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

Daniel Roth

.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)" />
        <h1>Page not found</h1>
        <p>Sorry, but there's nothing here!</p>

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)" />
        <LayoutView Layout="typeof(MainLayout)">
            <h1>Page not found</h1>
            <p>Sorry, but there's nothing here!</p>

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)" />
            <LayoutView Layout="@typeof(MainLayout)">
                <p>Sorry, there's nothing at this address.</p>

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)">
                <p>Nope, nope!</p>

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:

    AdditionalAssemblies="new[] { typeof(Component1).Assembly }>

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>
    @foreach (var user in Service.Users)

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.

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!


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

  • Peter albanese 0

    Quick note to those upgrading. Once you update Visual Studio to 2019 16.3 Preview, you don’t need to seperately install .NET Core 3 Preview 9, it appears this version of .NET Core is already included with Visual Studio 16.3

  • Alexandre Reyes 0

    @Daniel where is selector option?preview-8: .MapBlazorHub<App>(selector: “app”)

    • Daniel RothMicrosoft employee 0

      The selector based overload to `MapBlazorHub` is no longer needed and has been removed. Instead, you just need a single call to `MapBlazorHub()` and then use `Html.RenderComponentAsync` with the desired RenderMode.

      • Alexandre Reyes 0

        But I want to disable server side rendering @Daniel, i don’t like to keep validating if javascript is available.

        • Daniel RothMicrosoft employee 0

          Blazor Server apps are generally rendered on the server, so I’m not sure I understand what you mean by disabling it. Could you please clarify what you mean?

          Also, what specifically do you not want to validate? 

          • Daniel RothMicrosoft employee 0

            You can disable prerendering by using RenderMode.Server, which means that only a marker will be put in the initial HTML that will later be updated when the client connection has been established. This is very similar to how things worked when using a static HTML file.

            JS interop should be available in OnAfterRender. However, please note that JS interop calls may still fail in a Blazor Server app  due to other concerns like loss of network connectivity.

          • Guillaume ZAHRA 0

            Actually, i think removing the overload present in preview8 with MapBlazorHub is a mistake.

            In previous versions, you were able to:

            • Use client-side “index.html” file in server-side mode instead of the CSHTML file.
            • Use another component than “App.razor” as root component.

            Now there is some differences between the client-side and server-side on this point as:

            • Use client-side “index.html” file in server-side mode instead of the CSHTML file. => should still be ok with MapFallbackToClientSideBlazor, even if i didn’t re-tested.
            • Use another component than “App.razor” as root component.  => NOT OK

            But you can still do this apparently on the client-side Startup side.
            It seem to me inconsistent.

            The main problem on my side is that i was able to inherit from App.razor, adding some custom logic and then use my inherited component at startup easily, without decorating any layout or other rendering thing, for managing to load my BlazorMobileComponent class at startup for BlazorMobile.
            Now i cannot inherit this behavior on the server-side project type because of this breaking change.
            Well, i think i will have to search another easy way to do that.

          • Guillaume ZAHRA 0

            Just to clarify: You can actually do what was present before (even loading the component you want at Startup) in pure server-side mode, but you can’t with client-side fallback, as you cannot inject any @(await Html.RenderComponentAsync<App>(RenderMode.ServerPrerendered)) .

            EDIT: I ended to updating my plugin MSBuild task in order to generate a valid starting *.cshtml file sync from the index.html file in order to workaround this difference.

          • Daniel RothMicrosoft employee 0

            You should be able to render whatever component you want using RenderComponentAsync<TComponent>(…). You can also render multiple different components this way on different parts of the page (this was specifically enabled in Preview 9).

            It would be great to understand your requirements for static HTML in Blazor Server apps. Could you please open an issue on on that with details about your scenario and requirements?

          • Guillaume ZAHRA 0

            Thanks for you reply Dan.
            Actually, i think there is something missing with theses new change, and i think it is maybe an edge case that has been forgotten.
            I posted my issues and my actual workaround here:

  • Peter albanese 0

    Can someone clarify the use case difference between the Microsoft.AspNetCore.Mvc.Rendering.RenderMode Enums? Most notably, the difference between Html.RenderComponentAsync<App>(RenderMode.ServerPrerendered) and Html.RenderComponentAsync<App>(RenderMode.Server)

    • Daniel RothMicrosoft employee 0

      RenderMode.Server will only render a marker where the component should be eventually rendered once the connection has been established.

      RenderMode.ServerPrerendered will prerender the component on the server and add the marker. When the connection gets established, the component is then rerendered and setup for interactivity.

  • FillTable Admin 0

    For Authorization, need to add @using Microsoft.AspNetCore.Components.Authorization to top level _Imports.razor file

    • Daniel RothMicrosoft employee 0

      Thanks! I’ve added this to the migrations steps.

      • Mike Foster 0

        Edit – I’ve got things working. But as noted below the Microsoft.AspNetCore.Components.Authorization isn’t available through the NuGet manager. I had to add it to the .csproj file manually as a reference.

  • FillTable Admin 0

    Is OnAfterRenderAsync() replaced? 

    • Daniel RothMicrosoft employee 0

      It’s now OnAfterRenderAsync(bool firstRender)

      • FillTable Admin 0

        Thank you! 

  • Steve Buchok 0

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

    </Layout> should be </LayoutView> ... pretty sure ;)

    • Daniel RothMicrosoft employee 0

      Whoops! Thanks! Should be fixed now.

  • Bob Evoy 0

    No “@using Microsoft.AspNetCore.Components.Authorization” for client side blazor?
    previously followed
    and can’t seem to get “AuthenticationStateProvider” to work clientside?

    • Bob Evoy 0

      nm… i could not find the package on nuget… but just added it to my package references and it worked…
      <PackageReference Include=”Microsoft.AspNetCore.Components.Authorization” Version=”3.0.0-preview9.19424.4″ />

    • Daniel RothMicrosoft employee 0

      In Blazor WebAssembly apps you also need to add the Microsoft.AspNetCore.Components.Authorization package. I’ve updated the migration steps accordingly.

  • Mr. Charles 0

    Hello!! What would be the recommended way to read the parameters from a URL if a blazor component is inside a razor page?
    I have a razor page for example:
    How can I read these 2 parameters from inside a blazor component without parsing the URL myself?  Until now the razor page passed the parameters using stateful prerendering but now what would be the recommended way to do it?


  • Maxim Rybkov 0

    After updating SDK and Hosting to Preview 9 getting this error on the form submission InvalidDataException: Form key length limit 2147483647 or value length limit 2147483647 exceeded.Rolling back to Preview 8 solves the issue.

    • Daniel RothMicrosoft employee 0

      Thanks for trying out Preview 9! We’ll need some more details on your app to figure out what’s going on. Could you please open an issue on with details on how to reproduce the issue so that we can investigate?

  • Lars vT 0

    As the update to Preview 16.3 automatically installed the Preview9, how can I rollback to Preview8 to build my current blazor apps without the need to upgrade everything now asap?Problem:Seems to require Microsoft.CodeAnalysis.Common (>= 3.3.0) – but this nuget package is not available, only a “beta2” which wont be accepted. See belowFehler NU1605 Ein Downgrade des Pakets “Microsoft.CodeAnalysis.Common” von 3.3.0 auf 3.3.0-beta2-final wurde festgestellt. Verweisen Sie direkt aus dem Projekt auf das Paket, um eine andere Version auszuwählen. ProjectRatingApp -> Microsoft.VisualStudio.Web.CodeGeneration.EntityFrameworkCore 3.0.0-preview9-19453-02 -> Microsoft.VisualStudio.Web.CodeGeneration.Core 3.0.0-preview9-19453-02 -> Microsoft.VisualStudio.Web.CodeGeneration.Templating 3.0.0-preview9-19453-02 -> Microsoft.CodeAnalysis.Razor 3.0.0-preview9.19423.3 -> Microsoft.CodeAnalysis.Common (>= 3.3.0) ProjectRatingApp -> Microsoft.CodeAnalysis.Common (>= 3.3.0-beta2-final)

    • Krasimir Ivanov 0

      did you find any solution on intalling the previous version of visual studio?

    • Krasimir Ivanov 0

      I managed to solve the issue by first uninstalling vs and then downloading it again from: because they seem to not have updated the download version (it is still 16.2). This may change soon though.

    • Daniel RothMicrosoft employee 0

      Microsoft.CodeAnalysis.Common 3.3.0 is available on, it’s just unlisted. So restoring it as a dependency should work fine. Did you maybe have an issue with your nuget feed configuration?

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