ASP.NET Core and Blazor updates in .NET Core 3.0 Preview 6

Daniel Roth


.NET Core 3.0 Preview 6 is now available and it includes a bunch of new updates to ASP.NET Core and Blazor.

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

  • New Razor features: @attribute, @code, @key, @namespace, markup in @functions
  • Blazor directive attributes
  • Authentication & authorization support for Blazor apps
  • Static assets in Razor class libraries
  • Json.NET no longer referenced in project templates
  • Certificate and Kerberos Authentication
  • SignalR Auto-reconnect
  • Managed gRPC Client
  • gRPC Client Factory
  • gRPC Interceptors

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 6 install the .NET Core 3.0 Preview 6 SDK

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

For the latest client-side Blazor templates also install the latest Blazor extension from the Visual Studio Marketplace.

Upgrade an existing project

To upgrade an existing an ASP.NET Core app to .NET Core 3.0 Preview 6, 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 5 project to Preview 6:

  • Update Microsoft.AspNetCore.* package references to 3.0.0-preview6.19307.2
  • In Blazor apps:
    • Rename @functions to @code
    • Update Blazor specific attributes and event handlers to use the new directive attribute syntax (see below)
    • Remove any call to app.UseBlazor<TStartup>() and instead add a call to app.UseClientSideBlazorFiles<TStartup>() before the call to app.UseRouting(). Also add a call to endpoints.MapFallbackToClientSideBlazor<TStartup>("index.html") in the call to app.UseEndpoints().



app.UseEndpoints(endpoints =>





app.UseEndpoints(endpoints =>

New Razor features

We’ve added support for the following new Razor language features in this release.


The new @attribute directive adds the specified attribute to the generated class.

@attribute [Authorize]


The new @code directive is used in .razor files (not supported in .cshtml files) to specify a code block to add to the generated class as additional members. It’s equivalent to @functions, but now with a better name.

@code {
    int currentCount = 0;

    void IncrementCount()


The new @key directive attribute is used in .razor files to specify a value (any object or unique identifier) that the Blazor diffing algorithm can use to preserve elements or components in a list.

    @foreach (var flight in Flights)
        <DetailsCard @key="flight" Flight="@flight" />

To understand why this feature is needed, consider rendering a list of cards with flight details without this feature:

    @foreach (var flight in Flights)
        <DetailsCard Flight="@flight" />

If you add a new flight into the middle of the Flights list the existing DetailsCard instances should remain unaffected and one new DetailsCard should be inserted into the rendered output.

To visualize this, if Flights previously contained [F0, F1, F2], then this is the before state:

  • DetailsCard0, with Flight=F0
  • DetailsCard1, with Flight=F1
  • DetailsCard2, with Flight=F2

… and this is the desired after state, given we insert a new item FNew at index 1:

  • DetailsCard0, with Flight=F0
  • DetailsCardNew, with Flight=FNew
  • DetailsCard1, with Flight=F1
  • DetailsCard2, with Flight=F2

However, the actual after state this:

  • DetailsCard0, with Flight=F0
  • DetailsCard1, with Flight=FNew
  • DetailsCard2, with Flight=F1
  • DetailsCardNew, with Flight=F2

The system has no way to know that DetailsCard2 or DetailsCard3 should preserve their associations with their older Flight instances, so it just re-associates them with whatever Flight matches their position in the list. As a result, DetailsCard1 and DetailsCard2 rebuild themselves completely using new data, which is wasteful and sometimes even leads to user-visible problems (e.g., input focus is unexpectedly lost).

By adding keys using @key the diffing algorithm can associate the old and new elements or components.


Specifies the namespace for the generated class or the namespace prefix when used in an _Imports.razor file. The @namespace directive works today in pages and views (.cshtml) apps, but is now it is also supported with components (.razor).

@namespace MyNamespace

Markup in @functions and local functions

In views and pages (.cshtml files) you can now add markup inside of methods in the @functions block and in local functions.

@{ GreetPerson(person); }

@functions {
    void GreetPerson(Person person)
        <p>Hello, <em>@person.Name!</em></p>

Blazor directive attributes

Blazor uses a variety of attributes for influencing how components get compiled (e.g. ref, bind, event handlers, etc.). These attributes have been added organically to Blazor over time and use different syntaxes. In this Blazor release we’ve standardized on a common syntax for directive attributes. This makes the Razor syntax used by Blazor more consistent and predictable. It also paves the way for future extensibility.

Directive attributes all follow the following syntax where the values in parenthesis are optional:


Some valid examples:

<!-- directive -->
<div @directive>...</div>
<div @directive="value"></div>

<!-- directive with key/value arg-->
<div @directive:key>...</div>
<div @directive:key="value"></div>

<!-- directive with suffix -->
<div @directive-suffix></div>
<div @directive-suffix="value"></div>

<!-- directive with suffix and key/value arg-->
<div @directive-suffix:key></div>
<div @directive-suffix:key="value"></div>

All of the Blazor built-in directive attributes have been updated to use this new syntax as described below.

Event handlers

Specifying event handlers in Blazor now uses the new directive attribute syntax instead of the normal HTML syntax. The syntax is similar to the HTML syntax, but now with a leading @ character. This makes C# event handlers distinct from JS event handlers.

<button @onclick="@Clicked">Click me!</button>

When specifying a delegate for C# event handler the @ prefix is currently still required on the attribute value, but we expect to remove this requirement in a future update.

In the future we also expect to use the directive attribute syntax to support additional features for event handlers. For example, stopping event propagation will likely look something like this (not implemented yet, but it gives you an idea of scenarios now enabled by directive attributes):

<button @onclick="Clicked" @onclick:stopPropagation>Click me!</button>


<input @bind="myValue">...</input>
<input @bind="myValue" @bind:format="mm/dd">...</input>
<MyButton @bind-Value="myValue">...</MyButton>


<div @key="id">...</div>


<button @ref="myButton">...</button>

Authentication & authorization support for Blazor apps

Blazor now has built-in support for handling authentication and authorization. The server-side Blazor template now supports options for enabling all of the standard authentication configurations using ASP.NET Core Identity, Azure AD, and Azure AD B2C. We haven’t updated the Blazor WebAssembly templates to support these options yet, but we plan to do so after .NET Core 3.0 has shipped.

To create a new Blazor app with authentication enabled:

  1. Create a new Blazor (server-side) project and select the link to change the authentication configuration. For example, select “Individual User Accounts” and “Store user accounts in-app” to use Blazor with ASP.NET Core Identity:

    Blazor authentication

  2. Run the app. The app includes links in the top row for registering as a new user and logging in.

    Blazor authentication running

  3. Select the Register link to register a new user.

    Blazor authentication register

  4. Select “Apply Migrations” to apply the ASP.NET Core Identity migrations to the database.

    Blazor authentication apply migrations

  5. You should now be logged in.

    Blazor authentication logged in

  6. Select your user name to edit your user profile.

    Blazor authentication edit profile

In the Blazor app, authentication and authorization are configured in the Startup class using the standard ASP.NET Core middleware.



app.UseEndpoints(endpoints =>

When using ASP.NET Core Identity all of the identity related UI concerns are handled by the framework provided default identity UI.


The authentication related links in top row of the app are rendered using the new built-in AuthorizeView component, which displays different content depending on the authentication state.


        <a href="Identity/Account/Manage">Hello, @context.User.Identity.Name!</a>
        <a href="Identity/Account/LogOut">Log out</a>
        <a href="Identity/Account/Register">Register</a>
        <a href="Identity/Account/Login">Log in</a>

The AuthorizeView component will only display its child content when the user is authorized. Alternatively, the AuthorizeView takes parameters for specifying different templates when the user is Authorized, NotAuthorized, or Authorizing. The current authentication state is passed to these templates through the implicit context parameter. You can also specify specific roles or an authorization policy on the AuthorizeView that the user must satisfy to see the authorized view.

To authorize access to specific pages in a Blazor app, use the normal [Authorize] attribute. You can apply the [Authorize] attribute to a component using the new @attribute directive.

@using Microsoft.AspNetCore.Authorization
@attribute [Authorize]
@page "/fetchdata"

To specify what content to display on a page that requires authorization when the user isn’t authorized or is still in the processing of authorizing, use the NotAuthorizedContent and AuthorizingContent parameters on the Router component. These Router parameters are only support in client-side Blazor for this release, but they will be enabled for server-side Blazor in a future update.

The new AuthenticationStateProvider service make the authentication state available to Blazor apps in a uniform way whether they run on the server or client-side in the browser. In server-side Blazor apps the AuthenticationStateProvider surfaces the user from the HttpContext that established the connection to the server. Client-side Blazor apps can configure a custom AuthenticationStateProvider as appropriate for that application. For example, it might retrieve the current user information by querying an endpoint on the server.

The authentication state is made available to the app as a cascading value (Task<AuthenticationState>) using the CascadingAuthenticationState component. This cascading value is then used by the AuthorizeView and Router components to authorize access to specific parts of the UI.


    <Router AppAssembly="typeof(Startup).Assembly">
            <p>Sorry, there's nothing at this address.</p>

Static assets in Razor class libraries

Razor class libraries can now include static assets like JavaScript, CSS, and images. These static assets can then be included in ASP.NET Core apps by referencing the Razor class library project or via a package reference.

To include static assets in a Razor class library add a wwwroot folder to the Razor class library and include any required files in that folder.

When a Razor class library with static assets is referenced either as a project reference or as a package, the static assets from the library are made available to the app under the path prefix _content/{LIBRARY NAME}/. The static assets stay in their original folders and any changes to the content of static assets in the Razor class libraries are reflected in the app without rebuilding.

When the app is published, the companion assets from all referenced Razor class libraries are copied into the wwwroot folder of the published app under the same prefix.

To try out using static assets from a Razor class library:

  1. Create a default ASP.NET Core Web App.

    dotnet new webapp -o WebApp1
  2. Create a Razor class library and reference it from the web app.

    dotnet new razorclasslib -o RazorLib1
    dotnet add WebApp1 reference RazorLib1
  3. Add a wwwroot folder to the Razor class library and include a JavaScript file that logs a simple message to the console.

    cd RazorLib1
    mkdir wwwroot


    console.log("Hello from RazorLib1!");
  4. Reference the script file from Index.cshtml in the web app.

    <script src="_content/RazorLib1/hello.js"></script>
  5. Run the app and look for the output in the browser console.

    Hello from RazorLib1!

Projects now use System.Text.Json by default

New ASP.NET Core projects will now use System.Text.Json for JSON handling by default. In this release we removed Json.NET (Newtonsoft.Json) from the project templates. To enable support for using Json.NET, add the Microsoft.AspNetCore.Mvc.NewtonsoftJson package to your project and add a call to AddNewtonsoftJson() following code in your Startup.ConfigureServices method. For example:


Certificate and Kerberos authentication

Preview 6 brings Certificate and Kerberos authentication to ASP.NET Core.

Certificate authentication requires you to configure your server to accept certificates, and then add the authentication middleware in Startup.Configure and the certificate authentication service in Startup.ConfigureServices.

public void ConfigureServices(IServiceCollection services)
    // All the other service configuration.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    // All the other app configuration.

Options for certificate authentication include the ability to accept self-signed certificates, check for certificate revocation, and check that the proffered certificate has the right usage flags in it. A default user principal is constructed from the certificate properties, with an event that enables you to supplement or replace the principal. All the options, and instructions on how to configure common hosts for certificate authentication can be found in the documentation.

We’ve also extended “Windows Authentication” onto Linux and macOS. Previously this authentication type was limited to IIS and HttpSys, but now Kestrel has the ability to use Negotiate, Kerberos, and NTLM on Windows, Linux, and macOS for Windows domain joined hosts by using the Microsoft.AspNetCore.Authentication.Negotiate nuget package. As with the other authentication services you configure authentication app wide, then configure the service:

public void ConfigureServices(IServiceCollection services)

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    // All the other app configuration.

Your host must be configured correctly. Windows hosts must have SPNs added to the user account hosting the application. Linux and macOS machines must be joined to the domain, then SPNs must be created for the web process, as well as keytab files generated and configured on the host machine. Full instructions are given in the documentation.

SignalR Auto-reconnect

This preview release, available now via npm install @aspnet/signalr@next and in the .NET Core SignalR Client, includes a new automatic reconnection feature. With this release we’ve added the withAutomaticReconnect() method to the HubConnectionBuilder. By default, the client will try to reconnect immediately and after 2, 10, and 30 seconds. Enlisting in automatic reconnect is opt-in, but simple via this new method.

const connection = new signalR.HubConnectionBuilder()

By passing an array of millisecond-based durations to the method, you can be very granular about how your reconnection attempts occur over time.

.withAutomaticReconnect([0, 3000, 5000, 10000, 15000, 30000])
//.withAutomaticReconnect([0, 2000, 10000, 30000]) yields the default behavior

Or you can pass in an implementation of a custom reconnect policy that gives you full control.

If the reconnection fails after the 30-second point (or whatever you’ve set as your maximum), the client presumes the connection is offline and stops trying to reconnect. During these reconnection attempts you’ll want to update your application UI to provide cues to the user that the reconnection is being attempted.

Reconnection Event Handlers

To make this easier, we’ve expanded the SignalR client API to include onreconnecting and onreconnected event handlers. The first of these handlers, onreconnecting, gives developers a good opportunity to disable UI or to let users know the app is offline.

connection.onreconnecting((error) => {
    const status = `Connection lost due to error "${error}". Reconnecting.`;
    document.getElementById("messageInput").disabled = true;
    document.getElementById("sendButton").disabled = true;
    document.getElementById("connectionStatus").innerText = status;

Likewise, the onreconnected handler gives developers an opportunity to update the UI once the connection is reestablished.

connection.onreconnected((connectionId) => {
    const status = `Connection reestablished. Connected.`;
    document.getElementById("messageInput").disabled = false;
    document.getElementById("sendButton").disabled = false;
    document.getElementById("connectionStatus").innerText = status;

Learn more about customizing and handling reconnection

Automatic reconnect has been partially documented already in the preview release. Check out the deeper docs on the topic, with more examples and details on usage, at

Managed gRPC Client

In prior previews, we relied on the Grpc.Core library for client support. The addition of HTTP/2 support in HttpClient in this preview has allowed us to introduce a fully managed gRPC client.

To begin using the new client, add a package reference to Grpc.Net.Client and then you can create a new client.

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

gRPC Client Factory

Building on the opinionated pattern we introduced in HttpClientFactory, we’ve added a gRPC client factory for creating gRPC client instances in your project. There are two flavors of the factory that we’ve added: Grpc.Net.ClientFactory and Grpc.AspNetCore.Server.ClientFactory.

The Grpc.Net.ClientFactory is designed for use in non-ASP.NET app models (such as Worker Services) that still use the Microsoft.Extensions.* primitives without a dependency on ASP.NET Core.

In applications that perform service-to-service communication, we often observe that most servers are also clients that consume other services. In these scenarios, we recommend the use of Grpc.AspNetCore.Server.ClientFactory which features automatic propagation of gRPC deadlines and cancellation tokens.

To use the client factory, add the appropriate package reference to your project (Grpc.AspNetCore.Server.Factory or Grpc.Net.ClientFactory) before adding the following code to ConfigureServices().

    .AddGrpcClient<GreeterClient>(options =>
        options.BaseAddress = new Uri("https://localhost:5001");

gRPC Interceptors

gRPC exposes a mechanism to intercept RPC invocations on both the client and the server. Interceptors can be used in conjunction with existing HTTP middleware. Unlike HTTP middleware, interceptors give you access to actual request/response objects before serialization (on the client) and after deserialization (on the server) and vice versa for the response. All middlewares run before interceptors on the request side and vice versa on the response side.

Client interceptors

When used in conjunction with the client factory, you can add a client interceptor as shown below.

    .AddGrpcClient<GreeterClient>(options =>
        options.BaseAddress = new Uri("https://localhost:5001");

Server interceptors

Server interceptors can be registered in ConfigureServices() as shown below.

    .AddGrpc(options =>
        // This registers a global interceptor
    .AddServiceOptions<GreeterService>(options =>
        // This registers an interceptor for the Greeter service

For examples on how to author an interceptors, take a look at these examples in the grpc-dotnet repo.

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!

Daniel Roth
Daniel Roth

Principal Program Manager, ASP.NET

Follow Daniel   

Alexandre Reyes 2019-06-12 11:38:07
Looks like DotNetObjectRef has changed too
Michael DeMond 2019-06-12 11:53:00
Looks like I jumped into the previews at the right time.  This looks amazing!  Now let's get this sucker to the place where we can deploy it into store-hosted environments and call it a day:
Hallam Etragu 2019-06-12 14:49:33
Hi Daniel,  First of all I have to say I've never been so excited in a while after getting to code Blazor, I just hope it gets more investment and more publicity. It really looks great. Some questions: While working with lists in UWP apps is important to virtualize the collection, is there any easy way to do this in Blazor like a virtualizing container that only draws items depending on the viewport width of that component? Blazor serverside has loads of sense to be deployed as an Electron app, are there any plans to facilitate this to the users? Regards.
Øyvind Habberstad
Øyvind Habberstad 2019-06-12 14:55:40
Hi Daniel, I assume that you are refering to a nuget package, when you write "add a package reference to Grpc.Net.Client and then you can create a new client", but I cannot a package named "Grpc.Net.Client" on nuget. I tried to add the "Grpc.Core", but I cannot find any "Grpc.Net.Client" namespace in that package. Is there something I have misunderstood? (I have ticked the "Include prerelease") Thanks in advance.
anonymous 2019-06-12 15:27:59
This comment has been deleted.
Serge S
Serge S 2019-06-12 15:34:32
Hi Daniel,  Re: "When specifying a delegate for C# event handler the @ prefix is currently still required on the attribute value, but we expect to remove this requirement in a future update."I have a possibility to run preview 6 today and it was diificult to findout (5 minutes) that you have changed @Something. It is looks better code now and it is easy to distinguish in *razor code between HTML and C#. I like it! Please keep current syntax: It is will consistent @ - means Blazor! <button @onclick="@Clicked">Click me!</button>Thank you, Serge
moshe habas 2019-06-13 00:15:35
Wonderful work Daniel, Thanks to you and the rest of the team!
Brown, Stephen 2019-06-13 01:40:28
This is fantastic. Is it possible to use the gRPC client from client-side WebAssembly Blazor?
Issac Vale 2019-06-13 03:55:28
It should be app.UseClientSideBlazorFiles<TStartup> instead of app.UseClientSideBlazor<TStartup>() Hope this helps...
Stefan Over 2019-06-13 04:51:51
"We haven’t updated the Blazor WebAssembly templates to support these options yet, but we plan to do so after .NET Core 3.0 has shipped." Does this mean that the client-side templates don't include it? Or are the authentication features not supported by client-side projects?What about client-side projects with ASP.NET Core backend, which will probably the most successful ones?
JJ Bussert 2019-06-13 09:36:02
Finally we have easier to use Auth integration and templates to spin up the projects faster.  Can't wait to give this a try!
Steven Sullivan
Steven Sullivan 2019-06-13 15:00:08
I have a Blazor Server-Side project that has now been updated to reference 3.0.0-preview6.19307.2 as well as the most recent version of VS2019. However, it's still unable to recognize @code, or really implement any of the new features.Am I missing something?
Luis Palacios 2019-06-13 20:56:13
Thanks a lot, Daniel, I'm highly excited for Blazor, and I'm no javascript hater, in fact, I've been doing a lot of typescript with react and angular but I'm still excited about the possibilities that are opening with Blazor 
Tahsin Kasap
Tahsin Kasap 2019-06-14 04:10:50
preview6 is still missing at releases.json Because of this Our builds are failed. And also there is no site extension for preview6 at Azure Portal. Am I missing something to build with preview6?
Ra D 2019-06-14 08:25:47
Hi Daniel - excellent work and thanks for including the authentication in this release. I understand this is a preview release but I tried extending the user registration with custom attributes to a sample project by using the Scaffold Identity steps - the end result is a mess; I ended up discarding the project. Steps to reproduce: create a sample Blazor server-side with single user authentication. Project gets generated, run the project - works like a charm. Now extend the app with Scaffold Identity - select only Account\Registration page, select the dbcontext. VS adds additional code and pages - try running the project - either build fails or app wont run. Could you please streamline this to work with new Blazor libraries. Thank you.
Serge S
Serge S 2019-06-14 10:49:40
Hi Daniel, I have a question regarding injecting: Server-Side or Client-Side Blazor: Client-Side Blazor: If I correctly understand, when we injecting a Class is it transferring an entire content of a Class (a lot’s of data in memory) to Client over SignalR? Server-Side Blazor: it is local by definition. No transferring. From this point of view the Server-Side should be much faster for an embedded server application, which is supposed to handle the device activities and, because number of clients are limited, also preferred. I am using the Server-Side Blazor and it is really fast. But I don’t know how fast it will be using Client-Side Blazor, probably it is required a granular data control. Thank you, Serge
Rod Macdonald
Rod Macdonald 2019-06-15 04:08:00
Blazor just seems to be getting better and better. What's the client side story for a local database? Have been saying for years SQL Server could do with a client side companion (non-Jet, non-SQLite, kinda SQL Server'ish, you know?)  :-)
D S 2019-06-15 16:10:12
Hi Daniel: Excellent work of your Blazor team. I am trying to learn about the authentication feature of Blazor Server Side.  Is it possible to create a custom login template instead of using the default page. Thanks David
Jesse Sander 2019-06-18 13:37:49
Hi Daniel, Thanks for the update! Really impressed with Blazor so far. Just wanted to say, keep up the good work and cheers to the team from Holland!
Shaun Sales
Shaun Sales 2019-06-18 20:39:30
Hi Daniel, very happy to see great features continuing to be added to SignalR Core. Can we expect to see a SignalR C# client compatible with client-side Blazor any time soon?
Wilson, Joel 2019-06-20 11:03:06
Hosting Blazor apps in IIS doesn't seem to work under Preview 6. I always get the message "This localhost page can’t be found" (Chrome). The app runs fine in Visual Studio 2019 Preview (16.2.0 Preview 2.0). It also works if I run the server executable from the command line (i.e., WebApplication1.Server.exe) and then navigate the browser to http://localhost:5000. But when I publish the code to an IIS website I get the error. I'm thinking it has something to do with the new MapFallbackToClientSideBlazor call in Startup.Configure (but that's just a guess). Wondering if anyone else is having the same problem OR if anyone else has successfully hosted a Preview 6 Blazor app in IIS (and, yes, I installed the Preview 6 Runtime & Hosting Bundle).
anonymous 2019-06-21 20:37:30
This comment has been deleted.
Mark Lynn
Mark Lynn 2019-06-22 03:12:12
Looks like this.StateHasChanged isn't behaving in a threadsafe manner?This version now throws in the following,- countdown example from button click var timer = new Timer(new TimerCallback(_ =>{if (Count <= 0) return;Count--; // recognize the state change and refresh the UIthis.StateHasChanged();}), null, 1000, 1000); or a ticking clock example?
Dimitri Joosten 2019-06-23 08:59:41
Hi Daniel, my question is about Deployment to Azure Web App. For .Net Core Preview 5 there is an extention provided in Azure whereas for preview 6 is none provided (yet). So I tried to deploy self-contained. I used the project template "Blazor (ASPNet Core hosted)". The startup target is "xxxx.Server" When I try to publish I get the following error: Severity Code Description Project File Line Suppression StateError  Assets file '....Client\obj\project.assets.json' doesn't have a target for '.NETStandard,Version=v2.0'. Ensure that restore has run and that you have included 'netstandard2.0' in the TargetFrameworks for your project. xxx.Server  0  I tried to remove / reinstall the preview (.net core and visual studio preview). Created the project from scretch but that didn't help...  Thank you for helping me solving this issue. Br. Dimitri
Alexandre Reyes 2019-06-24 08:05:52
Hey Daniel, does "Static assets in Razor class libraries" works when you disable server pre-rendering? endpoints.MapBlazorHub().AddComponent(typeof(App), selector: "app");endpoints.MapFallbackToFile("index.html");