder) of the app contains the components that must adopt static SSR and only run on the server. The components in the folder share the same route path prefix. For example, the Identity Razor components of the Blazor Web App project template are in the Components/Account/Pages folder and share the root path prefix /Account.

The folder also contains an _Imports.razor file, which applies a custom account layout to the components in the folder:

@using BlazorSample.Components.Account.Shared
@layout AccountLayout

The Shared folder maintains the AccountLayout layout component. The component makes use of HttpContext to determine if the component is rendering on the server. Identity components must render on the server with static SSR because they set Identity cookies. If the value of HttpContext is null, the component is rendering interactively, and a full-page reload is performed by calling NavigationManager.Refresh with forceLoad set to true. This forces a full rerender of the page using static SSR.

Components/Account/Shared/AccountLayout.razor:

@inherits LayoutComponentBase
@layout BlazorSample.Components.Layout.MainLayout
@inject NavigationManager NavigationManager

@if (HttpContext is null)
{
    <p>Loading...</p>
}
else
{
    @Body
}

@code {
    [CascadingParameter]
    private HttpContext? HttpContext { get; set; }

    protected override void OnParametersSet()
    {
        if (HttpContext is null)
        {
            NavigationManager.Refresh(forceReload: true);
        }
    }
}

Note

In the Blazor Web App project template, there's a second layout file (ManageLayout.razor in the Components/Account/Shared folder) for Identity components in the Components/Account/Pages/Manage folder. The Manage folder has its own _Imports.razor file to apply to the ManageLayout to components in the folder. In your own apps, using nested _Imports.razor files is a useful approach for applying custom layouts to groups of pages.

In the App component, any request for a component in the Account folder applies a null render mode, which enforces static SSR. Other component requests receive a global application of the interactive SSR render mode (InteractiveServer).

Important

Applying a null render mode doesn't always enforce static SSR. It just happens to behave that way using the approach shown in this section.

A null render mode is effectively the same as not specifying a render mode, which results in the component inheriting its parent's render mode. In this case, the App component is rendered using static SSR, so a null render mode results in the Routes component inheriting static SSR from the App component. If a null render mode is specified for a child component whose parent uses an interactive render mode, the child inherits the same interactive render mode.

Components/App.razor:

<Routes @rendermode="RenderModeForPage" />

...

@code {
    [CascadingParameter]
    private HttpContext HttpContext { get; set; } = default!;

    private IComponentRenderMode? RenderModeForPage => 
        HttpContext.Request.Path.StartsWithSegments("/Account")
            ? null
            : {INTERACTIVE RENDER MODE};
}

In the preceding code, change the {INTERACTIVE RENDER MODE} placeholder to the appropriate value, depending on if the rest of the application should adopt global InteractiveServer, InteractiveWebAssembly, or InteractiveAuto rendering.

The components that must adopt static SSR in the Account folder aren't required to set the layout because it's applied via the _Imports.razor file, and the components do not set a render mode because they should render with static SSR. Nothing further must be done for the components in the Account folder to enforce static SSR.

Static SSR components spread out across the app

In the preceding subsection, the app controls the render mode of the components by setting the render mode globally in the App component. Alternatively, the App component can also adopt per-component render modes for setting the render mode, which permits components spread around the app to enforce adoption of static SSR. This subsection describes the approach.

The app has a custom layout that can be applied to components around the app. Usually, a shared component for the app is placed in the Components/Layout folder. The component makes use of HttpContext to determine if the component is rendering on the server. If the value of HttpContext is null, the component is rendering interactively, and a full-page reload is performed by calling NavigationManager.Refresh with forceLoad set to true. This triggers a request to the server for the component.

Components/Layout/StaticSsrLayout.razor:

@inherits LayoutComponentBase
@layout MainLayout
@inject NavigationManager NavigationManager

@if (HttpContext is null)
{
    <p>Loading...</p>
}
else
{
    @Body
}

@code {
    [CascadingParameter]
    private HttpContext? HttpContext { get; set; }

    protected override void OnParametersSet()
    {
        if (HttpContext is null)
        {
            NavigationManager.Refresh(forceReload: true);
        }
    }
}

In the App component, reflection is used to set the render mode. Whatever render mode is assigned to the individual component definition file is applied to the Routes component.

Components/App.razor:

<Routes @rendermode="RenderModeForPage" />

...

@code {
    [CascadingParameter]
    private HttpContext HttpContext { get; set; } = default!;

    private IComponentRenderMode? RenderModeForPage =>
        HttpContext.GetEndpoint()?.Metadata.GetMetadata<RenderModeAttribute>()?
            .Mode;
}

Each component that must adopt static SSR sets the custom layout and does not specify a render mode. Not specifying a render mode results in a null value of RenderModeAttribute.Mode in the App component, which results in no render mode assigned to the Routes component instance and enforcement of static SSR.

Important

Applying a null render mode doesn't always enforce static SSR. It just happens to behave that way using the approach shown in this section.

A null render mode is effectively the same as not specifying a render mode, which results in the component inheriting its parent's render mode. In this case, the App component is rendered using static SSR, so a null render mode results in the Routes component inheriting static SSR from the App component. If a null render mode is specified for a child component whose parent uses an interactive render mode, the child inherits the same interactive render mode.

Nothing further must be done for the components to enforce static SSR than applying the custom layout:

@layout BlazorSample.Components.Layout.StaticSsrLayout

Other components around the app set an appropriate interactive render mode, which upon reflection in the App component is applied to the Routes component. Interactive components avoid applying the custom static SSR layout:

@rendermode {INTERACTIVE RENDER MODE}

In the preceding code, change the {INTERACTIVE RENDER MODE} placeholder to the appropriate value, depending on if the component should adopt InteractiveServer, InteractiveWebAssembly, or InteractiveAuto rendering.

Client-side services fail to resolve during prerendering

Assuming that prerendering isn't disabled for a component or for the app, a component in the .Client project is prerendered on the server. Because the server doesn't have access to registered client-side Blazor services, it isn't possible to inject these services into a component without receiving an error that the service can't be found during prerendering.

For example, consider the following Home component in the .Client project in a Blazor Web App with global Interactive WebAssembly or Interactive Auto rendering. The component attempts to inject IWebAssemblyHostEnvironment to obtain the environment's name.

@page "/"
@inject IWebAssemblyHostEnvironment Environment

<PageTitle>Home</PageTitle>

<h1>Home</h1>

<p>
    Environment: @Environment.Environment
</p>

No compile time error occurs, but a runtime error occurs during prerendering:

Cannot provide a value for property 'Environment' on type 'BlazorSample.Client.Pages.Home'. There is no registered service of type 'Microsoft.AspNetCore.Components.WebAssembly.Hosting.IWebAssemblyHostEnvironment'.

This error occurs because the component must compile and execute on the server during prerendering, but IWebAssemblyHostEnvironment isn't a registered service on the server.

If the app doesn't require the value during prerendering, this problem can be solved by injecting IServiceProvider to obtain the service instead of the service type itself:

@page "/"
@using Microsoft.AspNetCore.Components.WebAssembly.Hosting
@inject IServiceProvider Services

<PageTitle>Home</PageTitle>

<h1>Home</h1>

<p>
    <b>Environment:</b> @environmentName
</p>

@code {
    private string? environmentName;

    protected override void OnInitialized()
    {
        if (Services.GetService<IWebAssemblyHostEnvironment>() is { } env)
        {
            environmentName = env.Environment;
        }
    }
}

However, the preceding approach isn't useful if your logic requires a value during prerendering.

You can also avoid the problem if you disable prerendering for the component, but that's an extreme measure to take in many cases that may not meet your component's specifications.

There are a three approaches that you can take to address this scenario. The following are listed from most recommended to least recommended:

  • Recommended for shared framework services: For shared framework services that merely aren't registered server-side in the main project, register the services in the main project, which makes them available during prerendering. For an example of this scenario, see the guidance for HttpClient services in Call a web API from an ASP.NET Core Blazor app.

  • Recommended for services outside of the shared framework: Create a custom service implementation for the service on the server. Use the service normally in interactive components of the .Client project. For a demonstration of this approach, see ASP.NET Core Blazor environments.

  • Create a service abstraction and create implementations for the service in the .Client and server projects. Register the services in each project. Inject the custom service in the component.

  • You might be able to add a .Client project package reference to a server-side package and fall back to using the server-side API when prerendering on the server.

Discover components from additional assemblies

Additional assemblies must be disclosed to the Blazor framework to discover routable Razor components in referenced projects. For more information, see ASP.NET Core Blazor routing and navigation.

Closure of circuits when there are no remaining Interactive Server components

Interactive Server components handle web UI events using a real-time connection with the browser called a circuit. A circuit and its associated state are created when a root Interactive Server component is rendered. The circuit is closed when there are no remaining Interactive Server components on the page, which frees up server resources.

Custom shorthand render modes

The @rendermode directive takes a single parameter that's a static instance of type IComponentRenderMode. The @rendermode directive attribute can take any render mode instance, static or not. The Blazor framework provides the RenderMode static class with some predefined render modes for convenience, but you can create your own.

Normally, a component uses the following @rendermode directive to disable prerendering:

@rendermode @(new InteractiveServerRenderMode(prerender: false))

However, consider the following example that creates a shorthand interactive server-side render mode without prerendering via the app's _Imports file (Components/_Imports.razor):

public static IComponentRenderMode InteractiveServerWithoutPrerendering { get; } = 
    new InteractiveServerRenderMode(prerender: false);

Use the shorthand render mode in components throughout the Components folder:

@rendermode InteractiveServerWithoutPrerendering

Alternatively, a single component instance can define a custom render mode via a private field:

@rendermode interactiveServerWithoutPrerendering

...

@code {
    private static IComponentRenderMode interactiveServerWithoutPrerendering = 
        new InteractiveServerRenderMode(prerender: false);
}

At the moment, the shorthand render mode approach is probably only useful for reducing the verbosity of specifying the prerender flag. The shorthand approach might be more useful in the future if additional flags become available for interactive rendering and you would like to create shorthand render modes with different combinations of flags.

Service injection via a top-level imports file (_Imports.razor)

This section only applies to Blazor Web Apps.

A top-level imports file in the Components folder (Components/_Imports.razor) injects its references into all of the components in the folder hierarchy, which includes the App component (App.razor). The App component is always rendered statically even if prerendering of a page component is disabled. Therefore, injecting services via the top-level imports file results in resolving two instances of the service in page components.

To address this scenario, inject the service in a new imports file placed in the Pages folder (Components/Pages/_Imports.razor). From that location, the service is only resolved once in page components.

Additional resources