ASP.NET Core updates in .NET 7 Preview 3

Daniel Roth

.NET 7 Preview 3 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:

  • Support for route handler filters in minimal APIs
  • Improved unit testability for minimal route handlers
  • Bind using TryParse in MVC and API controllers
  • New Results.Stream() overloads
  • Improved HTTP/2 performance when using many streams on a connection
  • New ServerReady event for measuring startup time
  • Developer exception page dark mode

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

Get started

To get started with ASP.NET Core in .NET 7 Preview 3, 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

Note: Building .NET 6 Blazor projects with the .NET 7 SDK and the .NET 7 WebAssembly build tools is currently not supported. This will be addressed in a future .NET 7 update: dotnet/runtime#65211.

Upgrade an existing project

To upgrade an existing ASP.NET Core app from .NET 7 Preview 2 to .NET 7 Preview 3:

  • Update all Microsoft.AspNetCore.* package references to 7.0.0-preview.3.*.
  • Update all Microsoft.Extensions.* package references to 7.0.0-preview.3.*.

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

Support for route handler filters in minimal APIs

In this preview, we introduce support for filters in route handlers in minimal applications. Filters are executed before the core route handler logic and can be used to inspect and modify handler parameters or intercept handler execution.

Filters can be registered onto a handler using a variety of strategies. For example, you can register a filter using a RouteHandlerFilterDelegate and the AddFilter extension method as follows:

string HelloName(string name) => $"Hello, {name}!";

app.MapGet("/hello/{name}", HelloName)
    .AddFilter(async (routeHandlerInvocationContext, next) =>
        var name = (string) routeHandlerInvocationContext.Parameters[0];
        if (name == "Bob")
            return Results.Problem("No Bob's allowed");
        return await next(routeHandlerInvocationContext);

Filters can also be registered via a filter factory strategy that gives access to a RouteHandlerContext which gives access to the MethodInfo associated with the handler and the metadata registered on the endpoint.

app.MapGet("/hello/{name}", HelloName)
    .AddFilter((routeHandlerContext, next) =>
        var parameters = routeHandlerContext.MethodInfo.GetParameters();
        var hasCorrectSignature = parameters.Length == 1 && parameters[0].ParameterType == typeof(string);
        return async (routeHandlerInvocationContext) =>
            if (hasCorrectSignature)
                var name = (string) routeHandlerInvocationContext.Parameters[0];
                if (name == "Bob")
                    return Results.Problem("No Bob's allowed");
            return await next(routeHandlerInvocationContext);

Finally, filters can implement the IRouteHandlerFilter interface and be resolved from DI or passed as an instance.

app.MapGet("/hello/{name}", HelloName)

Improved unit testability for minimal route handlers

IResult implementation types are now publicly available in the namespace Microsoft.AspNetCore.Http with the suffix HttpResult (OkObjectHttpResult, ProblemHttpResult, etc.). With these types, you can now more easily unit test your minimal route handlers when using named methods instead of lambdas.

public async Task GetTodoReturnsTodoFromDatabase()
    var todo = new Todo { Id = 42, Name = "Improve Results testability!" };
    var mockDb = new MockTodoDb(new[] { todo });

    var result = (OkObjectHttpResult)await TodoEndpoints.GetTodo(mockDb, todo.Id);

    Assert.Equal(200, result.StatusCode);

    var foundTodo = Assert.IsAssignableFrom<Models.Todo>(result.Value);
    Assert.Equal(id, foundTodo.Id);

public void CreateTodoWithValidationProblems()
    var newTodo = default(Todo);
    var mockDb = new MockTodoDb();

    var result = TodoEndpoints.CreateTodo(mockDb, newTodo);

    var problemResult = Assert.IsAssignableFrom<ProblemHttpResult>(result);
    Assert.Equal(400, problemResult.StatusCode);

Bind using TryParse in MVC and API Controllers

You can now bind controller action parameter values using a TryParse method that has one of the following signatures:

public static bool TryParse(string value, T out result);
public static bool TryParse(string value, IFormatProvider provider, T out result);

For example, the Get action in the following controller binds data from the query string using a TryParse method on the parameter type:

public class TryParseController : ControllerBase
    // GET /tryparse?data=MyName
    public ActionResult Get([FromQuery]CustomTryParseObject data) => Ok();

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

        public static bool TryParse(string s, out CustomTryParseObject result)
            if (s is null) 
                result = default;
                return false;

            result = new CustomTryParseObject { Name = s };
            return true;

New Results.Stream() overloads

We introduced new Results.Stream(...) overloads to accommodate scenarios where you need access to the underlying HTTP response stream without buffering. These overloads also improve cases where your API wants to stream data to the HTTP response stream, like from Azure Blob Storage. The example below demonstrates using Results.Stream() when doing image manipulation with ImageSharp.

app.MapGet("/process-image", async (HttpContext http) =>
    using var image = await Image.LoadAsync("puppy.jpeg");
    int width = image.Width / 2;
    int height = image.Height / 2;
    image.Mutate(x => x.Resize(width, height));
    http.Response.Headers.CacheControl = $"public,max-age={FromHours(24).TotalSeconds}";
    return Results.Stream(stream => image.SaveAsync(stream, PngFormat.Instance), "image/png");

Improved HTTP/2 performance when using many streams on a connection

We made a change in our HTTP/2 frame writing code that improves performance when there are multiple streams trying to write data on a single HTTP/2 connection. We now dispatch TLS work to the thread pool and more quickly release a write lock that other streams can acquire to write their data. The reduction in wait times can yield significant performance improvements in cases where there is contention for this write lock. A gRPC benchmark with 70 streams on a single connection (with TLS) showed a ~15% improvement in requests per second (RPS) with this change.

New ServerReady event for measuring startup time

If you use EventSource for metrics/diagnostics and want to measure the startup time of your ASP.NET Core app, you can now use the new ServerReady event in the Microsoft.AspNetCore.Hosting source that represents the point where your server is up and running.

Developer exception page dark mode

The ASP.NET Core developer exception page now supports dark mode:

ASP.NET Core developer exception page dark mode

Thank you @poke for contributing this improvement!

Give feedback

We hope you enjoy this preview release of ASP.NET Core in .NET 7. Let us know what you think about these new improvements by filing issues on GitHub.

Thanks for trying out ASP.NET Core!


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

  • Angius 0

    Unironically, I’m the most excited for the dark mode exception pages lol

    Now if only Swagger UI supported dark mode it’d be a dream. But seeing how their answer to a PR that adds dark mode was “we won’t add it and for a good reason, no, we won’t tell you the reason” I’m not holding my breath

  • Lohith GN 0


    in the section – if I use the code snippet as is, we have 2 issues:
    – It is not

    • but – it is
    • If we use Results.Problem() – we get an error IResult cannot be converted to ValueTask<Object?>

    I looked at examples on MinimalApiFilter by @captainsofia (who seems to be the dev for this feature) here:

    It has a completely different format of the usage:

    .AddFilter((routeHandlerContext, next) => async (context) =>
            if (context.HttpContext.Response.StatusCode == 400)
                return Results.Problem("New response", statusCode: 400);
            return await next(context);

    Notice the sub lambda. This code snippet works. Also, I know this is preview – but there is no docs to comb through to understand the feature.

    thoughts ?

    • Daniel RothMicrosoft employee 0

      Hi Lohith. Thanks for pointing out this issue! I’ve updated the blog post to fix the Results typo and to put in a workaround for We’ll get this fixed in the next preview. And docs are coming soon!

  • Peter Dräxler 0

    I’m curious – is there a dev team focused specifically on Blazor? Are you actively working on Blazor improvements right now? Or is Blazor scheduled for later?

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