Can only set the KeepClean property before the start

websocket-sharp/Server/HttpServer.cs

@@ -244,6 +244,12 @@ namespace WebSocketSharp.Server
       }
 
       set {
+        var msg = _state.CheckIfStartable ();
+        if (msg != null) {
+          _logger.Error (msg);
+          return;
+        }
+
         _services.KeepClean = value;
       }
     }

websocket-sharp/Server/WebSocketServer.cs

@@ -372,6 +372,12 @@ namespace WebSocketSharp.Server
       }
 
       set {
+        var msg = _state.CheckIfStartable ();
+        if (msg != null) {
+          _logger.Error (msg);
+          return;
+        }
+
         _services.KeepClean = value;
       }
     }

websocket-sharp/Server/WebSocketServiceHost.cs

@@ -187,6 +187,12 @@ namespace WebSocketSharp.Server
       }
 
       set {
+        var msg = _sessions.State.CheckIfStartable ();
+        if (msg != null) {
+          _logger.Error (msg);
+          return;
+        }
+
         _sessions.KeepClean = value;
       }
     }