[Modify] Lock it

Author: sta

websocket-sharp/Net/HttpListener.cs

@@ -822,13 +822,15 @@ public HttpListenerContext EndGetContext (IAsyncResult asyncResult)
         throw new ArgumentException (msg, "asyncResult");
       }
 
-      if (ares.EndCalled) {
-        var msg = "This IAsyncResult instance cannot be reused.";
+      lock (ares.SyncRoot) {
+        if (ares.EndCalled) {
+          var msg = "This IAsyncResult instance cannot be reused.";
 
-        throw new InvalidOperationException (msg);
-      }
+          throw new InvalidOperationException (msg);
+        }
 
-      ares.EndCalled = true;
+        ares.EndCalled = true;
+      }
 
       if (!ares.IsCompleted)
         ares.AsyncWaitHandle.WaitOne ();