October 15th, 2019

ASP.NET Core updates in .NET Core 3.1 Preview 1

Daniel Roth
Daniel Roth
Principal Product Manager

.NET Core 3.1 Preview 1 is now available. This release is primarily focused on bug fixes, but it contains a few new features as well.

Here’s what’s new in this release for ASP.NET Core:

  • Partial class support for Razor components
  • Pass parameters to top-level components
  • Support for shared queues in HttpSysServer
  • Breaking changes for SameSite cookies

Alongside this .NET Core 3.1 Preview 1 release, we’ve also released a Blazor WebAssembly update, which now requires .NET Core 3.1. To use Blazor WebAssembly you will need to install .NET Core 3.1 Preview 1 as well as the latest preview of Visual Studio.

See the release notes for additional details and known issues.

Get started

To get started with ASP.NET Core in .NET Core 3.1 Preview 1 install the .NET Core 3.1 Preview 1 SDK.

If you’re on Windows using Visual Studio, for the best experience we recommend installing the latest preview of Visual Studio 2019 16.4. Installing Visual Studio 2019 16.4 will also install .NET Core 3.1 Preview 1, so you don’t need to separately install it. For Blazor development with .NET Core 3.1, Visual Studio 2019 16.4 is required.

To install the latest Blazor WebAssembly template run the following command:

dotnet new -i Microsoft.AspNetCore.Blazor.Templates::3.1.0-preview1.19508.20

Upgrade an existing project

To upgrade an existing ASP.NET Core 3.0 project to 3.1 Preview 1:

  • Update any projects targeting netcoreapp3.0 to target netcoreapp3.1
  • Update all Microsoft.AspNetCore.* package references to 3.1.0-preview1.19506.1

See also the full list of breaking changes in ASP.NET Core 3.1.

That’s it! You should now be all set to use .NET Core 3.1 Preview 1!

Partial class support for Razor components

Razor components are now generated as partial classes. You can author the code for a Razor component using a code-behind file defined as a partial class instead of defining all the code for the component in a single file.

For example, instead of defining the default Counter component with an @code block, like this:

Counter.razor

@page "/counter"

<h1>Counter</h1>

<p>Current count: @currentCount</p>

<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>

@code {
    int currentCount = 0;

    void IncrementCount()
    {
        currentCount++;
    }
}

You can now separate out the code into a code-behind file using a partial class like this:

Counter.razor

@page "/counter"

<h1>Counter</h1>

<p>Current count: @currentCount</p>

<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>

Counter.razor.cs

namespace BlazorApp1.Pages
{
    public partial class Counter
    {
        int currentCount = 0;

        void IncrementCount()
        {
            currentCount++;
        }
    }
}

Pass parameters to top-level components

Blazor Server apps can now pass parameters to top-level components during the initial render. Previously you could only pass parameters to a top-level component with RenderMode.Static. With this release, both RenderMode.Server and RenderModel.ServerPrerendered are now supported. Any specified parameter values are serialized as JSON and included in the initial response.

For example, you could prerender a Counter component with a specific current count like this:

@(await Html.RenderComponentAsync<Counter>(RenderMode.ServerPrerendered, new { CurrentCount = 123 }))

Support for shared queues in HttpSysServer

In addition to the existing behavior where HttpSysServer created anonymous request queues, we’ve added to ability to create or attach to an existing named HTTP.sys request queue. This should enable scenarios where the HTTP.Sys controller process that owns the queue is independent to the listener process making it possible to preserve existing connections and enqueued requests between across listener process restarts.

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            // ...
            webBuilder.UseHttpSys(options =>
            {
                options.RequestQueueName = "MyExistingQueue",
                options.RequestQueueMode = RequestQueueMode.CreateOrAttach
            })
        });

Breaking changes for SameSite cookies

This release updates the behavior of SameSite cookies in ASP.NET Core to conform to the latest standards being enforced by browsers. For details on these changes and their impact on existing apps see https://github.com/aspnet/Announcements/issues/390.

Give feedback

We hope you enjoy the new features in this preview release of ASP.NET Core! Please let us know what you think by filing issues on GitHub.

Thanks for trying out ASP.NET Core!

Category
.NET CoreASP.NETASP.NET CoreBlazor
Topics
.net frameworkannouncementasp.netdiagnosticsfundamentalsnugetreleases

Author

Daniel Roth
Daniel Roth
Principal Product Manager

Daniel Roth is a Program Manager on the ASP.NET team at Microsoft.

37 comments

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  • taurius litvinavicius

    Are there any plans to improve file handling? Have some native file picker/file stream? To me this seems to be the biggest issue, to really exploit the technology to the fullest extent.

      • taurius litvinavicius

        That does seem like a big restriction, but I still believe something is possible. After all, this would save millions of dollars in some cases.

  • Daniel Pach

    Hi,

    Since Blazor is such a new technology, it is sometimes challenging to figure out how to handle more complex UI scenarios. The official documentation covers basic concepts, but I'd like to see a few examples of best practices for designing large enterprise applications - e.g. proper way to handle communication between sibling/distant components, implementing multi-tabbed UI, how to implement state management for an application with hundreds of components etc. Keep in mind that there is...

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    • Rod Macdonald

      I hope MS releases something like the old IBuySpy app that was available for ASP.NET when it came out - that was a pretty useful and substantial reference app. I'm not sure if something similar was released at .NETConf this year. There was mention of an e-commerce demo on day 3, I think, but I missed those presentations and couldn't find a catch up anywhere. Ditto Ed Charbeneau's talk but Ed kindly sent me a...

      Read more
  • 志远 马

    Can I Translate Chinese?

  • Jing Zhe Shan

    Hey Daniel, this is very exciting, and I tried using blazor web assembly build few components, seems very promising 👏🎉 I just wondering can we start using this to build up sth, are there any significant breaking changes going to happen in next few months? Thanks.

    • Daniel RothMicrosoft employee Author

      Hi Jing,

      Blazor WebAssembly is still in preview, so some changes are expected. The good news is that Blazor WebAssembly uses exactly the same component model as Blazor Server, so we don't expect breaking changes with how you author components. However, there will likely be changes in how Blazor WebAssembly apps are hosted, like what happens on build & publish, how static files are handled, what the WebAssembly host looks like, etc.

      I hope this helps!

      Read more
      • Jing Zhe Shan

        That’s very helpful! Thanks.

  • Keith Frechette

    Is there any thought being given to allowing the code within a @code section to not be highlighted as it is in normal Razor code, without turning it off for all Razor code? While I appreciate the different background color of code when used in normal Razor code where there is lots of mixed HTML and C#, I find it distracting within a @code section, where the content is expected to be code. If anything,...

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    • Daniel RothMicrosoft employee Author

      That’s great feedback! Please open a Visual Studio Feedback issue with this feedback, and we’ll take a look.

  • FrankJeff

    Installed VS 16.4, I create a new Blazer Web Assembly project and build it. Then Ctrl + F5 to execute … nothing happens. If I copy paste the localhost link from the outpane the web app is displayed. Is anyone else seeing this?

    • Daniel RothMicrosoft employee Author

      Thanks for sharing with us this feedback! Could you please create a Visual Studio Feedback issue for the behavior you are seeing so we can investigate?

  • Jason Chen

    This release is kind of unstable?

    issue #1:
    I have several pages that will get System.NullReferenceException: ‘Object reference not set to an instance of an object.’ upon loading. It seems the html part is being rendered before OnInitializedAsync() completes.

    • Daniel RothMicrosoft employee Author

      Hi Jason,

      You get a NullReferenceException because you're initializing cProfile in OnInitializedAsync using an async call. The call to OnInitializedAsync completes while the GetProfile call is still in progress and the component is rendered while cProfile is still null. The common pattern for dealing with this is to put a null check in you rendering logic and display some placeholder UI while the async initialization logic is still in flight:

      @if (cProfile == null)
      {

      Loading...

      }
      else {Read more

  • Gauthier M.

    Ah ! Partial class is cool. I will move to this instead of inherits from customs componnent 🙂

  • Alex Bedel Miaffo Zefack

    Hi,
    It is great. But unfortunately my visual studio is not generating the backend file for the view. C-Sharp code and the view code are still in the single page after all configurations.
    The more, I am getting the following error:
    Severity Code Description Project File Line Suppression State Suppression State
    Error rzc generate exited...

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      • Alex Bedel Miaffo Zefack

        I am using the latest version of VS.
        I changed the configuration to use the previews sdk
        The more, I realized that when create a new project the version remain 3.0 but the templates are 3.1 previews.
        The problem pop up when changing the version from 3.0 to 3.1

        as you can see below

        Project Sdk="Microsoft.NET.Sdk.Web"

        PropertyGroup
        TargetFramework netstandard2.0 /TargetFramework
        RazorLangVersion 3.0/RazorLangVersion
        /PropertyGroup

        ItemGroup
        PackageReference...

        Read more
      • Daniel RothMicrosoft employee Author

        I am using the latest version of VS. I changed the configuration to use the previews sdk

        Which specific VS version are you using? You can find the version number under Help -> About. It sounds like you're using the latest stable version of VS (16.3) when you need to use the latest preview version when working with the .NET Core 3.1 SDK. Doing Blazor development with .NET Core 3.1 in VS 2019 16.3...

        Read more
  • Ben Hayat

    Daniel, I have one important question to ask;

    When we learned about the future of .Net (.Net 5), one thing that became clear that .Net 5 will be more compact and will replace Mono in the cases of devices, mobile, Xamarin and Client Blazor which will be available in Nov. of 2020
    Hearing that, has given me a lot motivation to consider Client Blazor with a smaller app's footprint.

    However, we also recently heard by Scott Hunter...

    Read more
    • Daniel RothMicrosoft employee Author

      Hi Ben,

      The initial release of Blazor WebAssembly will be based on the existing mono.wasm runtime. In the .NET 5 time frame we expect Blazor WebAssembly to start using the core framework libraries from .NET Core instead of the existing mono based implementation. Not much more is know about the WebAssembly runtime implementation for .NET 5 at this point. It's too early to know what impact .NET 5 will have on app size, but the the...

      Read more

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