NetMQPoller.Dispose throws if inner socket is disposed

src/NetMQ.Tests/NetMQPollerTest.cs

@@ -402,6 +402,24 @@ public void RemoveSocket()
             }
         }
 
+        [Test]
+        public void DisposeThrowsIfSocketAlreadyDisposed()
+        {
+            var socket = new RouterSocket();
+
+            var poller = new NetMQPoller { socket };
+
+            // Dispose the socket.
+            // It is incorrect to have a disposed socket in a poller.
+            // Disposed sockets can throw into the poller's thread.
+            socket.Dispose();
+
+            // Dispose throws if a polled socket is disposed
+            var ex = Assert.Throws<NetMQException>(() => poller.Dispose());
+
+            Assert.AreEqual("Invalid state detected: NetMQPoller contains a disposed NetMQSocket. Sockets must be either removed before being disposed, or disposed after the poller is disposed.", ex.Message);
+        }
+
         [Test]
         public void SimpleTimer()
         {

src/NetMQ/NetMQPoller.cs

@@ -600,6 +600,7 @@ private void CheckDisposed()
         /// <remarks>
         /// Note that you cannot dispose the poller on the poller's thread. Doing so results in a deadlock.
         /// </remarks>
+        /// <exception cref="NetMQException">A socket in the poller has been disposed.</exception>
         public void Dispose()
         {
             // Attempting to dispose from the poller thread would cause a deadlock.
@@ -625,7 +626,11 @@ public void Dispose()
 #endif
 
             foreach (var socket in m_sockets)
+            {
+                if (socket.IsDisposed)
+                    throw new NetMQException($"Invalid state detected: {nameof(NetMQPoller)} contains a disposed {nameof(NetMQSocket)}. Sockets must be either removed before being disposed, or disposed after the poller is disposed.");
                 socket.EventsChanged -= OnSocketEventsChanged;
+            }
 
             m_disposeState = (int)DisposeState.Disposed;
         }