ASP.NET Core updates in .NET 5 Preview 8

Daniel Roth

.NET 5 Preview 8 is now available and is ready for evaluation. Here’s what’s new in this release:

  • Azure Active Directory authentication with Microsoft.Identity.Web
  • CSS isolation for Blazor components
  • Lazy loading in Blazor WebAssembly
  • Updated Blazor WebAssembly globalization support
  • New InputRadio Blazor component
  • Set UI focus in Blazor apps
  • Influencing the HTML head in Blazor apps
  • Control Blazor component instantiation
  • Protected browser storage
  • Model binding and validation with C# 9 record types
  • Improvements to DynamicRouteValueTransformer
  • Auto refresh with dotnet watch
  • Console Logger Formatter
  • JSON Console Logger

See the .NET 5 release notes for additional details and known issues.

Get started

To get started with ASP.NET Core in .NET 5 Preview 8 install the .NET 5 SDK.

You need to use Visual Studio 2019 16.8 Preview 2 or newer to use .NET 5 Preview 8. .NET 5 is also supported with the latest preview of Visual Studio for Mac. To use .NET 5 with Visual Studio Code, install the latest version of the C# extension.

Upgrade an existing project

To upgrade an existing ASP.NET Core app from .NET 5 Preview 7 to .NET 5 Preview 8:

  • Update all Microsoft.AspNetCore.* package references to 5.0.0-preview.8.*.
  • Update all Microsoft.Extensions.* package references to 5.0.0-preview.8.*.
  • Update System.Net.Http.Json package references to 5.0.0-preview.8.*.

That’s it! You should be all ready to go.

See also the full list of breaking changes in ASP.NET Core for .NET 5.

What’s new?

Azure Active Directory authentication with Microsoft.Identity.Web

The ASP.NET Core project templates now integrate with Microsoft.Identity.Web to handle authentication with Azure Activity Directory (Azure AD). The Microsoft.Identity.Web package provides a better experience for authentication through Azure AD as well as an easier way to access Azure resources on behalf of your users, including Microsoft Graph. Check out the Microsoft.Identity.Web sample that take you from a simple login through multi-tenancy, using Azure APIs, using Microsoft Graph, and protecting your own APIs. Microsoft.Identity.Web will be generally available alongside .NET 5.

CSS isolation for Blazor components

Blazor now supports defining CSS styles that are scoped to a given component. Component specific CSS styles make it easier to reason about the styles in your app and to avoid unintentional side effects from global styles. You define component specific styles in a .razor.css file the matches the name of the .razor file for the component.

For example, let’s say you have a component MyComponent.razor file that looks like this:


<h1>My Component</h1>

<ul class="cool-list">

You can then define a MyComponent.razor.css with the styles for MyComponent:


h1 {
    font-family: 'Comic Sans MS'

.cool-list li {
    color: red;

The styles in MyComponent.razor.css will only get applied to the rendered output of MyComponent; the h1 elements rendered by other components, for example, are not affected.

To write a selector in component specific styles that affects child components, use the ::deep combinator.

.parent ::deep .child {
    color: red;

By using the ::deep combinator, only the .parent class selector is scoped to the component; the .child class selector is not scoped, and will match content from child components.

Blazor achieves CSS isolation by rewriting the CSS selectors as part of the build so that they only match markup rendered by the component. Blazor then bundles together the rewritten CSS files and makes the bundle available to the app as a static web asset at the path _framework/scoped.styles.css.

While Blazor doesn’t natively support CSS preprocessors like Sass or Less, you can still use CSS preprocessors to generate component specific styles before they are rewritten as part of the building the project.

Lazy loading in Blazor WebAssembly

Lazy loading enables you to improve the load time of your Blazor WebAssembly app by deferring the download of specific app dependencies until they are required. Lazy loading may be helpful if your Blazor WebAssembly app has large dependencies that are only used for specific parts of the app.

To delay the loading of an assembly, you add it to the BlazorWebAssemblyLazyLoad item group in your project file:

Assemblies marked for lazy loading must be explicitly loaded by the app before they are used. To lazy load assemblies at runtime, use the LazyAssemblyLoader service:

@inject LazyAssemblyLoader LazyAssemblyLoader

@code {
    var assemblies = await LazyAssemblyLoader.LoadAssembliesAsync(new string[] { "Lib1.dll" });

To lazy load assemblies for a specific page, use the OnNavigateAsync event on the Router component. The OnNavigateAsync event is fired on every page navigation and can be used to lazy load assemblies for a particular route. You can also lazily load the entire page for a route by passing any lazy loaded assemblies as additional assemblies to the Router.

The following examples demonstrates using the LazyAssemblyLoader service to lazy load a specific dependency (Lib1.dll) when the user navigates to /page1. The lazy loaded assembly is then added to the additional assemblies list passed to the Router component, so that it can discover any routable components in that assembly.

@using System.Reflection
@using Microsoft.AspNetCore.Components.Routing
@using Microsoft.AspNetCore.Components.WebAssembly.Services
@inject LazyAssemblyLoader LazyAssemblyLoader

<Router AppAssembly="typeof(Program).Assembly" AdditionalAssemblies="lazyLoadedAssemblies" OnNavigateAsync="@OnNavigateAsync">
            <p>Loading the requested page...</p>
    <Found Context="routeData">
        <RouteView RouteData="@routeData" DefaultLayout="typeof(MainLayout)" />
        <LayoutView Layout="typeof(MainLayout)">
            <p>Sorry, there is nothing at this address.</p>

@code {
    private List<Assembly> lazyLoadedAssemblies = 
        new List<Assembly>();

    private async Task OnNavigateAsync(NavigationContext args)
        if (args.Path.EndsWith("/page1"))
            var assemblies = await LazyAssemblyLoader.LoadAssembliesAsync(new string[] { "Lib1.dll"  });

Updated Blazor WebAssembly globalization support

.NET 5 Preview 8 reintroduces globalization support for Blazor WebAssembly based on International Components for Unicode (ICU). Part of introducing the ICU data and logic is optimizing these payloads for download size. This work is not fully completed yet. We expect to reduce the size of the ICU data in future .NET 5 preview updates.

New InputRadio Blazor component

Blazor in .NET 5 now includes built-in InputRadio and InputRadioGroup components. These components simplify data binding to radio button groups with integrated validation alongside the other Blazor form input components.

Opinion about blazor:
<InputRadioGroup @bind-Value="survey.OpinionAboutBlazor">
    @foreach (var opinion in opinions)
        <div class="form-check">
            <InputRadio class="form-check-input" id="" Value="" />
            <label class="form-check-label" for="">@opinion.label</label>

Set UI focus in Blazor apps

Blazor now has a FocusAsync convenience method on ElementReference for setting the UI focus on that element.

<button @onclick="() => textInput.FocusAsync()">Set focus</button>
<input @ref="textInput"/>

Control Blazor component instantiation

You can now control how Blazor components are instantiated by providing your own IComponentActivator service implementation.

Thank you Mladen Macanović for this Blazor feature contribution!

Influencing the HTML head in Blazor apps

Use the new Title, Link, and Meta components to programmatically set the title of a page and dynamically add link and meta tags to the HTML head in a Blazor app.

To use the new Title, Link, and Meta components:

  1. Add a package reference to the Microsoft.AspNetCore.Components.Web.Extensions package.
  2. Include a script reference to _content/Microsoft.AspNetCore.Components.Web.Extensions/headManager.js.
  3. Add a @using directive for Microsoft.AspNetCore.Components.Web.Extensions.Head.

The following example programmatically sets the page title to show the number of unread user notifications, and updates the page icon a as well:

@if (unreadNotificationsCount > 0)
    var title = $"Notifications ({unreadNotificationsCount})";
    <Title Value="title"></Title>
    <Link rel="icon" href="icon-unread.ico" />

Protected browser storage

In Blazor Server apps, you may want to persist app state in local or session storage so that the app can rehydrate it later if needed. When storing app state in the user’s browser, you also need to ensure that it hasn’t been tampered with.

Blazor in .NET 5 helps solve this problem by providing two new services: ProtectedLocalStorage and ProtectedSessionStorage. These services help you store state in local and session storage respectively, and they take care of protecting the stored data using the ASP.NET Core data protection APIs.

To use the new protected browser storage services:

  1. Add a package reference to Microsoft.AspNetCore.Components.Web.Extensions.
  2. Configure the services by calling services.AddProtectedBrowserStorage() from Startup.ConfigureServcies.
  3. Inject either ProtectedLocalStorage and ProtectedSessionStorage into your component implementation:
    @inject ProtectedLocalStorage ProtectedLocalStorage
    @inject ProtectedSessionStorage ProtectedSessionStorage
  4. Use the desired service to get, set, and delete state asynchronously:
    private async Task IncrementCount()
        await ProtectedLocalStorage.SetAsync("count", ++currentCount);

Model binding and validation with C# 9 record types

You can now use C# 9 record types with model binding in an MVC controller or a Razor Page. Record types are a great way to model data being transmitted over the wire.

For example, the PersonController below uses the Person record type with model binding and form validation:

public record Person([Required] string Name, [Range(0, 150)] int Age);

public class PersonController
   public IActionResult Index() => View();

   public IActionResult Index(Person person)
          // ...


@model Person

Name: <input asp-for="Model.Name" />
<span asp-validation-for="Model.Name" />

Age: <input asp-for="Model.Age" />
<span asp-validation-for="Model.Age" />

Improvements to DynamicRouteValueTransformer

ASP.NET Core in .NET Core 3.1 introduced DynamicRouteValueTransformer as a way to use use a custom endpoint to dynamically select an MVC controller action or a razor page. In .NET 5 Preview 8 you can now pass state to your DynamicRouteValueTransformer and filter the set of endpoints chosen.

Auto refresh with dotnet watch

In .NET 5, running dotnet watch on an ASP.NET Core project will now both launch the default browser and auto refresh the browser as you make changes to your code. This means you can open an ASP.NET Core project in your favorite text editor, run dotnet watch run once, and then focus on your code changes while the tooling handles rebuilding, restarting, and reloading your app. We expect to bring the auto refresh functionality to Visual Studio in the future as well.

Console Logger Formatter

We’ve made improvements to the console log provider in the Microsoft.Extensions.Logging library. Developers can now implement a custom ConsoleFormatter to exercise complete control over formatting and colorization of the console output. The formatter APIs allow for rich formatting by implementing a subset of the VT-100 (supported by most modern terminals) escape sequences. The console logger can parse out escape sequences on unsupported terminals allowing you to author a single formatter for all terminals.

JSON Console Logger

In addition to support for custom formatters, we’ve also added a built-in JSON formatter that emits structured JSON logs to the console. You can switch from the default simple logger to JSON, add to following snippet to your Program.cs:

public static IHostBuilder CreateHostBuilder(string[] args) =>
+       .ConfigureLogging(logging =>
+       {
+           logging.AddJsonConsole(options =>
+           {
+               options.JsonWriterOptions = new JsonWriterOptions() { Indented = true };
+           });
+       })
        .ConfigureWebHostDefaults(webBuilder =>

Once enabled, log messages emitted to the console are now JSON formatted.

  "EventId": 0,
  "LogLevel": "Information",
  "Category": "Microsoft.Hosting.Lifetime",
  "Message": "Now listening on: https://localhost:5001",
  "State": {
    "Message": "Now listening on: https://localhost:5001",
    "address": "https://localhost:5001",
    "{OriginalFormat}": "Now listening on: {address}"

Give feedback

We hope you enjoy this release of ASP.NET Core in .NET 5! We are eager to hear about your experiences with this latest .NET 5 release. Let us know what you think by filing issues on GitHub.

Thanks for trying out ASP.NET Core!


Discussion is closed. Login to edit/delete existing comments.

  • Craig Johnson 0

    Blazor is winning all the things!

    • Andrew Dinh 0


  • Patrick Hintermayer 0

    I’m excited about dotnet watch and can’t wait to use it with Visual Studio. Although I’m a bit skeptical about the time it takes for a full refresh (detecting change, rebuilding, refreshing page etc.). Most (web) developers will compare the times with watch tools for Node / JS. Hopefully there won’t be a big difference in the future.

    PS: there are some code formatting errors at Model binding and validation with C# 9 record types

    • Daniel RothMicrosoft employee 0

      Hi Patrick.

      I’m excited about dotnet watch and can’t wait to use it with Visual Studio. Although I’m a bit skeptical about the time it takes for a full refresh (detecting change, rebuilding, refreshing page etc.). Most (web) developers will compare the times with watch tools for Node / JS. Hopefully there won’t be a big difference in the future.

      We are doing work to make rebuild times faster. We also expect to look at true hot reload support in .NET 6.

      PS: there are some code formatting errors at Model binding and validation with C# 9 record types

      Thanks for bringing this to our attention! Should be fixed now.

  • Matt Beckius 0

    Does dotnet watch pickup changes made to css?

    • Gábor Szabó 0

      Most probably, currently it detects any and all file changes, regardless of whether they are relevant to the build or not. I started using dotnet watch with Blazor today, and now this update supports hot reload(ish) out of the box, nice 🙂

  • Jack Della 0

    Blazor on preview7 and 8 do not build for me:
    error NETSDK1073: The FrameworkReference ‘Microsoft.AspNetCore.App’ was not recognized.
    Not referencing that thing anywhere…

    • Ivan Montilla Paniagua 0

      Hi Jack!

      Same problem for me, this problem is because your Blazor project is referencing some project that use Microsoft.NET.Sdk.Web that references all framework. Change SDK of your Blazor project dependency to Microsoft.NET.Sdk.Razor.

      More info:

  • Lee McPherson 0

    CSS isolation works pretty well. (Once I figured out the difference between the paths for server-side and client-side) However, I can’t get any ::after and ::before pseudo elements to work in the css. Not sure if I’m doing something wrong or if this is a bug…

    • Yasser SASSI 0

      Do you have to recompile your project to apply the css changes or you only refresh the page ?

      • Lee McPherson 0

        Honestly, I haven’t noticed. I’ve been recompiling every time. However, I think you do need to recompile. It is creating a new file (scoped.styles.css) when you do it.

  • Mihai Dumitru 0

    When will virtualization be included?

    • Daniel RothMicrosoft employee 0

      We’ve added a Virtualize component to help with handling virtualization in .NET 5 RC1, which should be available in a couple of weeks.. You can try out a .NET 5 RC1 build from

  • Manoj Kumar 0

    After updating to preview 8, blazor webassembly is throwing error like this System.Security.Cryptography is not supported on Browser
    same working fine in 3.2

  • Indudhar Gowda 0

    kodus to steve sanderson @stevensanderson for his brainchild Alias Blazor..

  • Mi. Ri. 0

    Do the custom ConsoleFormatter and the JSON Console Logger also work with the WebAssemblyConsoleLogger (the default Logger implementation in Blazor WebAssembly)?

    • Safia AbdallaMicrosoft employee 0

      Unfortunately not. The ConsoleFormatter and JSON Console Logger are designed to work with terminal-based consoles. The WebAssemblyConsoleLogger is a wrapper around the JavaScript console in the browser (which is usually accessed via commands like console.log or console.warn).

      If you have any feedback on ways the web-based console logging experience can be improved, let us know with a suggestion at


  • Josh McCallMicrosoft employee 0

    FocusAsync but does it ScrollToAsync?

    • Daniel RothMicrosoft employee 0

      Hi Josh. I don’t believe that’s functionality that we provide currently, but if you’d like to see it added please file an issue with the feature suggestion:

Feedback usabilla icon