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

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ASP.NET Core and Blazor updates in .NET Core 3.0 Preview 9 - xSicKxBot - 09-11-2019

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

Daniel

September 4th, 2019

.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:

Code:
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
  Code:
  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
  Code:
  Microsoft.AspNetCore.Components.Authorization
  in your top level _Imports.razor file.
- Update all Blazor component parameters to be public.
- Update implementations of
  Code:
  IJSRuntime
  to return
  Code:
  ValueTask<T>
  .
- Replace calls to
  Code:
  MapBlazorHub<TComponent>
  with a single call to
  Code:
  MapBlazorHub
  .
- Update calls to
  Code:
  RenderComponentAsync
  and
  Code:
  RenderStaticComponentAsync
  to use the new overloads to
  Code:
  RenderComponentAsync
  that take a
  Code:
  RenderMode
  parameter (see Render multiple Blazor components from MVC views or pages below for details).
- Update App.razor to use the updated
  Code:
  Router
  component (see Blazor routing improvements below for details).
- (Optional) Remove page specific _Imports.razor file with the
  Code:
  @layout
  directive to use the default layout specified through the router instead.
- Remove any use of the
  Code:
  PageDisplay
  component and replace with
  Code:
  LayoutView
  ,
  Code:
  RouteView
  , or
  Code:
  AuthorizeRouteView
  as appropriate (see Blazor routing improvements below for details).
- Replace uses of
  Code:
  IUriHelper
  with
  Code:
  NavigationManager
  .
- Remove any use of
  Code:
  @ref:suppressField
  .
- Replace the previous
  Code:
  RevalidatingAuthenticationStateProvider
  code with the new
  Code:
  RevalidatingIdentityAuthenticationStateProvider
  code from the project template.
- Replace
  Code:
  Microsoft.AspNetCore.Components.UIEventArgs
  with
  Code:
  System.EventArgs
  and remove the “UI” prefix from all
  Code:
  EventArgs
  derived types (
  Code:
  UIChangeEventArgs
  ->
  Code:
  ChangeEventArgs
  , etc.).
- Replace
  Code:
  DotNetObjectRef
  with
  Code:
  DotNetObjectReference
  .
- Replace
  Code:
  OnAfterRender()
  and
  Code:
  OnAfterRenderAsync()
  implementations with
  Code:
  OnAfterRender(bool firstRender)
  or
  Code:
  OnAfterRenderAsync(bool firstRender)
  .
In gRPC projects:
- Update calls to
  Code:
  GrpcClient.Create
  with a call
  Code:
  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
  Code:
  ClientBase
  instead of
  Code:
  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

Code:
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

Code:
@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

Code:
@using Microsoft.AspNetCore.Components.Web

.

Troubleshooting guidance

With the correct references and using statement for

Code:
Microsoft.AspNetCore.Components.Web

, event handlers like

Code:
@onclick

and

Code:
@bind

should be bold font and colorized as shown below when using Visual Studio.

If

Code:
@bind

Code:
@onclick

are colorized as a normal HTML attribute, then the

Code:
@using

statement is missing.

If you’re missing a using statement for the

Code:
Microsoft.AspNetCore.Components.Web

namespace, you may see build failures. For example, the following build error for the code shown above indicates that the

Code:
@bind

attribute wasn’t recognized:

Code:
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

Code:
@onclick

attribute wasn’t recognized:

Code:
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

Code:
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

Code:
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:

Code:
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

Code:
Router

component to make it more flexible and to enable new scenarios. The

Code:
Router

component in Blazor handles rendering the correct component that matches the current address. Routable components are marked with the

Code:
@page

directive, which adds the

Code:
RouteAttribute

to the generated component classes. If the current address matches a route, then the

Code:
Router

renders the contents of its

Code:
Found

parameter. If no route matches, then the

Code:
Router

component renders the contents of its

Code:
NotFound

parameter.

To render the component with the matched route, use the new

Code:
RouteView

component passing in the supplied

Code:
RouteData

from the

Code:
Router

along with any desired parameters. The

Code:
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.

Code:
<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

Code:
LayoutView

component. This is useful when specifying content for not found pages that you still want to use the app’s layout.

Code:
<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

Code:
Router

. Instead, you use the

Code:
AuthorizeRouteView

component. The

Code:
AuthorizeRouteView

component is a

Code:
RouteView

that will only render the matched component if the user is authorized. Authorization rules for specific components are specified using the

Code:
AuthorizeAttribute

. The

Code:
AuthorizeRouteView

component also sets up the

Code:
AuthenticationState

as a cascading value if there isn’t one already. Otherwise, you can still manually setup the

Code:
AuthenticationState

as a cascading value using the

Code:
CascadingAuthenticationState

component.

Code:
<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

Code:
NotAuthorized

and

Code:
Authorizing

parameters of the

Code:
AuthorizedRouteView

component to specify content to display if the user is not authorized or authorization is still in progress.

Code:
<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

Code:
Router

component to consider when searching for routable components. These assemblies will be considered in addition to the specified

Code:
AppAssembly

. You specify these assemblies using the

Code:
AdditionalAssemblies

parameter. For example, if

Code:
Component1

is a routable component defined in a referenced class library, then you can support routing to this component like this:

Code:
<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

Code:
Html.RenderComponentAsync<TComponent>(RenderMode renderMode, object parameters)

HTML helper method with the desired

Code:
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

Code:
Configure

method. Only a single call to

Code:
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.

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

Code:
OwningComponentBase

and

Code:
OwningComponentBase<TService>

base classes. These base classes expose a

Code:
ScopedServices

property of type

Code:
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

Code:
@inherits

directive.

Code:
@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

Code:
@inject

or the

Code:
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

Code:
Problem

and

Code:
ValidationProblem

method overloads to controllers that use optional parameters to simplify returning Problem Detail responses.

Code:
[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

Code:
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

Code:
Grpc.Core

, making it easier to switch between using the two libraries.

Code:
// 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

Code:
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.

Code:
// 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!

Daniel Roth

Principal Program Manager, ASP.NET

Follow Daniel https://devblogs.microsoft.com/aspnet/author/danroth27/feed/