September 4th, 2019

ASP.NET Core and Blazor updates in .NET Core 3.0 Preview 9

Daniel Roth
Daniel Roth
Principal Product Manager

.NET Core 3.0 Preview 9 is now available and it contains a number of improvements and updates to ASP.NET Core and Blazor.

Here’s the list of what’s new in this preview:

  • Blazor event handlers and data binding attributes moved to Microsoft.AspNetCore.Components.Web
  • Blazor routing improvements
    • Render content using a specific layout
    • Routing decoupled from authorization
    • Route to components from multiple assemblies
  • Render multiple Blazor components from MVC views or pages
  • Smarter reconnection for Blazor Server apps
  • Utility base component classes for managing a dependency injection scope
  • Razor component unit test framework prototype
  • Helper methods for returning Problem Details from controllers
  • New client API for gRPC
  • Support for async streams in streaming gRPC responses

Please see the release notes for additional details and known issues.

Get started

To get started with ASP.NET Core in .NET Core 3.0 Preview 9 install the .NET Core 3.0 Preview 9 SDK.

If you’re on Windows using Visual Studio, install the latest preview of Visual Studio 2019.

.NET Core 3.0 Preview 9 requires Visual Studio 2019 16.3 Preview 3 or later.

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

dotnet new -i Microsoft.AspNetCore.Blazor.Templates::3.0.0-preview9.19424.4

Upgrade an existing project

To upgrade an existing ASP.NET Core app to .NET Core 3.0 Preview 9, follow the migrations steps in the ASP.NET Core docs.

Please also see the full list of breaking changes in ASP.NET Core 3.0.

To upgrade an existing ASP.NET Core 3.0 Preview 8 project to Preview 9:

  • Update all Microsoft.AspNetCore.* package references to 3.0.0-preview9.19424.4
  • In Blazor apps and libraries:
    • Add a using statement for Microsoft.AspNetCore.Components.Web in your top level _Imports.razor file (see Blazor event handlers and data binding attributes moved to Microsoft.AspNetCore.Components.Web below for details)
    • Add a using statement for Microsoft.AspNetCore.Components.Authorization in your top level _Imports.razor file. In Blazor WebAssembly apps also add a package reference to Microsoft.AspNetCore.Components.Authorization.
    • Update all Blazor component parameters to be public.
    • Update implementations of IJSRuntime to return ValueTask<T>.
    • Replace calls to MapBlazorHub<TComponent> with a single call to MapBlazorHub.
    • Update calls to RenderComponentAsync and RenderStaticComponentAsync to use the new overloads to RenderComponentAsync that take a RenderMode parameter (see Render multiple Blazor components from MVC views or pages below for details).
    • Update App.razor to use the updated Router component (see Blazor routing improvements below for details).
    • (Optional) Remove page specific _Imports.razor file with the @layout directive to use the default layout specified through the router instead.
    • Remove any use of the PageDisplay component and replace with LayoutView, RouteView, or AuthorizeRouteView as appropriate (see Blazor routing improvements below for details).
    • Replace uses of IUriHelper with NavigationManager.
    • Remove any use of @ref:suppressField.
    • Replace the previous RevalidatingAuthenticationStateProvider code with the new RevalidatingIdentityAuthenticationStateProvider code from the project template.
    • Replace Microsoft.AspNetCore.Components.UIEventArgs with System.EventArgs and remove the “UI” prefix from all EventArgs derived types (UIChangeEventArgs -> ChangeEventArgs, etc.).
    • Replace DotNetObjectRef with DotNetObjectReference.
    • Replace OnAfterRender() and OnAfterRenderAsync() implementations with OnAfterRender(bool firstRender) or OnAfterRenderAsync(bool firstRender).
    • Remove any usage of IComponentContext and move any logic that should not run during prerendering into OnAfterRender or OnAfterRenderAsync.
  • In gRPC projects:
    • Update calls to GrpcClient.Create with a call GrpcChannel.ForAddress to create a new gRPC channel and new up your typed gRPC clients using this channel.
    • Rebuild any project or project dependency that uses gRPC code generation for an ABI change in which all clients inherit from ClientBase instead of LiteClientBase. There are no code changes required for this change.
    • Please also see the grpc-dotnet announcement for all changes.

You should now be all set to use .NET Core 3.0 Preview 9!

Blazor event handlers and data binding attributes moved to Microsoft.AspNetCore.Components.Web

In this release we moved the set of bindings and event handlers available for HTML elements into the Microsoft.AspNetCore.Components.Web.dll assembly and into the Microsoft.AspNetCore.Components.Web namespace. This change was made to isolate the web specific aspects of the Blazor programming from the core programming model. This section provides additional details on how to upgrade your existing projects to react to this change.

Blazor apps

Open the application’s root _Imports.razor and add @using Microsoft.AspNetCore.Components.Web. Blazor apps get a reference to the Microsoft.AspNetCore.Components.Web package implicitly without any additional package references, so adding a reference to this package isn’t necessary.

Blazor libraries

Add a package reference to the Microsoft.AspNetCore.Components.Web package package if you don’t already have one. Then open the root _Imports.razor file for the project (create the file if you don’t already have it) and add @using Microsoft.AspNetCore.Components.Web.

Troubleshooting guidance

With the correct references and using statement for Microsoft.AspNetCore.Components.Web, event handlers like @onclick and @bind should be bold font and colorized as shown below when using Visual Studio.

Events and binding working in Visual Studio

If @bind or @onclick are colorized as a normal HTML attribute, then the @using statement is missing.

Events and binding not recognized

If you’re missing a using statement for the Microsoft.AspNetCore.Components.Web namespace, you may see build failures. For example, the following build error for the code shown above indicates that the @bind attribute wasn’t recognized:

CS0169  The field 'Index.text' is never used
CS0428  Cannot convert method group 'Submit' to non-delegate type 'object'. Did you intend to invoke the method?

In other cases you may get a runtime exception and the app fails to render. For example, the following runtime exception seen in the browser console indicates that the @onclick attribute wasn’t recognized:

Error: There was an error applying batch 2.
DOMException: Failed to execute 'setAttribute' on 'Element': '@onclick' is not a valid attribute name.

Add a using statement for the Microsoft.AspNetCore.Components.Web namespace to address these issues. If adding the using statement fixed the problem, consider moving to the using statement app’s root _Imports.razor so it will apply to all files.

If you add the Microsoft.AspNetCore.Components.Web namespace but get the following build error, then you’re missing a package reference to the Microsoft.AspNetCore.Components.Web package:

CS0234  The type or namespace name 'Web' does not exist in the namespace 'Microsoft.AspNetCore.Components' (are you missing an assembly reference?)

Add a package reference to the Microsoft.AspNetCore.Components.Web package to address the issue.

Blazor routing improvements

In this release we’ve revised the Blazor Router component to make it more flexible and to enable new scenarios. The Router component in Blazor handles rendering the correct component that matches the current address. Routable components are marked with the @page directive, which adds the RouteAttribute to the generated component classes. If the current address matches a route, then the Router renders the contents of its Found parameter. If no route matches, then the Router component renders the contents of its NotFound parameter.

To render the component with the matched route, use the new RouteView component passing in the supplied RouteData from the Router along with any desired parameters. The RouteView component will render the matched component with its layout if it has one. You can also optionally specify a default layout to use if the matched component doesn’t have one.

<Router AppAssembly="typeof(Program).Assembly">
    <Found Context="routeData">
        <RouteView RouteData="routeData" DefaultLayout="typeof(MainLayout)" />
    </Found>
    <NotFound>
        <h1>Page not found</h1>
        <p>Sorry, but there's nothing here!</p>
    </NotFound>
</Router>

Render content using a specific layout

To render a component using a particular layout, use the new LayoutView component. This is useful when specifying content for not found pages that you still want to use the app’s layout.

<Router AppAssembly="typeof(Program).Assembly">
    <Found Context="routeData">
        <RouteView RouteData="routeData" DefaultLayout="typeof(MainLayout)" />
    </Found>
    <NotFound>
        <LayoutView Layout="typeof(MainLayout)">
            <h1>Page not found</h1>
            <p>Sorry, but there's nothing here!</p>
        </LayoutView>
    </NotFound>
</Router>

Routing decoupled from authorization

Authorization is no longer handled directly by the Router. Instead, you use the AuthorizeRouteView component. The AuthorizeRouteView component is a RouteView that will only render the matched component if the user is authorized. Authorization rules for specific components are specified using the AuthorizeAttribute. The AuthorizeRouteView component also sets up the AuthenticationState as a cascading value if there isn’t one already. Otherwise, you can still manually setup the AuthenticationState as a cascading value using the CascadingAuthenticationState component.

<Router AppAssembly="@typeof(Program).Assembly">
    <Found Context="routeData">
        <AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
    </Found>
    <NotFound>
        <CascadingAuthenticationState>
            <LayoutView Layout="@typeof(MainLayout)">
                <p>Sorry, there's nothing at this address.</p>
            </LayoutView>
        </CascadingAuthenticationState>
    </NotFound>
</Router>

You can optionally set the NotAuthorized and Authorizing parameters of the AuthorizedRouteView component to specify content to display if the user is not authorized or authorization is still in progress.

<Router AppAssembly="@typeof(Program).Assembly">
    <Found Context="routeData">
        <AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)">
            <NotAuthorized>
                <p>Nope, nope!</p>
            </NotAuthorized>
        </AuthorizeRouteView>
    </Found>
</Router>

Route to components from multiple assemblies

You can now specify additional assemblies for the Router component to consider when searching for routable components. These assemblies will be considered in addition to the specified AppAssembly. You specify these assemblies using the AdditionalAssemblies parameter. For example, if Component1 is a routable component defined in a referenced class library, then you can support routing to this component like this:

<Router
    AppAssembly="typeof(Program).Assembly"
    AdditionalAssemblies="new[] { typeof(Component1).Assembly }>
    ...
</Router>

Render multiple Blazor components from MVC views or pages

We’ve reenabled support for rendering multiple components from a view or page in a Blazor Server app. To render a component from a .cshtml file, use the Html.RenderComponentAsync<TComponent>(RenderMode renderMode, object parameters) HTML helper method with the desired RenderMode.

RenderMode Description Supports parameters?
Static Statically render the component with the specified parameters. Yes
Server Render a marker where the component should be rendered interactively by the Blazor Server app. No
ServerPrerendered Statically prerender the component along with a marker to indicate the component should later be rendered interactively by the Blazor Server app. No

Support for stateful prerendering has been removed in this release due to security concerns. You can no longer prerender components and then connect back to the same component state when the app loads. We may reenable this feature in a future release post .NET Core 3.0.

Blazor Server apps also no longer require that the entry point components be registered in the app’s Configure method. Only a single call to MapBlazorHub() is required.

Smarter reconnection for Blazor Server apps

Blazor Server apps are stateful and require an active connection to the server in order to function. If the network connection is lost, the app will try to reconnect to the server. If the connection can be reestablished but the server state is lost, then reconnection will fail. Blazor Server apps will now detect this condition and recommend the user to refresh the browser instead of retrying to connect.

Blazor Server reconnect rejected

Utility base component classes for managing a dependency injection scope

In ASP.NET Core apps, scoped services are typically scoped to the current request. After the request completes, any scoped or transient services are disposed by the dependency injection (DI) system. In Blazor Server apps, the request scope lasts for the duration of the client connection, which can result in transient and scoped services living much longer than expected.

To scope services to the lifetime of a component you can use the new OwningComponentBase and OwningComponentBase<TService> base classes. These base classes expose a ScopedServices property of type IServiceProvider that can be used to resolve services that are scoped to the lifetime of the component. To author a component that inherits from a base class in Razor use the @inherits directive.

@page "/users"
@attribute [Authorize]
@inherits OwningComponentBase<Data.ApplicationDbContext>

<h1>Users (@Service.Users.Count())</h1>
<ul>
    @foreach (var user in Service.Users)
    {
        <li>@user.UserName</li>
    }
</ul>

Note: Services injected into the component using @inject or the InjectAttribute are not created in the component’s scope and will still be tied to the request scope.

Razor component unit test framework prototype

We’ve started experimenting with building a unit test framework for Razor components. You can read about the prototype in Steve Sanderson’s Unit testing Blazor components – a prototype blog post. While this work won’t ship with .NET Core 3.0, we’d still love to get your feedback early in the design process. Take a look at the code on GitHub and let us know what you think!

Helper methods for returning Problem Details from controllers

Problem Details is a standardized format for returning error information from an HTTP endpoint. We’ve added new Problem and ValidationProblem method overloads to controllers that use optional parameters to simplify returning Problem Detail responses.

[Route("/error")]
public ActionResult<ProblemDetails> HandleError()
{
    return Problem(title: "An error occurred while processing your request", statusCode: 500);
}

New client API for gRPC

To improve compatibility with the existing Grpc.Core implementation, we’ve changed our client API to use gRPC channels. The channel is where gRPC configuration is set and it is used to create strongly typed clients. The new API provides a more consistent client experience with Grpc.Core, making it easier to switch between using the two libraries.

// Old
using var httpClient = new HttpClient() { BaseAddress = new Uri("https://localhost:5001") };
var client = GrpcClient.Create<GreeterClient>(httpClient);

// New
var channel = GrpcChannel.ForAddress("https://localhost:5001");
var client = new GreeterClient(channel);

var reply = await client.GreetAsync(new HelloRequest { Name = "Santa" });

Support for async streams in streaming gRPC responses

gRPC streaming responses return a custom IAsyncStreamReader type that can be iterated on to receive all response messages in a streaming response. With the addition of async streams in C# 8, we’ve added a new extension method that makes for a more ergonomic API while consuming streaming responses.

// Old
while (await requestStream.MoveNext(CancellationToken.None))
{
  var message = requestStream.Current;
  // …
}

// New and improved
await foreach (var message in requestStream.ReadAllAsync())
{
  // …
}

Give feedback

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

Thanks for trying out ASP.NET Core and Blazor!

Category
.NET CoreASP.NETASP.NET CoreBlazor

Author

Daniel Roth
Daniel Roth
Principal Product Manager

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

116 comments

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  • Felipe Almeida

    Hey, just installed the latest version of vs along of the web assembly preview and created a webassembly project but the compiler does not found “mapFallbackToClientSideBlazor, along with other functions. Any help?

    • Daniel RothMicrosoft employee Author

      Hi Felipe,

      Please make sure you’ve followed all of the Get Started steps listed on https://blazor.net. You need the .NET Core 3.0 SDK (3.0.100), the latest version of the Blazor WebAssembly templates, and Visual Studio 2019 16.3. If you’ve checked that you have all of the right versions of everything then please open an issue on GitHub and we’ll take a look: https://github.com/aspnet/aspnetcore/issues.

  • Maciej Misztal

    @Daniel I may have missed this, but is the “Microsoft.AspNetCore” not a part of the ASP.NET Core 3.0? Latest published version is 2.2.0 ?

      • Ian Sharp

        does NuGet support Microsoft.NET.Sdk.Web ? When I search for Microsoft.NET.Sdk.Web in the NuGet package manager I am not finding a match.

      • Ian Sharp

        oh nevermind I see you are referring to the $projectname.csproj file and the fact that the definition is in there already… still getting the error mentioned on this thread though about UIKeyboardEventArgs – on Core 3.0.100 (not using preview)

      • Daniel RothMicrosoft employee Author

        UIKeyboardEventArgs was renamed to KeyboardEventArgs.

  • Riaan G

    Seems that NavigationManager has “BaseUri” instead of “GetBaseUri()”If you’re using :Blazored.LocalStorageBlazored.SessionStorage
    Just upgrade to the latest versions. The change Task<> to ValueTask<> has been updated I believe.Prev 8 to 9 was by far the easiest upgrade for me.Back to having fun!

    • Daniel RothMicrosoft employee Author

      Nice! That’s great to hear. And thank you for your work on the Blazored project!

  • Darrell Tunnell

    Nothing mentioned about this breaking change:
    In a .razor file, if you used a generic component with a type argument it used to be <MyComonent T=”SomeType” /> but that will now error out. You have to change it to <MyComonent TValue=”SomeType” />

  • Rajni Kumar

    After upgrading to preview 9 my ApiAuthenticationStateProvider class does not compile. I get error “no suitable method to override on GetAuthenticationStateAsync and NotifyAuthenticationStateChanged. I have installed package Microsoft.AspNetCore.Components.Authorization3.0.0-preview9.19424.4. Is there anything i am missing?

    • Daniel RothMicrosoft employee Author

      Hmm…are you perhaps also missing a using statement for Microsoft.AspNetCore.Components.Authorization? What kind of project are you building this with?

      • Rajni Kumar

        It is asp.net core hosted webassembly project. I have using statement Microsoft.AspNetCore.Components.Authorization but it turns gray

      • marouen kharraz

        I think you need to add “using Microsoft.AspNetCore.Components.Authorization” in the .cs file too and that would correct it.

  • Bartho Bernsmann

    Was Type property removed from ChangeEventArgs?

  • Charles Rouhier

    Right off the bat ... I'm new to Blazor and just learning it.  So far, I like what I see and think it will offer some big benefits for a number of projects we need to convert from Flash.  So with that, I've been trying to get the Blazor events to work now for a couple of weeks without success.  I just upgraded to Preview9 and they still don't work (Yes.  VS Preview 3.0).  I...

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    • Daniel RothMicrosoft employee Author

      Also could you please share what OS version and webbrowser you are using?

      • Charles Rouhier

        Daniel,  I believe I've found the issue.  It appears Blazor doesn't like IE.  I found a statemen on this page indicating you could only debug in Chrome or Edge. https://docs.microsoft.com/en-us/aspnet/core/blazor/debug?view=aspnetcore-3.0I switched to Edge and now they work.  I've been trying to get EventCallback's to work too, which led me going back to the basics, hopefully this solves the issues I've been having with the callback's.BTW, I don't know if you're aware, but the tree for...

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      • Daniel RothMicrosoft employee Author

        Yeah, Blazor Server apps can be made to work in IE, but you need to add the appropriate polyfills yourself. Checkout https://github.com/Daddoon/Blazor.Polyfill for a community based solution. Blazor WebAssembly apps are not supported at all in IE, because IE doesn't support WebAssembly.

        Thanks for bringing the Blazor docs issue to our attention! Unfortunately, I'm not able to reproduce this. The docs seem fine in both Edge and Edge (dev) as far as I can tell. Could...

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      • Charles Rouhier

        Thanks for the link.
        The Edge thing probably has something to do with where I work.  We have issues with other things in Edge, too.  So, I’ll just chalk it up to that for now.
        Thanks again for the assist.
        Chuck

      • Charles Rouhier

        We develop on Windows 10 VM’s and I’m using IE and/or Edge.  Right now IE.

    • Daniel RothMicrosoft employee Author

      Hi Charles,

      I’m sorry to hear you’re having issues! Could you please check the brower console and and the server logs to see if there are any errors that show up there when you click the Counter button?

      Thanks for taking the time to learn Blazor! Hopefully we’ll get you sorted out soon.

      • Charles Rouhier

        The Windows Logs/Application event log shows the following warning when the debug session starts up in VS:The directory specified for caching compressed content C:\inetpub\temp\IIS Temporary Compressed Files\BlazorServerTestApp AppPool is invalid. Static compression is being disabled.I don't see any errors in the event logs or browser console when I press the 'Click Me' button.  Someplace else you want me to look?
        I see one error in the browser console when I click on the Counter menu...

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      • Charles Rouhier

        I created the C:\inetpub\temp\IIS temporary Compressed Files structure, but the button still doesn’t work.  But the warning is gone.

      • AlanW

        This comment has been deleted.

  • Frédéric Taillefer

    Really enjoying blazor lately. Does this version has a json serializer that knows how to deserialize to class with no parameterless contructor (like KeyValuePair for instance ?). If not, will it be fixed in the final version ? tks

    • Daniel RothMicrosoft employee Author

      Hi Frédéric,

      It’s great to hear you are enjoying Blazor! Blazor uses the new System.Text.Json serializer for its JSON serialization needs. If you’re interested in seeing new features added to System.Text.Json, please submit your feedback as GitHub issues at https://github.com/dotnet/corefx/issues.&nbsp;

  • Enis Necipoğlu

    Injecting NavigationManager returns null. How can I use new NavigationManager ??

    • Enis Necipoğlu

      I’ve just solved, the library I tried to use it, is not updated to latest preview

  • Mark Shortt

    Hi
    I had been using Microsoft.AspNetCore.Blazor.Services.WebAssemblyUriHelper to get the base URI, but with Preview 9 the Microsoft.AspNetCore.Blazor.Services doesn’t appear to be present
    What’s the correct way to to get the base Uri in a Web Assembly app? 

    Thanks

    • Daniel RothMicrosoft employee Author

      NavigationManager is the new type for this.

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