diff --git a/websocket-sharp/Server/WebSocketServer.cs b/websocket-sharp/Server/WebSocketServer.cs
index 85fdba26..4e95f9a2 100644
--- a/websocket-sharp/Server/WebSocketServer.cs
+++ b/websocket-sharp/Server/WebSocketServer.cs
@@ -558,6 +558,35 @@ namespace WebSocketSharp.Server
       _state = ServerState.Stop;
     }
 
+    private bool checkIfAvailable (
+      bool ready, bool start, bool shutting, bool stop, out string message
+    )
+    {
+      message = null;
+
+      if (!ready && _state == ServerState.Ready) {
+        message = "This operation is not available in: ready";
+        return false;
+      }
+
+      if (!start && _state == ServerState.Start) {
+        message = "This operation is not available in: start";
+        return false;
+      }
+
+      if (!shutting && _state == ServerState.ShuttingDown) {
+        message = "This operation is not available in: shutting down";
+        return false;
+      }
+
+      if (!stop && _state == ServerState.Stop) {
+        message = "This operation is not available in: stop";
+        return false;
+      }
+
+      return true;
+    }
+
     private string checkIfCertificateExists ()
     {
       return _secure && (_sslConfig == null || _sslConfig.ServerCertificate == null)