How do you implement private messaging between two users in a SignalR chat application?

Question

Ravi Vishwakarma · Answer

Identify users uniquely.Map each connected user to a SignalR connection.Send a message to a specific user’s connection instead of broadcasting to all clients.
Here’s a step-by-step guide:
1. Setup SignalR Hub
public class ChatHub : Hub
{
    // Static dictionary to keep track of userId and connectionId
    private static readonly Dictionaryandlt;string, stringandgt; _userConnections = new();
    // Called when a client connects
    public override Task OnConnectedAsync()
    {
        var userId = Context.UserIdentifier; // Must be set by authentication middleware
        if (!string.IsNullOrEmpty(userId))
        {
            lock (_userConnections)
            {
                _userConnections[userId] = Context.ConnectionId;
            }
        }
        return base.OnConnectedAsync();
    }
    // Called when a client disconnects
    public override Task OnDisconnectedAsync(Exception exception)
    {
        var userId = Context.UserIdentifier;
        if (!string.IsNullOrEmpty(userId))
        {
            lock (_userConnections)
            {
                _userConnections.Remove(userId);
            }
        }
        return base.OnDisconnectedAsync(exception);
    }
    // Private messaging method
    public async Task SendPrivateMessage(string recipientUserId, string message)
    {
        if (_userConnections.TryGetValue(recipientUserId, out string connectionId))
        {
            await Clients.Client(connectionId).SendAsync("ReceivePrivateMessage", Context.UserIdentifier, message);
        }
    }
}
2. Configure Authentication Middleware
Ensure that UserIdentifier is set — typically the username or user ID from your authentication system.
In Startup.cs (for .NET Core):
services.AddSignalR();
services.AddAuthentication(...);
services.AddAuthorization();
app.UseAuthentication();
app.UseAuthorization();
app.UseEndpoints(endpoints =andgt;
{
    endpoints.MapHubandlt;ChatHubandgt;("/chatHub");
});
If you're not using full authentication, override UserIdentifier manually:
public class CustomUserIdProvider : IUserIdProvider
{
    public string GetUserId(HubConnectionContext connection)
    {
        // Use connection.User.Identity.Name or a query string, etc.
        return connection.User?.Identity?.Name;
    }
}
Register it:
services.AddSingletonandlt;IUserIdProvider, CustomUserIdProviderandgt;();
3. Client-Side JavaScript Example
const connection = new signalR.HubConnectionBuilder()
    .withUrl("/chatHub")
    .build();
connection.on("ReceivePrivateMessage", (senderUserId, message) =andgt; {
    console.log(`Message from ${senderUserId}: ${message}`);
});
connection.start().then(() =andgt; {
    // Example: send a private message
    const recipientUserId = "user456";
    connection.invoke("SendPrivateMessage", recipientUserId, "Hello in private!");
});
Tips
Use a persistent store instead of an in-memory dictionary (_userConnections) for scalability.For multiple connections per user (multiple browser tabs), use
ConcurrentDictionaryandlt;string, HashSetandlt;stringandgt;andgt;.For group chat, use Groups.AddToGroupAsync(connectionId, groupName).

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How do you implement private messaging between two users in a SignalR chat application?

Ravi Vishwakarma

Can you answer this question?

1 Answers

1. Setup SignalR Hub

2. Configure Authentication Middleware

3. Client-Side JavaScript Example

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