ASP.NET Core Blazor state management

Server-side Blazor is a stateful app framework. Most of the time, the app maintains a connection to the server. The user's state is held in the server's memory in a circuit.

Examples of user state held in a circuit include:

• The hierarchy of component instances and their most recent render output in the rendered UI.
• The values of fields and properties in component instances.
• Data held in dependency injection (DI) service instances that are scoped to the circuit.

User state might also be found in JavaScript variables in the browser's memory set via JavaScript interop calls.

If a user experiences a temporary network connection loss, Blazor attempts to reconnect the user to their original circuit with their original state. However, reconnecting a user to their original circuit in the server's memory isn't always possible:

• The server can't retain a disconnected circuit forever. The server must release a disconnected circuit after a timeout or when the server is under memory pressure.
• In multi-server, load-balanced deployment environments, individual servers may fail or be automatically removed when no longer required to handle the overall volume of requests. The original server processing requests for a user may become unavailable when the user attempts to reconnect.
• The user might close and reopen their browser or reload the page, which removes any state held in the browser's memory. For example, JavaScript variable values set through JavaScript interop calls are lost.

When a user can't be reconnected to their original circuit, the user receives a new circuit with an empty state. This is equivalent to closing and reopening a desktop app.

Persist state across circuits

Generally, maintain state across circuits where users are actively creating data, not simply reading data that already exists.

To preserve state across circuits, the app must persist the data to some other storage location than the server's memory. State persistence isn't automatic. You must take steps when developing the app to implement stateful data persistence.

Data persistence is typically only required for high-value state that users expended effort to create. In the following examples, persisting state either saves time or aids in commercial activities:

• Multi-step web forms: It's time-consuming for a user to re-enter data for several completed steps of a multi-step web form if their state is lost. A user loses state in this scenario if they navigate away from the form and return later.
• Shopping carts: Any commercially important component of an app that represents potential revenue can be maintained. A user who loses their state, and thus their shopping cart, may purchase fewer products or services when they return to the site later.

An app can only persist app state. UIs can't be persisted, such as component instances and their render trees. Components and render trees aren't generally serializable. To persist UI state, such as the expanded nodes of a tree view control, the app must use custom code to model the behavior of the UI state as serializable app state.

Where to persist state

Common locations exist for persisting state:

• Server-side storage
• URL
• Browser storage
• In-memory state container service

Server-side storage

For permanent data persistence that spans multiple users and devices, the app can use server-side storage. Options include:

• Blob storage
• Key-value storage
• Relational database
• Table storage

After data is saved, the user's state is retained and available in any new circuit.

For more information on Azure data storage options, see the following:

• Azure Databases
• Azure Storage Documentation

URL

For transient data representing navigation state, model the data as a part of the URL. Examples of user state modeled in the URL include:

• The ID of a viewed entity.
• The current page number in a paged grid.

The contents of the browser's address bar are retained:

• If the user manually reloads the page.
• If the web server becomes unavailable, and the user is forced to reload the page in order to connect to a different server.

For information on defining URL patterns with the @page directive, see ASP.NET Core Blazor routing and navigation.

Browser storage

For transient data that the user is actively creating, a commonly used storage location is the browser's localStorage and sessionStorage collections:

• localStorage is scoped to the browser's instance. If the user reloads the page or closes and reopens the browser, the state persists. If the user opens multiple browser tabs, the state is shared across the tabs. Data persists in localStorage until explicitly cleared. The localStorage data for a document loaded in a "private browsing" or "incognito" session is cleared when the last "private" tab is closed.
• sessionStorage is scoped to the browser tab. If the user reloads the tab, the state persists. If the user closes the tab or the browser, the state is lost. If the user opens multiple browser tabs, each tab has its own independent version of the data.

Generally, sessionStorage is safer to use. sessionStorage avoids the risk that a user opens multiple tabs and encounters the following:

• Bugs in state storage across tabs.
• Confusing behavior when a tab overwrites the state of other tabs.

localStorage is the better choice if the app must persist state across closing and reopening the browser.

Caveats for using browser storage:

• Similar to the use of a server-side database, loading and saving data are asynchronous.
• The requested page doesn't exist in the browser during prerendering, so local storage isn't available during prerendering.
• Storage of a few kilobytes of data is reasonable to persist for server-side Blazor apps. Beyond a few kilobytes, you must consider the performance implications because the data is loaded and saved across the network.
• Users may view or tamper with the data. ASP.NET Core Data Protection can mitigate the risk. For example, ASP.NET Core Protected Browser Storage uses ASP.NET Core Data Protection.

Third-party NuGet packages provide APIs for working with localStorage and sessionStorage. It's worth considering choosing a package that transparently uses ASP.NET Core Data Protection. Data Protection encrypts stored data and reduces the potential risk of tampering with stored data. If JSON-serialized data is stored in plain text, users can see the data using browser developer tools and also modify the stored data. Securing trivial data isn't a problem. For example, reading or modifying the stored color of a UI element isn't a significant security risk to the user or the organization. Avoid allowing users to inspect or tamper with sensitive data.

ASP.NET Core Protected Browser Storage

ASP.NET Core Protected Browser Storage leverages ASP.NET Core Data Protection for localStorage and sessionStorage.

Save and load data within a component

In any component that requires loading or saving data to browser storage, use the @inject directive to inject an instance of either of the following:

• ProtectedLocalStorage
• ProtectedSessionStorage

The choice depends on which browser storage location you wish to use. In the following example, sessionStorage is used:

@using Microsoft.AspNetCore.Components.Server.ProtectedBrowserStorage
@inject ProtectedSessionStorage ProtectedSessionStore

@using Microsoft.AspNetCore.ProtectedBrowserStorage
@inject ProtectedSessionStorage ProtectedSessionStore

The @using directive can be placed in the app's _Imports.razor file instead of in the component. Use of the _Imports.razor file makes the namespace available to larger segments of the app or the whole app.

To persist the currentCount value in the Counter component of an app based on the Blazor project template, modify the IncrementCount method to use ProtectedSessionStore.SetAsync:

private async Task IncrementCount()
{
    currentCount++;
    await ProtectedSessionStore.SetAsync("count", currentCount);
}

In larger, more realistic apps, storage of individual fields is an unlikely scenario. Apps are more likely to store entire model objects that include complex state. ProtectedSessionStore automatically serializes and deserializes JSON data to store complex state objects.

In the preceding code example, the currentCount data is stored as sessionStorage['count'] in the user's browser. The data isn't stored in plain text but rather is protected using ASP.NET Core Data Protection. The encrypted data can be inspected if sessionStorage['count'] is evaluated in the browser's developer console.

To recover the currentCount data if the user returns to the Counter component later, including if the user is on a new circuit, use ProtectedSessionStore.GetAsync:

protected override async Task OnInitializedAsync()
{
    var result = await ProtectedSessionStore.GetAsync<int>("count");
    currentCount = result.Success ? result.Value : 0;
}

protected override async Task OnInitializedAsync()
{
    currentCount = await ProtectedSessionStore.GetAsync<int>("count");
}

If the component's parameters include navigation state, call ProtectedSessionStore.GetAsync and assign a non-null result in OnParametersSetAsync, not OnInitializedAsync. OnInitializedAsync is only called once when the component is first instantiated. OnInitializedAsync isn't called again later if the user navigates to a different URL while remaining on the same page. For more information, see ASP.NET Core Razor component lifecycle.

Handle the loading state

Since browser storage is accessed asynchronously over a network connection, there's always a period of time before the data is loaded and available to a component. For the best results, render a message while loading is in progress instead of displaying blank or default data.

private int? currentCount;

@if (currentCount.HasValue)
{
    <p>Current count: <strong>@currentCount</strong></p>
    <button @onclick="IncrementCount">Increment</button>
}
else
{
    <p>Loading...</p>
}

Handle prerendering

During prerendering:

• An interactive connection to the user's browser doesn't exist.
• The browser doesn't yet have a page in which it can run JavaScript code.

localStorage or sessionStorage aren't available during prerendering. If the component attempts to interact with storage, an error is generated explaining that JavaScript interop calls cannot be issued because the component is being prerendered.

One way to resolve the error is to disable prerendering. This is usually the best choice if the app makes heavy use of browser-based storage. Prerendering adds complexity and doesn't benefit the app because the app can't prerender any useful content until localStorage or sessionStorage are available.

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

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

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

Prerendering might be useful for other pages that don't use localStorage or sessionStorage. To retain prerendering, defer the loading operation until the browser is connected to the circuit. The following is an example for storing a counter value:

@using Microsoft.AspNetCore.Components.Server.ProtectedBrowserStorage
@inject ProtectedLocalStorage ProtectedLocalStore

@if (isConnected)
{
    <p>Current count: <strong>@currentCount</strong></p>
    <button @onclick="IncrementCount">Increment</button>
}
else
{
    <p>Loading...</p>
}

@code {
    private int currentCount;
    private bool isConnected;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            isConnected = true;
            await LoadStateAsync();
            StateHasChanged();
        }
    }

    private async Task LoadStateAsync()
    {
        var result = await ProtectedLocalStore.GetAsync<int>("count");
        currentCount = result.Success ? result.Value : 0;
    }

    private async Task IncrementCount()
    {
        currentCount++;
        await ProtectedLocalStore.SetAsync("count", currentCount);
    }
}

@using Microsoft.AspNetCore.ProtectedBrowserStorage
@inject ProtectedLocalStorage ProtectedLocalStore

@if (isConnected)
{
    <p>Current count: <strong>@currentCount</strong></p>
    <button @onclick="IncrementCount">Increment</button>
}
else
{
    <p>Loading...</p>
}

@code {
    private int currentCount = 0;
    private bool isConnected = false;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            isConnected = true;
            await LoadStateAsync();
            StateHasChanged();
        }
    }

    private async Task LoadStateAsync()
    {
        currentCount = await ProtectedLocalStore.GetAsync<int>("count");
    }

    private async Task IncrementCount()
    {
        currentCount++;
        await ProtectedLocalStore.SetAsync("count", currentCount);
    }
}

Factor out state preservation to a common provider

If many components rely on browser-based storage, implementing state provider code many times creates code duplication. One option for avoiding code duplication is to create a state provider parent component that encapsulates the state provider logic. Child components can work with persisted data without regard to the state persistence mechanism.

In the following example of a CounterStateProvider component, counter data is persisted to sessionStorage, and it handles the loading phase by not rendering its child content until state loading is complete.

The CounterStateProvider component deals with prerendering by not loading state until after component rendering in the OnAfterRenderAsync lifecycle method, which doesn't execute during prerendering.

The approach in this section isn't capable of triggering rerendering of multiple subscribed components on the same page. If one subscribed component changes the state, it rerenders and can display the updated state, but a different component on the same page displaying that state displays stale data until its own next rerender. Therefore, the approach described in this section is best suited to using state in a single component on the page.

CounterStateProvider.razor:

@using Microsoft.AspNetCore.Components.Server.ProtectedBrowserStorage
@inject ProtectedSessionStorage ProtectedSessionStore

@if (isLoaded)
{
    <CascadingValue Value="this">
        @ChildContent
    </CascadingValue>
}
else
{
    <p>Loading...</p>
}

@code {
    private bool isLoaded;

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

    public int CurrentCount { get; set; }

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            isLoaded = true;
            await LoadStateAsync();
            StateHasChanged();
        }
    }

    private async Task LoadStateAsync()
    {
        var result = await ProtectedSessionStore.GetAsync<int>("count");
        CurrentCount = result.Success ? result.Value : 0;
        isLoaded = true;
    }

    public async Task IncrementCount()
    {
        CurrentCount++;
        await ProtectedSessionStore.SetAsync("count", CurrentCount);
    }
}

@using Microsoft.AspNetCore.ProtectedBrowserStorage
@inject ProtectedSessionStorage ProtectedSessionStore

@if (isLoaded)
{
    <CascadingValue Value="this">
        @ChildContent
    </CascadingValue>
}
else
{
    <p>Loading...</p>
}

@code {
    private bool isLoaded;

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

    public int CurrentCount { get; set; }

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            isLoaded = true;
            await LoadStateAsync();
            StateHasChanged();
        }
    }

    private async Task LoadStateAsync()
    {
        CurrentCount = await ProtectedSessionStore.GetAsync<int>("count");
        isLoaded = true;
    }

    public async Task IncrementCount()
    {
        CurrentCount++;
        await ProtectedSessionStore.SetAsync("count", CurrentCount);
    }
}

<Routes @rendermode="InteractiveServer" />

<CounterStateProvider>
    <Router ...>
        ...
    </Router>
</CounterStateProvider>

Wrapped components receive and can modify the persisted counter state. The following Counter component implements the pattern:

@page "/counter"

<p>Current count: <strong>@CounterStateProvider?.CurrentCount</strong></p>
<button @onclick="IncrementCount">Increment</button>

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

    private async Task IncrementCount()
    {
        if (CounterStateProvider is not null)
        {
            await CounterStateProvider.IncrementCount();
        }
    }
}

The preceding component isn't required to interact with ProtectedBrowserStorage, nor does it deal with a "loading" phase.

In general, the state provider parent component pattern is recommended:

• To consume state across many components.
• If there's just one top-level state object to persist.

To persist many different state objects and consume different subsets of objects in different places, it's better to avoid persisting state globally.

User state created in a Blazor WebAssembly app is held in the browser's memory.

Examples of user state held in browser memory include:

• The hierarchy of component instances and their most recent render output in the rendered UI.
• The values of fields and properties in component instances.
• Data held in dependency injection (DI) service instances.
• Values set through JavaScript interop calls.

When a user closes and reopens their browser or reloads the page, user state held in the browser's memory is lost.

Persist state across browser sessions

Generally, maintain state across browser sessions where users are actively creating data, not simply reading data that already exists.

To preserve state across browser sessions, the app must persist the data to some other storage location than the browser's memory. State persistence isn't automatic. You must take steps when developing the app to implement stateful data persistence.

Data persistence is typically only required for high-value state that users expended effort to create. In the following examples, persisting state either saves time or aids in commercial activities:

• Multi-step web forms: It's time-consuming for a user to re-enter data for several completed steps of a multi-step web form if their state is lost. A user loses state in this scenario if they navigate away from the form and return later.
• Shopping carts: Any commercially important component of an app that represents potential revenue can be maintained. A user who loses their state, and thus their shopping cart, may purchase fewer products or services when they return to the site later.

An app can only persist app state. UIs can't be persisted, such as component instances and their render trees. Components and render trees aren't generally serializable. To persist UI state, such as the expanded nodes of a tree view control, the app must use custom code to model the behavior of the UI state as serializable app state.

Where to persist state

Common locations exist for persisting state:

• Server-side storage
• URL
• Browser storage
• In-memory state container service

Server-side storage

For permanent data persistence that spans multiple users and devices, the app can use independent server-side storage accessed via a web API. Options include:

• Blob storage
• Key-value storage
• Relational database
• Table storage

After data is saved, the user's state is retained and available in any new browser session.

Because Blazor WebAssembly apps run entirely in the user's browser, they require additional measures to access secure external systems, such as storage services and databases. Blazor WebAssembly apps are secured in the same manner as single-page applications (SPAs). Typically, an app authenticates a user via OAuth/OpenID Connect (OIDC) and then interacts with storage services and databases through web API calls to a server-side app. The server-side app mediates the transfer of data between the Blazor WebAssembly app and the storage service or database. The Blazor WebAssembly app maintains an ephemeral connection to the server-side app, while the server-side app has a persistent connection to storage.

For more information, see the following resources:

• Call a web API from an ASP.NET Core Blazor app
• Secure ASP.NET Core Blazor WebAssembly
• Blazor Security and Identity articles

For more information on Azure data storage options, see the following:

• Azure Databases
• Azure Storage Documentation

URL

For transient data representing navigation state, model the data as a part of the URL. Examples of user state modeled in the URL include:

• The ID of a viewed entity.
• The current page number in a paged grid.

The contents of the browser's address bar are retained if the user manually reloads the page.

For information on defining URL patterns with the @page directive, see ASP.NET Core Blazor routing and navigation.

Browser storage

For transient data that the user is actively creating, a commonly used storage location is the browser's localStorage and sessionStorage collections:

• localStorage is scoped to the browser's instance. If the user reloads the page or closes and reopens the browser, the state persists. If the user opens multiple browser tabs, the state is shared across the tabs. Data persists in localStorage until explicitly cleared. The localStorage data for a document loaded in a "private browsing" or "incognito" session is cleared when the last "private" tab is closed.
• sessionStorage is scoped to the browser tab. If the user reloads the tab, the state persists. If the user closes the tab or the browser, the state is lost. If the user opens multiple browser tabs, each tab has its own independent version of the data.

Generally, sessionStorage is safer to use. sessionStorage avoids the risk that a user opens multiple tabs and encounters the following:

• Bugs in state storage across tabs.
• Confusing behavior when a tab overwrites the state of other tabs.

localStorage is the better choice if the app must persist state across closing and reopening the browser.