What is "OnDisconnected" in SignalR?

In SignalR, OnDisconnected (or OnDisconnectedAsync in ASP.NET Core SignalR) is a hub lifecycle method that is called when a client connection is terminated.

ASP.NET Core SignalR

You override OnDisconnectedAsync in your Hub:

using Microsoft.AspNetCore.SignalR;

public class ChatHub : Hub
{
    public override async Task OnDisconnectedAsync(Exception? exception)
    {
        string connectionId = Context.ConnectionId;

        Console.WriteLine($"Client disconnected: {connectionId}");

        await base.OnDisconnectedAsync(exception);
    }
}

When it is called

The method runs when a client disconnects for reasons such as:

• The user closes the browser/tab.
• The client explicitly stops the SignalR connection.
• A network interruption causes the connection to end.
• The server disconnects the client.
• A connection timeout occurs.

The exception parameter

public override async Task OnDisconnectedAsync(Exception? exception)
{
    if (exception != null)
    {
        Console.WriteLine($"Disconnected due to error: {exception.Message}");
    }

    await base.OnDisconnectedAsync(exception);
}

exception == null usually indicates a normal disconnect.

A non-null exception may indicate an unexpected termination or transport error.

Common uses

Track online users

public override async Task OnDisconnectedAsync(Exception? exception)
{
    OnlineUsers.Remove(Context.ConnectionId);

    await base.OnDisconnectedAsync(exception);
}

Remove users from custom tracking

public override async Task OnDisconnectedAsync(Exception? exception)
{
    _connectionManager.RemoveConnection(Context.ConnectionId);

    await base.OnDisconnectedAsync(exception);
}

Notify other clients

public override async Task OnDisconnectedAsync(Exception? exception)
{
    await Clients.All.SendAsync(
        "UserDisconnected",
        Context.ConnectionId);

    await base.OnDisconnectedAsync(exception);
}

Relationship with OnConnectedAsync

A typical connection lifecycle looks like:

Client connects
    ↓
OnConnectedAsync()
    ↓
Client invokes hub methods
    ↓
Connection ends
    ↓
OnDisconnectedAsync()

Older ASP.NET SignalR

In classic ASP.NET SignalR, the method was:

public override Task OnDisconnected(bool stopCalled)
{
    return base.OnDisconnected(stopCalled);
}

In ASP.NET Core SignalR, it became:

public override Task OnDisconnectedAsync(Exception? exception)
{
    return base.OnDisconnectedAsync(exception);
}

So OnDisconnectedAsync is the place to perform cleanup, update user presence, release resources, and notify other clients when a SignalR connection ends.