cancel_09.cs

class Client : IDisposable
{
    readonly TimeSpan timeout;
    readonly ObjectPool<CancellationTokenSource> timeoutTokenSourcePool;
    readonly CancellationTokenSource clientLifetimeTokenSource;

    public TimeSpan Timeout { get; }

    public Client(TimeSpan timeout)
    {
        this.Timeout = timeout;
        this.timeoutTokenSourcePool = ObjectPool.Create<CancellationTokenSource>();
        this.clientLifetimeTokenSource = new CancellationTokenSource();
    }

    public async Task SendAsync(CancellationToken cancellationToken = default)
    {
        var timeoutTokenSource = timeoutTokenSourcePool.Get();

        CancellationTokenRegistration externalCancellation = default;
        if (cancellationToken.CanBeCanceled)
        {
            // call Timeout's cancel from argument CancellationToken
            externalCancellation = cancellationToken.UnsafeRegister(static state =>
            {
                ((CancellationTokenSource)state!).Cancel();
            }, timeoutTokenSource);
        }

        // same as Client's lifetime
        var clientLifetimeCancellation = clientLifetimeTokenSource.Token.UnsafeRegister(static state =>
        {
            ((CancellationTokenSource)state!).Cancel();
        }, timeoutTokenSource);

        timeoutTokenSource.CancelAfter(Timeout);

        try
        {
            await SendCoreAsync(timeoutTokenSource.Token);
        }
        finally
        {
            // Unregisters
            externalCancellation.Dispose();
            clientLifetimeCancellation.Dispose();
            if (timeoutTokenSource.TryReset())
            {
                timeoutTokenSourcePool.Return(timeoutTokenSource);
            }
        }
    }

    async Task SendCoreAsync(CancellationToken cancellationToken)
    {
        // snip...
    }

    public void Dispose()
    {
        clientLifetimeTokenSource.Cancel();
        clientLifetimeTokenSource.Dispose();
    }
}