ASP.NET Core updates in .NET 8 Preview 4

Daniel Roth

.NET 8 Preview 4 is now available and includes many great new improvements to ASP.NET Core.

Here’s a summary of what’s new in this preview release:

  • Blazor
    • Streaming rendering with Blazor components
    • Handling form posts with Blazor SSR
    • Route to named elements in Blazor
    • Webcil packaging for Blazor WebAssembly apps
  • API authoring
    • Expanded support for form binding in minimal APIs
    • API project template includes .http file
  • Native AOT
    • Logging and exception handling in compile-time generated minimal APIs
    • ASP.NET Core top-level APIs annotated for trim warnings
    • Reduced app size with configurable HTTPS support
    • Worker Service template updates
    • Additional default services configured in the slim builder
    • API template JSON configuration changes
    • Support for JSON serialization of compiler-generated IAsyncEnumerable unspeakable types
  • Authentication and authorization
    • Identity API endpoints
    • Improved support for custom authorization policies with IAuthorizationRequirementData
  • ASP.NET Core metrics

For more details on the ASP.NET Core work planned for .NET 8 see the full ASP.NET Core roadmap for .NET 8 on GitHub.

Get started

To get started with ASP.NET Core in .NET 8 Preview 4, install the .NET 8 SDK.

If you’re on Windows using Visual Studio, we recommend installing the latest Visual Studio 2022 preview. Visual Studio for Mac support for .NET 8 previews isn’t available at this time.

Upgrade an existing project

To upgrade an existing ASP.NET Core app from .NET 8 Preview 3 to .NET 8 Preview 4:

  • Update the target framework of your app to net8.0.
  • Update all Microsoft.AspNetCore.* package references to 8.0.0-preview.4.*.
  • Update all Microsoft.Extensions.* package references to 8.0.0-preview.4.*.

See also the full list of breaking changes in ASP.NET Core for .NET 8.


Streaming rendering with Blazor components

You can now stream content updates on the response stream when using server-side rendering (SSR) with Blazor in .NET 8. Streaming rendering can improve the user experience for server-side rendered pages that need to perform long-running async tasks in order to render fully.

For example, to render a page you might need to make a long running database query or an API call. Normally all async tasks executed as part of rendering a page must complete before the rendered response can be sent, which can delay loading the page. Streaming rendering initially renders the entire page with placeholder content while async operations execute. Once the async operations completes, the updated content is sent to the client on the same response connection and then patched by Blazor into the DOM. The benefit of this approach is that the main layout of the app renders as quickly as possible and the page is updated as soon as the content is ready.

To enable streaming rendering, you’ll first need to add the new Blazor script.

<script src="_framework/blazor.web.js" suppress-error="BL9992"></script>

Note that if you’re adding this script to a Blazor component, like your layout component, you’ll need to add the suppress-error="BL9992" attribute to avoid getting an error about using script tags in components.

Then, to enable streaming rendering for a specific component, use the [StreamRendering(true)] attribute. Typically this is done using the @attribute Razor directive:

@page "/fetchdata"
@using BlazorSSR.Data
@inject WeatherForecastService ForecastService
@attribute [StreamRendering(true)]

<PageTitle>Weather forecast</PageTitle>

<h1>Weather forecast</h1>

@if (forecasts is null)
    // Render weather forecasts

@code {
    private string message;

    protected override async Task OnInitializedAsync()
        forecasts = await ForecastService.GetForecastAsync(DateOnly.FromDateTime(DateTime.Now));

The component will now initially render without waiting for any async tasks to complete using placeholder content (“Loading…”). As the async tasks complete, the updated content is streamed to the response and then patched by Blazor into the DOM.

Blazor streaming rendering

Handling form posts with Blazor SSR

You can now use Blazor components to handle form posts with server-side rendering.

To enable handling form submissions from the server, you first need to setup a model binding context using the CascadingModelBinder component. An easy way to do this is in the main layout of your app:


To define a form in Blazor you use the existing EditForm component and the corresponding input components, like InputText, InputSelect, etc.

The EditForm component will render a standard HTML form element, so you can use the method attribute to specify if the form should send POST request. The EditForm event handlers are not supported with GET requests.

When the form is submitted, the request will be routed to the corresponding page and then handled by the form with the matching form handler name as specified by the handler query string parameter. You can specify the form handler name for an EditForm using the FormHandlerName attribute. If there’s only one form on the page, then you don’t need to specify a name. You can then handle the form submission using the EditForm events.

Support for model binding and validating the request data hasn’t been implemented yet (it’s coming!), but you can manually handle the request data using the FormDataProvider service. The FormDataProvider.Entries property provides access to the form data and the FormDataProvider.Name property specifies the intended form handler.

Here’s what a simple server-side rendered form in Blazor looks like:

@inject FormDataProvider FormData

<EditForm method="POST" Model="exampleModel" OnValidSubmit="HandleSubmit">
    <InputText @bind-Value="exampleModel.Name" />
    <button type="submit">Submit</button>

@code {
    ExampleModel exampleModel = new();

    protected override void OnInitialized()
        // Manually model bind the form data using the FormDataProvider service
        if (FormData.Entries.TryGetValue("Name", out var nameValues))
            exampleModel.Name = nameValues.First();

    void HandleSubmit()
        // Handle the submitted form data

    public class ExampleModel
        public string? Name { get; set; }

Route to named elements in Blazor

Blazor now supports using client-side routing to navigate to a specific HTML element on a page using standard URL fragments. If you specify an identifier for an HTML element using the standard id attribute, Blazor will correctly scroll to that element when the URL fragment matches the element identifier.

Webcil packaging for Blazor WebAssembly apps

You can now try out the new Webcil package with Blazor WebAssembly apps. Webcil is web-friendly packaging of .NET assemblies that removes any content specific to native Windows execution to avoid issues when deploying to environments that block the download or use of .dll files.

To enable use of Webcil for your Blazor WebAssembly apps, add the WasmEnableWebcil property to your project file:


If you encounter issues with using .webcil files in your environment, please let us know by creating an issue on GitHub.

API authoring

Expanded support for form binding in minimal APIs

This preview introduces expanded support for binding to form types in minimal APIs. Form-based parameters are now inferred without the need of the FromForm attribute. Support for form-based parameters includes: IFormCollection, IFormFile, and IFormFileCollection. OpenAPI metadata is inferred for form parameters to support integration with Swagger UI.

The code sample below showcases implementing a minimal API that handles file uploads by leveraging inferred binding from the IFormFile type.

var app = WebApplication.Create();

string GetOrCreateFilePath(string fileName, string filesDirectory = "uploadFiles")
    var directoryPath = Path.Combine(app.Environment.ContentRootPath, filesDirectory);
    return Path.Combine(directoryPath, fileName);

async Task UploadFileWithName(IFormFile file, string fileSaveName)
    var filePath = GetOrCreateFilePath(fileSaveName);
    await using var fileStream = new FileStream(filePath, FileMode.Create);
    await file.CopyToAsync(fileStream);

app.MapPost("/upload", async (IFormFile file) => {
    var fileSaveName = Guid.NewGuid().ToString("N") + Path.GetExtension(file.FileName);
    await UploadFileWithName(file, fileSaveName);
    return TypedResults.Ok("File uploaded successfully!");


This feature is supported in minimal APIs that use runtime-based code generation and in minimal APIs leveraging the new compile-time code generation for Native AOT scenarios.

Note: It’s important to be defensive against XSRF attacks when implementing forms in an application. This code sample outlines how to use the anti-forgery services in ASP.NET to support the generation and validation of anti-forgery tokens in minimal APIs.

API project template includes .http file

The API project template (generated via dotnet new api) now includes an .http file that can be used to send requests to the endpoints defined within the application from the new HTTP editor in Visual Studio.

@MyApi_HostAddress = http://localhost:5233

GET {{MyApi_HostAddress}}/todos/
Accept: application/json


GET {{MyApi_HostAddress}}/todos/1
Accept: application/json


Native AOT

Logging and exception handling in compile-time generated minimal APIs

Minimal APIs generated at runtime support automatically logging (or throwing exceptions in Development environments) when parameter binding fails. In this preview, we introduce the same support for APIs generated at compile-time via the Request Delegate Generator (RDG).

Consider the following API endpoints where we’ve enabled using the RDG by setting <EnableRequestDelegateGenerator>true</EnableRequestDelegateGenerator>:

var app = WebApplication.Create();

app.MapGet("/hello/{name}", (string name)
    => $"Hello {name}!");
app.MapGet("/age", (DateTime birthDate)
    => $"You're about {DateTime.Now.Year - birthDate.Year} years old!");


Sending the following request will throw a BadHttpRequestException since the required name parameter is not provided in the route or query string.

curl "http://localhost:5056/hello"
Microsoft.AspNetCore.Http.BadHttpRequestException: Required parameter "string name" was not provided from route or query string.
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

Similarly, sending a request to the /age endpoint with an unparsable birthDate value will throw an exception.

curl "http://localhost:5056/age?birthDate=invalidDate"
Microsoft.AspNetCore.Http.BadHttpRequestException: Failed to bind parameter "DateTime birthDate" from "invalidDate".
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

ASP.NET Core top-level APIs annotated for trim warnings

To help developers understand which features are incompatible with Native AOT, we’ve annotated the main entry points to subsystems that do not work reliably with Native AOT. When these methods are called from an application with Native AOT enabled, developers will get a warning. For example, the following code snippet will produce a warning at the invocation of AddControllers to indicate that this API is not trim-safe.

Native AOT warnings for top-level APIs

Reduced app size with configurable HTTPS support

In Preview 4, we’ve further reduced Native AOT binary size for apps that don’t need HTTPS or HTTP/3 support. This is quite common for apps that run behind a TLS termination proxy (e.g. hosted on Azure).

When you use the new WebApplication.CreateSlimBuilder, by default this functionality won’t be included. It can be re-added by calling builder.WebHost.UseKestrelHttpsConfiguration() or builder.WebHost.UseQuic(), respectively.

As a result of these and other changes, we can update our table from Preview 3:

We ran a simple ASP.NET Core API app in our benchmarking lab to compare the differences in app size, memory use, startup time, and CPU load, published with and without native AOT:

Publish kind Startup time (ms) App size (MB)
Default 169 88.5
Native AOT – Preview 3 34 11.3
Native AOT – Preview 4 32 9.3

Note the 2 MB drop in app size.

You can explore these and more metrics on our public benchmarks dashboard.

Worker Service template updates

The Worker Service templates in ASP.NET Core (available via dotnet new worker) now include support for the --aot flag to enable creating a worker service project with AOT publishing enabled.

dotnet new worker -o WorkerWithAot --aot

The templates have also been updated to leverage the simplified HostApplicationBuilder for configuring the application host.

using WorkerWithAot;

var builder = Host.CreateApplicationBuilder(args);

var host = builder.Build();

Additional default services configured in the slim builder

The WebApplication.CreateSlimBuilder API, introduced in .NET 8 Preview 3, initializes the essential features in an app to minimize its deployed size. In .NET 8 Preview 4, we’ve update the SlimBuilder to include the following features for an improved development experience, while still maintaining a total app size below 10 MB.

  • JSON file configuration for appsettings.json and appsettings.{EnvironmentName}.json
  • User secrets configuration
  • Console logging
  • Logging configuration

API template JSON configuration changes

We introduced the new API project template in .NET 8 Preview 3. In Preview 4, projects created with this template using the --aot option, have changed to insert the app’s source-generated JsonSerializationContext into the beginning of the JsonSerializerOptions.TypeInfoResolverChain. The previously generated code used the now-obsolete JsonSerializerOptions.AddContext<T> API and any project created with the Preview 3 version of the template should be updated to call the new API.

You can read more about the new JsonSerializerOptions.TypeInfoResolverChain API in the .NET 8 Preview 4 blog post.

Support for JSON serialization of compiler-generated IAsyncEnumerable<T> unspeakable types

Support for JSON serialization of IAsyncEnumerable<T> implementations implemented by the C# compiler is now supported, opening up their use in ASP.NET Core projects configured to publish native AOT. This is useful in scenarios where your route handler returns the result of calling an API that utilizes IAsyncEnumerable<T> and yield return to asynchonously return an enumeration, e.g. materializing rows from a database query (example).

You can read more about the JSON serializer support for unspeakable types in the .NET 8 Preview 4 blog post.

Authentication and Authorization

Identity API endpoints

We’re excited to introduce MapIdentityApi<TUser>() which is an extension method that adds two new API endpoints (/register and /login). The main goal of the MapIdentityApi is to make it easy for developers to use ASP.NET Core Identity for authentication in JavaScript-based single page apps (SPA) or Blazor apps. Instead of using the default UI provided by ASP.NET Core Identity, which is based on Razor Pages, MapIdentityApi adds JSON API endpoints that are more suitable for SPA apps and non-browser apps.

In addition to user registration and login, the identity API endpoints will support features like two-factor authentication and email verification in upcoming previews. You can find a list of planned features in the issues labeled feature-token-identity on the ASP.NET Core GitHub repository.

The following shows the Program.cs of an ASP.NET Core app that uses MapIdentityApi to enable both opaque bearer token and cookie authentication. To individually enable cookie or token authentication, you can call the existing AddCookie or the new AddBearerToken AuthenticationBuilder extension methods directly. Both are done for you by the AddIdentityApiEndpoints method below:

// usings ... 

var builder = WebApplication.CreateBuilder(args);


    options => options.UseSqlite(builder.Configuration["ConnectionString"]));


var app = builder.Build();


app.MapGet("/requires-auth", (ClaimsPrincipal user) => $"Hello, {user.Identity?.Name}!").RequireAuthorization();


// public class ApplicationDbContext : IdentityDbContext<IdentityUser> ...

On the client, you can call the /register endpoint as follows assuming httpClient, username and password are already initialized in a .NET console app:

// Email confirmation will be added later.
// The request body is: { "username": "<username>", "password": "<password>" }
await httpClient.PostAsJsonAsync("/identity/register", new { username, password });

And you can sign in and get an opaque bearer token using the /login endpoint:

// 2fa flow will be added later.
// The request body is: { "username": "<username>", "password": "<password>" }
var loginResponse = await httpClient.PostAsJsonAsync("/identity/login", new { username, password });

// loginResponse is similar to the "Access Token Response" defined in the OAuth 2 spec
// {
//   "token_type": "Bearer",
//   "access_token": "...",
//   "expires_in": 3600
// }
// refresh token is likely to be added later
var loginContent = await loginResponse.Content.ReadFromJsonAsync<JsonElement>();
var accessToken = loginContent.GetProperty("access_token").GetString();

httpClient.DefaultRequestHeaders.Authorization = new("Bearer", accessToken);
Console.WriteLine(await httpClient.GetStringAsync("/requires-auth"));

Or, if you want to get a cookie instead, you can set ?cookieMode=true in the /login query string:

// HttpClientHandler.UseCookies is true by default on supported platforms.
// The request body is: { "username": "<username>", "password": "<password>" }
await httpClient.PostAsJsonAsync("/identity/login?cookieMode=true", new { username, password });

Console.WriteLine(await httpClient.GetStringAsync("/requires-auth"));

We’re looking forward to getting your feedback on our early work to improve the identity experience for SPA and mobile applications.

Improved support for custom authorization policies with IAuthorizationRequirementData

Prior to this preview, adding a parameterized authorization policy to an endpoint required writing a lot of code.

  • Implementing an AuthorizeAttribute for each policy
  • Implementing an AuthorizationPolicyProvider to process a custom policy from a string-based contract
  • Implementing an AuthorizationRequirement for the policy
  • Implementing an AuthorizationHandler for each requirement

A partial implementation of a custom parameterized policy is below. The unabbreviated version contains the complete code.

var builder = WebApplication.CreateBuilder();

builder.Services.AddSingleton<IAuthorizationPolicyProvider, MinimumAgePolicyProvider>();
builder.Services.AddSingleton<IAuthorizationHandler, MinimumAgeAuthorizationHandler>();

var app = builder.Build();



public class GreetingsController : Controller
    public string Hello(ClaimsPrincipal user) => $"Hello {(user.Identity?.Name ?? "world")}!";

class MinimumAgeAuthorizeAttribute : AuthorizeAttribute { }

class MinimumAgePolicyProvider : IAuthorizationPolicyProvider { }

class MinimumAgeRequirement : IAuthorizationRequirement { }

class MinimumAgeAuthorizationHandler : AuthorizationHandler<MinimumAgeRequirement> { }

This preview introduces the IAuthorizationRequirementData interface which allows the attribute definition to also specify the requirements associated with the authorization policy. By leveraging this change, we can reimplement our custom authorization policy with fewer lines of code. The unabbreviated version contains the complete code.

var builder = WebApplication.CreateBuilder();

builder.Services.AddSingleton<IAuthorizationHandler, MinimumAgeAuthorizationHandler>();

var app = builder.Build();



public class GreetingsController : Controller
    public string Hello(ClaimsPrincipal user) => $"Hello {(user.Identity?.Name ?? "world")}!";

class MinimumAgeAuthorizeAttribute : AuthorizeAttribute, IAuthorizationRequirement, IAuthorizationRequirementData
    public MinimumAgeAuthorizeAttribute(int age) => Age =age;
    public int Age { get; }

    public IEnumerable<IAuthorizationRequirement> GetRequirements()
        yield return this;

class MinimumAgeAuthorizationHandler : AuthorizationHandler<MinimumAgeAuthorizeAttribute>
    protected override Task HandleRequirementAsync(AuthorizationHandlerContext context, MinimumAgeAuthorizeAttribute requirement) { ... }

ASP.NET Core metrics

Metrics are measurements reported over time and are most often used to monitor the health of an app and to generate alerts. For example, a counter that reports failed HTTP requests could be displayed in dashboards or generate alerts when failures pass a threshold.

This preview adds new metrics throughout ASP.NET Core using System.Diagnostics.Metrics. Metrics is a modern API for reporting and collecting information about your apps.

Metrics offers a number of improvements compared to existing event counters:

  • New kinds of measurements with counters, gauges and histograms
  • Powerful reporting with multi-dimensional values
  • Integration into the wider cloud native eco-system by aligning with OpenTelemetry standards

Metrics have been added for ASP.NET Core hosting, Kestrel and SignalR. Expect more APIs across .NET to get metrics in the future.

If you’re interested in trying out metrics, we’ve put together Grafana dashboards that report ASP.NET Core metrics collected by Prometheus. You can get the dashboards at the aspnetcore-grafana repository and import them into your own Grafana environment.

ASP.NET Core Grafana dashboard

Give feedback

We hope you enjoy this preview release of ASP.NET Core in .NET 8. Let us know what you think about these new improvements by filing issues on GitHub.

Thanks for trying out ASP.NET Core!


Comments are closed. Login to edit/delete your existing comments

  • Tim Altonji 0

    Your articles detailing and summarizing the developments in ASP.NET core are terrific. So thank you for your blog posts. We’re porting a large C# WinForms app to Blazor on DotNet7. Using C# and Blazor we’re able to reuse a lot of code that we otherwise would not have been able to reuse had we elected to use another web application framework such as angular/react, etc.

    One issue we’re having though is that our WinForms app is multithreaded. We’re trying to figure out how to adapt it to the single threaded environment in Blazor WASM.

    A previous blog post of yours discussed the experimental .NET multithreading on WebAssembly in DotNet7.
    “WebAssembly multithreading (experimental)”

    But I don’t think this made it into the final DotNet7 release because when we try to start a thread we get a PlatformNotSupportedException.
    System.PlatformNotSupportedException: Operation is not supported on this platform.
    at System.Threading.Thread.ThrowIfNoThreadStart(Boolean internalThread)
    at System.Threading.Thread.Start(Boolean captureContext, Boolean internalThread)
    at System.Threading.Thread.Start()

    I see no mention of the WebAssembly multithreading in DotNet8. Did it get dropped?
    Thank you,

    • Daniel RothMicrosoft employee 1

      Hi Tim. We started work on the .NET runtime support for multithreading on WebAssembly in .NET 7, but it wasn’t completed or integrated into Blazor. We’re actively working on support for multithreading in Blazor WebAssembly apps for .NET 8: You can see our full ASP.NET Core & Blazor roadmap here: Please note that multithreading on WebAssembly is based on shared array buffers, so it will require running the app with cross origin isolation.

  • Ezequiel Regaldo 0

    I have the feeling of minimals apis should never exist, they are like nerfed version of controllers, same with blazor. It feels so redundant. Ok there is SSR and CSR but youre using a socket for send states refresh and doesnt scale well .. and i prefer web components compatibility, btw nice try to be modern. But AOT compiling is not good enought implemented, why i have to make a different version for get AOT ? Not worth, java can do it without code changes and reach better performance, no, im not java lover and i hate to compare it but i like C# and i dont want to see it losing in the future (still waiting for minimal api data validation)

    • Daniel RothMicrosoft employee 0

      Hi Ezequiel. We’re tracking adding support for parameter validation for minimal APIs here: Be sure the give the issue a 👍 if this is something you’re interested in.

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