Asp.Net Core SignalR on Blazor Server-Side never calls OnDisconnectedAsync

[francescocristallo]

Describe the bug

OnDisconnectedAsync is never called when using Asp.net Core SignalR with Blazor Server-Side

To Reproduce

https://github.com/francescocristallo/BlazorAppServerChat

Launch the app, open two tabs, the chat works.
When closing one of the browsers/tab, OnDisconnectedAsync is not called.

The same code calls OnDisconnectedAsync if used with Blazor Webassembly (Client Side)

Further technical details

ASP.NET Core 3.1 on Visual Studio Professional 2019

.NET Core SDK (reflecting any global.json):
Version: 3.1.200-preview-014883
Commit: 4e2a0ee959

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64

Host (useful for support):
Version: 3.1.1
Commit: a1388f194c

[SteveSandersonMS]

Hub connections are disposable, and you do need to dispose them when you're finished with them. Blazor components get disposed when the user navigates away or terminates their connection. So, to avoid problems you should add code to your Index.razor component similar to the following:

@implements IDisposable

... rest of component code ...

@code {
    public void Dispose()
    {
        _hubConnection.DisposeAsync();
    }
}

As you can see, one oddity with this is that currently Blazor doesn’t support IAsyncDisposable, so the call to DisposeAsync isn't awaited. Adding IAsyncDisposable is tracked in #9960

Note that this isn't just for Blazor Server. You also need to dispose your hub connection on Blazor WebAssembly if it lives within a single component like yours does. Otherwise if the user keeps navigating back and forth between Index.razor and other components, you'll still be accumulating undisposed hub connections.