aspnet-whats-new

What's new in ASP.NET for .NET 10 Preview 3

• Validation support in Minimal APIs
• Support for SSE (Server-Sent Events) in Minimal APIs
• Setting the Description in a ProducesResponseTypeAttribute works correctly for Minimal API
  • dotnet/aspnetcore#60539
• Add Microsoft.AspNetCore.OpenApi to the webapiaot template
  • dotnet/aspnetcore#60337

Validation support in Minimal APIs

Support for validation in minimal APIs is now available. This feature allows you to request validation of data sent to your API endpoints. When validation is enabled, the ASP.NET Core runtime will perform any validations defined on query, header, and route parameters, as well as on the request body. Validations can be defined using attributes in the System.ComponentModel.DataAnnotations namespace. Developers can customize the behavior of the validation system by:

• creating custom ValidationAttribute implementations
• implement the IValidatableObject interface for complex validation logic

When validation fails, the runtime will return a 400 Bad Request response with details of the validation errors.

To enable built-in validation support for minimal APIs, call the AddValidation extension method to register the required services into the service container for your application.

builder.Services.AddValidation();

The implementation automatically discovers types that are defined in minimal API handlers or as base types of types defined in minimal API handlers. Validation is then performed on these types by an endpoint filter that is added for each endpoint.

Validation can be disabled for specific endpoints by using the DisableValidation extension method.

app.MapPost("/products",
    ([EvenNumber(ErrorMessage = "Product ID must be even")] int productId, [Required] string name)
        => TypedResults.Ok(productId))
    .DisableValidation();

Support for Server-Sent Events (SSE)

ASP.NET Core now supports returning a ServerSentEvents result using the TypedResults.ServerSentEvents API. This feature is supported in both Minimal APIs and controller-based apps.

Server-Sent Events (SSE) is a server push technology that allows a server to send a stream of event messages to a client over a single HTTP connection. In .NET the event messages are represented as SseItem objects, which may contain an event type, an ID, and a data payload of type T.

The TypedResults class has a new static method called ServerSentEvents that can be used to return a ServerSentEvents result. The first parameter to this method is an IAsyncEnumerable<SseItem<T>> that represents the stream of event messages to be sent to the client.

The following example illustrates how to the TypedResults.ServerSentEvents API to return a stream of heart rate events as JSON objects to the client:

app.MapGet("/json-item", (CancellationToken cancellationToken) =>
{
    async IAsyncEnumerable<HeartRateEvent> GetHeartRate(
        [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        while (!cancellationToken.IsCancellationRequested)
        {
            var heartRate = Random.Shared.Next(60, 100);
            yield return HeartRateEvent.Create(heartRate);
            await Task.Delay(2000, cancellationToken);
        }
    }

    return TypedResults.ServerSentEvents(GetHeartRate(cancellationToken), eventType: "heartRate");
});

When OpenAPI document generation is enabled, endpoints that return a ServerSentEvents result will be documented with the text/event-stream content type.

Setting Response Descriptions in Minimal API

The Description parameter of the ProducesAttribute, ProducesResponseTypeAttribute, and ProducesDefaultResponseAttribute is now supported in Minimal APIs. This allows you to set a description for the response type in the OpenAPI document.

Here's an example:

app.MapGet("/weatherforecast",
    [ProducesResponseType<IEnumerable<WeatherForecast>>(StatusCodes.Status200OK, Description = "The service is healthy.")]
    () =>

And the generated OpenAPI will be

        "responses": {
          "200": {
            "description": "The weather forecast for the next 5 days.",
            "content": {

This was a community contribution by @sander1095. Thanks for this contribution!

Add Microsoft.AspNetCore.OpenApi to the webapiaot template

Support for OpenAPI document generation with the Microsoft.AspNetCore.OpenApi packagte is now included by default in the webapiaot project template. This support can be disabled if desired by using the --no-openapi flag when creating a new project.

This was a community contribution by @sander1095. Thanks for this contribution!