ASP.NET Core updates in .NET 7 Preview 1

Daniel Roth

.NET 7 Preview 1 is now available!. This is the first preview of the next major version of .NET, which will include the next wave of innovations for web development with ASP.NET Core.

In .NET 7 we plan to make broad investments across ASP.NET Core. Below are some of the areas we plan to focus on:

  • Performance: .NET 6 contained many performance improvements for ASP.NET Core, and we’ll do work to make ASP.NET Core even faster and more efficient in .NET 7.
  • HTTP/3: HTTP/3 support shipped as a preview feature in .NET 6. For .NET 7, we want to finish it and make it a supported feature that’s enabled by default. In future previews, you can expect to see advanced TLS features and more performance improvements in our HTTP/3 support.
  • Minimal APIs: Add support for endpoint filters and route grouping as core primitives for minimal APIs. Also simplify authentication and authorization configurations for APIs in general.
  • gRPC: We’re investing in gRPC JSON transcoding. This feature allows gRPC services to be called like RESTful HTTP APIs with JSON requests and responses.
  • SignalR: Add support for strongly-typed clients and returning results from client invocations.
  • Razor: We’ll make various improvements to the Razor compiler to improve performance, resiliency, and to facilitate improved tooling.
  • Blazor: After finishing Blazor Hybrid support for .NET MAUI, WPF, and Windows Forms, we’ll make broad improvements to Blazor including:
    • New .NET WebAssembly capabilities: mixed-mode AOT, multithreading, web crypto.
    • Enhanced Hot Reload support.
    • Data binding improvements.
    • More flexible prerendering.
    • More control over the lifecycle of Blazor Server circuits.
    • Improved support for micro frontends.
  • MVC: Improvements to endpoint routing, link generation, and parameter binding.
  • Orleans: The ASP.NET Core and Orleans teams are investigating ways to further align and integrate the Orleans distributed programming model with ASP.NET Core. Orleans 4 will ship alongside .NET 7 and focuses on simplicity, maintainability, and performance, including human readable stream identities and a new optimized, version-tolerant serializer.

For more details on the specific ASP.NET Core work planned for .NET 7 see the full ASP.NET Core roadmap for .NET 7 on GitHub.

.NET 7 Preview 1 is the first of many .NET 7 preview releases in preparation for the .NET 7 release in November 2022.

I joined James Montemagno on a recent episode of On .NET to break down all of what is coming in .NET 7 and ASP.NET Core in .NET 7:

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

  • Minimal API improvements:
    • IFormFile and IFormFileCollection Support
    • Bind the request body as a Stream or PipeReader
    • JSON options configuration
  • SignalR client source generator
  • Support for nullable models in MVC views and Razor Pages
  • Use JSON property names in validation errors
  • Improved console output for dotnet watch
  • Configure dotnet watch to always restart for rude edits
  • Use dependency injection in a ValidationAttribute
  • Faster header parsing and writing
  • gRPC JSON transcoding

Get started

To get started with ASP.NET Core in .NET 7 Preview 1, install the .NET 7 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 7 previews isn’t available yet but is coming soon.

To install the latest .NET WebAssembly build tools, run the following command from an elevated command prompt:

dotnet workload install wasm-tools

Upgrade an existing project

To upgrade an existing ASP.NET Core app from .NET 6 to .NET 7 Preview 1:

  • Update the target framework for your app to net7.0.
  • Update all Microsoft.AspNetCore.* package references to 7.0.0-preview.1.*.
  • Update all Microsoft.Extensions.* package references to 7.0.0-preview.1.*.

See also the full list of breaking changes in ASP.NET Core for .NET 7.

Minimal API improvements

IFormFile and IFormFileCollection Support

You can now handle file uploads in minimal APIs using IFormFile and IFormFileCollection:

app.MapPost("/upload", async(IFormFile file) =>
    using var stream = System.IO.File.OpenWrite("upload.txt");
    await file.CopyToAsync(stream); 
app.MapPost("/upload", async (IFormFileCollection myFiles) => { ... });

Using this feature with authentication requires anti-forgery support, which isn’t yet implemented. Anti-forgery support for minimal APIs is on our roadmap for .NET 7. Binding to IFormFile or IFormFileCollection when the request contains an Authorization header, a client certificate, or a cookie header is currently disabled. This limitation will be addressed as soon as we complete the work on anti-forgery support.

Thanks to @martincostello for contributing this feature.

Bind the request body as a Stream or PipeReader

You can now bind the request body as a Stream or PipeReader to efficiently support scenarios where the user has to ingest data and either store it to a blob storage or enqueue the data to a queue provider (Azure Queue, etc.) for later processing by a worker or cloud function. The following example shows how to use the new binding:

app.MapPost("v1/feeds", async (QueueClient queueClient, Stream body, CancellationToken cancellationToken) =>
    await queueClient.CreateIfNotExistsAsync(cancellationToken: cancellationToken);
    await queueClient.SendMessageAsync(await BinaryData.FromStreamAsync(body), cancellationToken: cancellationToken);

When using the Stream or PipeReader there are a few things to take into consideration:

  • When ingesting data, the Stream will be the same object as HttpRequest.Body.
  • The request body isn’t buffered by default. After the body is read, it’s not rewindable (you can’t read the stream multiple times).
  • The Stream/PipeReader are not usable outside of the minimal action handler as the underlying buffers will be disposed and/or reused.

JSON options configuration

We’re introducing a new and cleaner API, ConfigureRouteHandlerJsonOptions, to configure JSON options for minimal API endpoints. This new API avoids confusion with Microsoft.AspNetCore.Mvc.JsonOptions.

var builder = WebApplication.CreateBuilder(args);
builder.Services.ConfigureRouteHandlerJsonOptions(options =>
    //Ignore Cycles
    options.SerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles; 

SignalR client source generator

We’ve added a new client source generator for SignalR thanks to a contribution by @mehmetakbulut.

The SignalR client source generator generates strongly-typed sending and receiving code based on interfaces that you define. You can reuse the same interfaces from strongly-typed SignalR hubs on the client in place of the loosely-typed .On("methodName", ...) methods. Similarly, your hub can implement an interface for its methods and the client can use that same interface to call the hub methods.

To use the SignalR client source generator:

internal class HubServerProxyAttribute : Attribute

internal class HubClientProxyAttribute : Attribute
  • Add a static partial class to your project and write static partial methods with the [HubClientProxy] and [HubServerProxy] attributes
internal static partial class MyCustomExtensions
    public static partial IDisposable ClientRegistration<T>(this HubConnection connection, T provider);

    public static partial T ServerProxy<T>(this HubConnection connection);
  • Use the partial methods from your code!
public interface IServerHub
    Task SendMessage(string message);
    Task<int> Echo(int i);

public interface IClient
    Task ReceiveMessage(string message);

public class Client : IClient
    // Equivalent to HubConnection.On("ReceiveMessage", (message) => {});
    Task ReceiveMessage(string message)
        return Task.CompletedTask;

HubConnection connection = new HubConnectionBuilder().WithUrl("...").Build();
var stronglyTypedConnection = connection.ServerProxy<IServerHub>();
var registrations = connection.ClientRegistration<IClient>(new Client());

await stronglyTypedConnection.SendMessage("Hello world");
var echo = await stronglyTypedConnection.Echo(10);

Support for nullable models in MVC views and Razor Pages

We enabled defining a nullable page or view model to improve the experience when using null state checking with ASP.NET Core apps:

@model Product?

Use JSON property names in validation errors

When model validation produces a ModelErrorDictionary it will by default use the property name as the error key ("MyClass.PropertyName"). Model property names are generally an implementation detail, which can make them difficult to handle from single-page apps. You can now configure validation to use the corresponding JSON property names instead with the new SystemTextJsonValidationMetadataProvider (or NewtonsoftJsonValidationMetadataProvider when using Json.NET).

services.AddControllers(options =>
    options.ModelMetadataDetailsProviders.Add(new SystemTextJsonValidationMetadataProvider())

Improved console output for dotnet watch

We cleaned up the console output from dotnet watch to better align with the log out of ASP.NET Core and to stand out with 😮emojis😍.

Here’s an example of what the new output looks like:

C:BlazorApp> dotnet watch
dotnet watch 🔥 Hot reload enabled. For a list of supported edits, see
  💡 Press "Ctrl + R" to restart.
dotnet watch 🔧 Building...
  Determining projects to restore...
  All projects are up-to-date for restore.
  You are using a preview version of .NET. See:
  BlazorApp -> C:UsersdarothDesktopBlazorAppbinDebugnet7.0BlazorApp.dll
dotnet watch 🚀 Started
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: https://localhost:7148
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:5041
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:UsersdarothDesktopBlazorApp
dotnet watch ⌚ File changed: .PagesIndex.razor.
dotnet watch 🔥 Hot reload of changes succeeded.
info: Microsoft.Hosting.Lifetime[0]
      Application is shutting down...
dotnet watch 🛑 Shutdown requested. Press Ctrl+C again to force exit.

Configure dotnet watch to always restart for rude edits

Configure dotnet watch to always restart without a prompt for rude edits (edits that can’t be hot reloaded) by setting the DOTNET_WATCH_RESTART_ON_RUDE_EDIT environment variable to true.

Inject services into custom validation attributes in Blazor

You can now inject services into custom validation attributes in Blazor. Blazor will setup the ValidationContext so it can be used as a service provider.

public class SaladChefValidatorAttribute : ValidationAttribute
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        var saladChef = validationContext.GetRequiredService<SaladChef>();
        if (saladChef.ThingsYouCanPutInASalad.Contains(value.ToString()))
            return ValidationResult.Success;
        return new ValidationResult("You should not put that in a salad!");

// Simple class configured as a service for dependency injection
public class SaladChef
    public string[] ThingsYouCanPutInASalad = { "Strawberries", "Pineapple", "Honeydew", "Watermelon", "Grapes" };

Thank you @MariovanZeist for this contribution!

Faster header parsing and writing

We made several improvements to the performance of header parsing and writing for HTTP/2 and HTTP/3. See the following pull requests for details:

gRPC JSON transcoding

gRPC JSON transcoding allows gRPC services to be used like a RESTful HTTP APIs. Once configured, gRPC JSON transcoding allows you to call gRPC methods with familiar HTTP concepts:

  • HTTP verbs
  • URL parameter binding
  • JSON requests/responses

Of course gRPC can continue to be used as well. RESTful APIs for your gRPC services. No duplication!

ASP.NET Core has experimental support for this feature using a library called gRPC HTTP API. For .NET 7 we plan to make this functionality a supported part of ASP.NET Core. This functionality isn’t included with .NET 7 yet, but you can try out the existing experimental packages. For more information, see the gRPC HTTP API getting started documentation.

Give feedback

We hope you enjoy this preview release of ASP.NET Core in .NET 7 and that you’re as excited about about our roadmap for .NET 7 as we are! We’re eager to hear about your experiences with this release and your thoughts on the roadmap. Let us know what you think by filing issues on GitHub and commenting on the roadmap issue.

Thanks for trying out ASP.NET Core!