var messageChannel = new FakeMessageChannel<string>();

await messageChannel.PostAsync(message: "foo bar");

try {
    var cts = new CancellationTokenSource(millisecondsDelay: 100);
    await messageChannel.SubscribeAsync((messages, results, cancellationToken) => {
        Debug.Assert(messages != null);
        Debug.Assert(results != null);
        Debug.Assert(messages.Count == results.Length);

        for (var i = 0; i < results.Length; i++) {
            Console.WriteLine(messages[i]);  // "foo bar"
            results[i] = true;
        }
        return Task.CompletedTask;
    }, cts.Token);
}
catch (OperationCanceledException) { }