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title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
Use hubs in ASP.NET Core SignalR	tdykstra	Learn how to use hubs in ASP.NET Core SignalR.	>= aspnetcore-2.1	tdykstra	mvc	09/12/2018	signalr/hubs

Use hubs in SignalR for ASP.NET Core

By Rachel Appel and Kevin Griffin

View or download sample code (how to download)

What is a SignalR hub

The SignalR Hubs API enables you to call methods on connected clients from the server. In the server code, you define methods that are called by client. In the client code, you define methods that are called from the server. SignalR takes care of everything behind the scenes that makes real-time client-to-server and server-to-client communications possible.

Configure SignalR hubs

The SignalR middleware requires some services, which are configured by calling services.AddSignalR.

[!code-csharpConfigure service]

When adding SignalR functionality to an ASP.NET Core app, setup SignalR routes by calling app.UseSignalR in the Startup.Configure method.

[!code-csharpConfigure routes to hubs]

Create and use hubs

Create a hub by declaring a class that inherits from Hub, and add public methods to it. Clients can call methods that are defined as public.

[!code-csharpCreate and use hubs]

You can specify a return type and parameters, including complex types and arrays, as you would in any C# method. SignalR handles the serialization and deserialization of complex objects and arrays in your parameters and return values.

The Context object

The Hub class has a Context property that contains the following properties with information about the connection:

Property	Description
`ConnectionId`	Gets the unique ID for the connection, assigned by SignalR. There is one connection ID for each connection.
`UserIdentifier`	Gets the user identifier. By default, SignalR uses the `ClaimTypes.NameIdentifier` from the `ClaimsPrincipal` associated with the connection as the user identifier.
`User`	Gets the `ClaimsPrincipal` associated with the current user.
`Items`	Gets a key/value collection that can be used to share data within the scope of this connection. Data can be stored in this collection and it will persist for the connection across different hub method invocations.
`Features`	Gets the collection of features available on the connection. For now, this collection isn't needed in most scenarios, so it isn't documented in detail yet.
`ConnectionAborted`	Gets a `CancellationToken` that notifies when the connection is aborted.

Hub.Context also contains the following methods:

Method	Description
`GetHttpContext`	Returns the `HttpContext` for the connection, or `null` if the connection is not associated with an HTTP request. For HTTP connections, you can use this method to get information such as HTTP headers and query strings.
`Abort`	Aborts the connection.

The Clients object

The Hub class has a Clients property that contains the following properties for communication between server and client:

Property	Description
`All`	Calls a method on all connected clients
`Caller`	Calls a method on the client that invoked the hub method
`Others`	Calls a method on all connected clients except the client that invoked the method

Hub.Clients also contains the following methods:

Method	Description
`AllExcept`	Calls a method on all connected clients except for the specified connections
`Client`	Calls a method on a specific connected client
`Clients`	Calls a method on specific connected clients
`Group`	Calls a method to all connections in the specified group
`GroupExcept`	Calls a method to all connections in the specified group, except the specified connections
`Groups`	Calls a method to multiple groups of connections
`OthersInGroup`	Calls a method to a group of connections, excluding the client that invoked the hub method
`User`	Calls a method to all connections associated with a specific user
`Users`	Calls a method to all connections associated with the specified users

Each property or method in the preceding tables returns an object with a SendAsync method. The SendAsync method allows you to supply the name and parameters of the client method to call.

Send messages to clients

To make calls to specific clients, use the properties of the Clients object. In the following example, the SendMessageToCaller method demonstrates sending a message to the connection that invoked the hub method. The SendMessageToGroups method sends a message to the groups stored in a List named groups.

[!code-csharpSend messages]

Strongly typed hubs

A drawback of using SendAsync is that it relies on a magic string to specify the client method to be called. This leaves code open to runtime errors if the method name is misspelled or missing from the client.

An alternative to using SendAsync is to strongly type the Hub with xref:Microsoft.AspNetCore.SignalR.Hub`1. In the following example, the ChatHub client methods have been extracted out into an interface called IChatClient.

[!code-csharpInterface for IChatClient]

This interface can be used to refactor the preceding ChatHub example.

[!code-csharpStrongly typed ChatHub]

Using Hub<IChatClient> enables compile-time checking of the client methods. This prevents issues caused by using magic strings, since Hub<T> can only provide access to the methods defined in the interface.

Using a strongly typed Hub<T> disables the ability to use SendAsync.

Handle events for a connection

The SignalR Hubs API provides the OnConnectedAsync and OnDisconnectedAsync virtual methods to manage and track connections. Override the OnConnectedAsync virtual method to perform actions when a client connects to the Hub, such as adding it to a group.

[!code-csharpHandle events]

Handle errors

Exceptions thrown in your hub methods are sent to the client that invoked the method. On the JavaScript client, the invoke method returns a JavaScript Promise. When the client receives an error with a handler attached to the promise using catch, it's invoked and passed as a JavaScript Error object.

[!code-javascriptError]

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