ASP.NET Core SignalR .NET Client

The ASP.NET Core SignalR .NET client library lets you communicate with SignalR hubs from .NET apps.

View or download sample code (how to download)

The code sample in this article is a WPF app that uses the ASP.NET Core SignalR .NET client.

Install the SignalR .NET client package

The Microsoft.AspNetCore.SignalR.Client package is needed for .NET clients to connect to SignalR hubs. To install the client library, run the following command in the Package Manager Console window:

Install-Package Microsoft.AspNetCore.SignalR.Client

Connect to a hub

To establish a connection, create a HubConnectionBuilder and call Build. The hub URL, protocol, transport type, log level, headers, and other options can be configured while building a connection. Configure any required options by inserting any of the HubConnectionBuilder methods into Build. Start the connection with StartAsync.

using System;
using System.Threading.Tasks;
using System.Windows;
using Microsoft.AspNetCore.SignalR.Client;

namespace SignalRChatClient
{
    public partial class MainWindow : Window
    {
        HubConnection connection;
        public MainWindow()
        {
            InitializeComponent();

            connection = new HubConnectionBuilder()
                .WithUrl("http://localhost:53353/ChatHub")
                .Build();

            connection.Closed += async (error) =>
            {
                await Task.Delay(new Random().Next(0,5) * 1000);
                await connection.StartAsync();
            };
        }

        private async void connectButton_Click(object sender, RoutedEventArgs e)
        {
            connection.On<string, string>("ReceiveMessage", (user, message) =>
            {
                this.Dispatcher.Invoke(() =>
                {
                   var newMessage = $"{user}: {message}";
                   messagesList.Items.Add(newMessage);
                });
            });

            try
            {
                await connection.StartAsync();
                messagesList.Items.Add("Connection started");
                connectButton.IsEnabled = false;
                sendButton.IsEnabled = true;
            }
            catch (Exception ex)
            {
                messagesList.Items.Add(ex.Message);
            }
        }

        private async void sendButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                await connection.InvokeAsync("SendMessage", 
                    userTextBox.Text, messageTextBox.Text);
            }
            catch (Exception ex)
            {                
                messagesList.Items.Add(ex.Message);                
            }
        }
    }
}

Handle lost connection

Automatically reconnect

The HubConnection can be configured to automatically reconnect using the WithAutomaticReconnect method on the HubConnectionBuilder. It won't automatically reconnect by default.

HubConnection connection= new HubConnectionBuilder()
    .WithUrl(new Uri("http://127.0.0.1:5000/chatHub"))
    .WithAutomaticReconnect()
    .Build();

Without any parameters, WithAutomaticReconnect() configures the client to wait 0, 2, 10, and 30 seconds respectively before trying each reconnect attempt, stopping after four failed attempts.

Before starting any reconnect attempts, the HubConnection will transition to the HubConnectionState.Reconnecting state and fire the Reconnecting event. This provides an opportunity to warn users that the connection has been lost and to disable UI elements. Non-interactive apps can start queuing or dropping messages.

connection.Reconnecting += error =>
{
    Debug.Assert(connection.State == HubConnectionState.Reconnecting);

    // Notify users the connection was lost and the client is reconnecting.
    // Start queuing or dropping messages.

    return Task.CompletedTask;
};

If the client successfully reconnects within its first four attempts, the HubConnection will transition back to the Connected state and fire the Reconnected event. This provides an opportunity to inform users the connection has been reestablished and dequeue any queued messages.

Since the connection looks entirely new to the server, a new ConnectionId will be provided to the Reconnected event handlers.

Warning

The Reconnected event handler's connectionId parameter will be null if the HubConnection was configured to skip negotiation.

connection.Reconnected += connectionId =>
{
    Debug.Assert(connection.State == HubConnectionState.Connected);

    // Notify users the connection was reestablished.
    // Start dequeuing messages queued while reconnecting if any.

    return Task.CompletedTask;
};

WithAutomaticReconnect() won't configure the HubConnection to retry initial start failures, so start failures need to be handled manually:

public static async Task<bool> ConnectWithRetryAsync(HubConnection connection, CancellationToken token)
{
    // Keep trying to until we can start or the token is canceled.
    while (true)
    {
        try
        {
            await connection.StartAsync(token);
            Debug.Assert(connection.State == HubConnectionState.Connected);
            return true;
        }
        catch when (token.IsCancellationRequested)
        {
            return false;
        }
        catch
        {
            // Failed to connect, trying again in 5000 ms.
            Debug.Assert(connection.State == HubConnectionState.Disconnected);
            await Task.Delay(5000);
        }
    }
}

If the client doesn't successfully reconnect within its first four attempts, the HubConnection will transition to the Disconnected state and fire the Closed event. This provides an opportunity to attempt to restart the connection manually or inform users the connection has been permanently lost.

connection.Closed += error =>
{
    Debug.Assert(connection.State == HubConnectionState.Disconnected);

    // Notify users the connection has been closed or manually try to restart the connection.

    return Task.CompletedTask;
};

In order to configure a custom number of reconnect attempts before disconnecting or change the reconnect timing, WithAutomaticReconnect accepts an array of numbers representing the delay in milliseconds to wait before starting each reconnect attempt.

HubConnection connection= new HubConnectionBuilder()
    .WithUrl(new Uri("http://127.0.0.1:5000/chatHub"))
    .WithAutomaticReconnect(new[] { TimeSpan.Zero, TimeSpan.Zero, TimeSpan.FromSeconds(10) })
    .Build();

    // .WithAutomaticReconnect(new[] { TimeSpan.Zero, TimeSpan.FromSeconds(2), TimeSpan.FromSeconds(10), TimeSpan.FromSeconds(30) }) yields the default behavior.

The preceding example configures the HubConnection to start attempting reconnects immediately after the connection is lost. This is also true for the default configuration.

If the first reconnect attempt fails, the second reconnect attempt will also start immediately instead of waiting 2 seconds like it would in the default configuration.

If the second reconnect attempt fails, the third reconnect attempt will start in 10 seconds which is again like the default configuration.

The custom behavior then diverges again from the default behavior by stopping after the third reconnect attempt failure. In the default configuration there would be one more reconnect attempt in another 30 seconds.

If you want even more control over the timing and number of automatic reconnect attempts, WithAutomaticReconnect accepts an object implementing the IRetryPolicy interface, which has a single method named NextRetryDelay.

NextRetryDelay takes a single argument with the type RetryContext. The RetryContext has three properties: PreviousRetryCount, ElapsedTime and RetryReason which are a long, a TimeSpan and an Exception respectively. Before the first reconnect attempt, both PreviousRetryCount and ElapsedTime will be zero, and the RetryReason will be the Exception that caused the connection to be lost. After each failed retry attempt, PreviousRetryCount will be incremented by one, ElapsedTime will be updated to reflect the amount of time spent reconnecting so far, and the RetryReason will be the Exception that caused the last reconnect attempt to fail.

NextRetryDelay must return either a TimeSpan representing the time to wait before the next reconnect attempt or null if the HubConnection should stop reconnecting.

public class RandomRetryPolicy : IRetryPolicy
{
    private readonly Random _random = new Random();

    public TimeSpan? NextRetryDelay(RetryContext retryContext)
    {
        // If we've been reconnecting for less than 60 seconds so far,
        // wait between 0 and 10 seconds before the next reconnect attempt.
        if (retryContext.ElapsedTime < TimeSpan.FromSeconds(60))
        {
            return TimeSpan.FromSeconds(_random.Next() * 10);
        }
        else
        {
            // If we've been reconnecting for more than 60 seconds so far, stop reconnecting.
            return null;
        }
    }
}
HubConnection connection = new HubConnectionBuilder()
    .WithUrl(new Uri("http://127.0.0.1:5000/chatHub"))
    .WithAutomaticReconnect(new RandomRetryPolicy())
    .Build();

Alternatively, you can write code that will reconnect your client manually as demonstrated in Manually reconnect.

Manually reconnect

Warning

Prior to 3.0, the .NET client for SignalR doesn't automatically reconnect. You must write code that will reconnect your client manually.

Use the Closed event to respond to a lost connection. For example, you might want to automate reconnection.

The Closed event requires a delegate that returns a Task, which allows async code to run without using async void. To satisfy the delegate signature in a Closed event handler that runs synchronously, return Task.CompletedTask:

connection.Closed += (error) => {
    // Do your close logic.
    return Task.CompletedTask;
};

The main reason for the async support is so you can restart the connection. Starting a connection is an async action.

In a Closed handler that restarts the connection, consider waiting for some random delay to prevent overloading the server, as shown in the following example:

connection.Closed += async (error) =>
{
    await Task.Delay(new Random().Next(0,5) * 1000);
    await connection.StartAsync();
};

Call hub methods from client

InvokeAsync calls methods on the hub. Pass the hub method name and any arguments defined in the hub method to InvokeAsync. SignalR is asynchronous, so use async and await when making the calls.

await connection.InvokeAsync("SendMessage", 
    userTextBox.Text, messageTextBox.Text);

The InvokeAsync method returns a Task which completes when the server method returns. The return value, if any, is provided as the result of the Task. Any exceptions thrown by the method on the server produce a faulted Task. Use await syntax to wait for the server method to complete and try...catch syntax to handle errors.

The SendAsync method returns a Task which completes when the message has been sent to the server. No return value is provided since this Task doesn't wait until the server method completes. Any exceptions thrown on the client while sending the message produce a faulted Task. Use await and try...catch syntax to handle send errors.

Note

If you're using Azure SignalR Service in Serverless mode, you cannot call hub methods from a client. For more information, see the SignalR Service documentation.

Call client methods from hub

Define methods the hub calls using connection.On after building, but before starting the connection.

connection.On<string, string>("ReceiveMessage", (user, message) =>
{
    this.Dispatcher.Invoke(() =>
    {
       var newMessage = $"{user}: {message}";
       messagesList.Items.Add(newMessage);
    });
});

The preceding code in connection.On runs when server-side code calls it using the SendAsync method.

public async Task SendMessage(string user, string message)
{
    await Clients.All.SendAsync("ReceiveMessage", user,message);
}

Error handling and logging

Handle errors with a try-catch statement. Inspect the Exception object to determine the proper action to take after an error occurs.

try
{
    await connection.InvokeAsync("SendMessage", 
        userTextBox.Text, messageTextBox.Text);
}
catch (Exception ex)
{                
    messagesList.Items.Add(ex.Message);                
}

Additional resources