public Task <SlackMessage> SendAccessoryMessage(string channel, string message, string imageUrl) { Console.WriteLine("[Send] " + message + " / " + imageUrl); var ts = new TaskCompletionSource <SlackMessage>(); Slack.PostBlock( _ => { ProcessCompletion(ts, channel, _); }, channel, message, as_user: true, accessory_image: imageUrl); return(ts.Task); }