[Fix] Throw invalid operation exception

websocket-sharp/Net/HttpListenerContext.cs

@@ -61,6 +61,7 @@ namespace WebSocketSharp.Net
     private HttpListenerRequest          _request;
     private HttpListenerResponse         _response;
     private IPrincipal                   _user;
+    private HttpListenerWebSocketContext _websocketContext;
 
     #endregion
 
@@ -220,8 +221,14 @@ namespace WebSocketSharp.Net
     ///   <paramref name="protocol"/> contains an invalid character.
     ///   </para>
     /// </exception>
+    /// <exception cref="InvalidOperationException">
+    /// This method has already been called.
+    /// </exception>
     public HttpListenerWebSocketContext AcceptWebSocket (string protocol)
     {
+      if (_websocketContext != null)
+        throw new InvalidOperationException ("The accepting is already in progress.");
+
       if (protocol != null) {
         if (protocol.Length == 0)
           throw new ArgumentException ("An empty string.", "protocol");
@@ -230,7 +237,8 @@ namespace WebSocketSharp.Net
           throw new ArgumentException ("Contains an invalid character.", "protocol");
       }
 
-      return new HttpListenerWebSocketContext (this, protocol);
+      _websocketContext = new HttpListenerWebSocketContext (this, protocol);
+      return _websocketContext;
     }
 
     #endregion