Implementation carried over from v1.

code/MessengerApi.QueryClient/QueryClient.cs

@@ -0,0 +1,71 @@
+using portaloggy;
+using MessengerApi.Model.Messages;
+using MessengerApi.Model;
+
+namespace MessengerApi
+{
+    public interface IQueryClient : IDisposable
+    {
+        Task<InboxMessage> Query(OutboxMessage message);
+    }
+
+    public class QueryClient : SubscriptionClient, IQueryClient
+    {
+        private readonly int _timeoutInSeconds;
+
+        public QueryClient(
+            Credentials credentials,
+            HttpClient httpClient = null, 
+            ILogger logger = null,
+            int maximumPermittedWaitTimeInSeconds = 10) :base(credentials, httpClient, logger)
+        {
+            this._timeoutInSeconds = maximumPermittedWaitTimeInSeconds;
+        }
+
+        public async Task<InboxMessage> Query(OutboxMessage message)
+        {
+            try
+            {
+                var messageTypeMask = $"{message.PayloadType}-{Guid.NewGuid()}";
+                var maskedMessage = new OutboxMessage
+                {
+                    Payload = message.Payload,
+                    LifespanInSeconds = message.LifespanInSeconds,
+                    PayloadType = messageTypeMask,
+                    ToUserId = message.ToUserId
+                };
+                
+                var cts = new CancellationTokenSource(TimeSpan.FromSeconds(this._timeoutInSeconds));
+                var sub = this.Subscribe(messageTypeMask);
+                var result = (InboxMessage)null;
+
+                sub.OnMessage += (source, message) =>
+                {
+                    result = message;
+                    cts.Cancel();
+                };
+
+                this.SendMessage(maskedMessage);
+
+                while (!cts.IsCancellationRequested)
+                {
+                    await Task.Delay(TimeSpan.FromSeconds(1));
+                }
+
+                this.Unsubscribe(sub);
+
+                if (result == null)
+                {
+                    throw new InvalidOperationException("Couldn't retrieve response.");
+                }
+
+                result.PayloadType = message.PayloadType;
+                return result;
+            }
+            catch
+            {
+                throw;
+            }
+        }
+    }
+}