diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index f73135ef..95737fe0 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -201,6 +201,16 @@ namespace WebSocketSharp
       }
     }
 
+    internal static string CheckIfAvailable (
+      this ServerState state, bool ready, bool start, bool shutting)
+    {
+      return (!ready && (state == ServerState.Ready || state == ServerState.Stop)) ||
+             (!start && state == ServerState.Start) ||
+             (!shutting && state == ServerState.ShuttingDown)
+             ? "This operation isn't available in: " + state
+             : null;
+    }
+
     internal static string CheckIfCanAccept (this WebSocketState state)
     {
       return state != WebSocketState.Connecting
@@ -238,40 +248,6 @@ namespace WebSocketSharp
              : null;
     }
 
-    internal static string CheckIfCanStart (this ServerState state)
-    {
-      return state == ServerState.Start || state == ServerState.ShuttingDown
-             ? "This operation has already been done."
-             : null;
-    }
-
-    internal static string CheckIfCanStop (this ServerState state)
-    {
-      return state != ServerState.Start
-             ? String.Format ("This operation isn't available ({0}).", state)
-             : null;
-    }
-
-    internal static string CheckIfStart (this ServerState state)
-    {
-      return state == ServerState.Ready
-             ? "The server hasn't yet started."
-             : state == ServerState.ShuttingDown
-               ? "The server is shutting down."
-               : state == ServerState.Stop
-                 ? "The server has already stopped."
-                 : null;
-    }
-
-    internal static string CheckIfStartable (this ServerState state)
-    {
-      return state == ServerState.Start
-             ? "The server has already started."
-             : state == ServerState.ShuttingDown
-               ? "The server is shutting down."
-               : null;
-    }
-
     internal static string CheckIfValidProtocols (this string[] protocols)
     {
       return protocols.Contains (
diff --git a/websocket-sharp/Server/HttpServer.cs b/websocket-sharp/Server/HttpServer.cs
index 01e3c3c7..d4ec6ad9 100644
--- a/websocket-sharp/Server/HttpServer.cs
+++ b/websocket-sharp/Server/HttpServer.cs
@@ -298,7 +298,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -346,7 +346,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -398,7 +398,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -426,7 +426,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -449,7 +449,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -473,7 +473,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -498,7 +498,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -521,7 +521,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable () ?? value.CheckIfValidWaitTime ();
+        var msg = _state.CheckIfAvailable (true, false, false) ?? value.CheckIfValidWaitTime ();
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -892,7 +892,7 @@ namespace WebSocketSharp.Server
     public void Start ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStart () ?? checkIfCertificateExists ();
+        var msg = _state.CheckIfAvailable (true, false, false) ?? checkIfCertificateExists ();
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -911,7 +911,7 @@ namespace WebSocketSharp.Server
     public void Stop ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStop ();
+        var msg = _state.CheckIfAvailable (false, true, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -939,7 +939,9 @@ namespace WebSocketSharp.Server
     public void Stop (ushort code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfAvailable (false, true, false) ??
+                  WebSocket.CheckCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -975,7 +977,9 @@ namespace WebSocketSharp.Server
     public void Stop (CloseStatusCode code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfAvailable (false, true, false) ??
+                  WebSocket.CheckCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
diff --git a/websocket-sharp/Server/WebSocketServer.cs b/websocket-sharp/Server/WebSocketServer.cs
index cd9833ec..457df8c6 100644
--- a/websocket-sharp/Server/WebSocketServer.cs
+++ b/websocket-sharp/Server/WebSocketServer.cs
@@ -303,7 +303,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -351,7 +351,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -403,7 +403,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -431,7 +431,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -455,7 +455,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -480,7 +480,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable ();
+        var msg = _state.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -503,7 +503,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfStartable () ?? value.CheckIfValidWaitTime ();
+        var msg = _state.CheckIfAvailable (true, false, false) ?? value.CheckIfValidWaitTime ();
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -795,7 +795,7 @@ namespace WebSocketSharp.Server
     public void Start ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStart () ?? checkIfCertificateExists ();
+        var msg = _state.CheckIfAvailable (true, false, false) ?? checkIfCertificateExists ();
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -814,7 +814,7 @@ namespace WebSocketSharp.Server
     public void Stop ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStop ();
+        var msg = _state.CheckIfAvailable (false, true, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -842,7 +842,9 @@ namespace WebSocketSharp.Server
     public void Stop (ushort code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfAvailable (false, true, false) ??
+                  WebSocket.CheckCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -877,7 +879,9 @@ namespace WebSocketSharp.Server
     public void Stop (CloseStatusCode code, string reason)
     {
       lock (_sync) {
-        var msg = _state.CheckIfCanStop () ?? WebSocket.CheckCloseParameters (code, reason, false);
+        var msg = _state.CheckIfAvailable (false, true, false) ??
+                  WebSocket.CheckCloseParameters (code, reason, false);
+
         if (msg != null) {
           _logger.Error (msg);
           return;
diff --git a/websocket-sharp/Server/WebSocketServiceHost.cs b/websocket-sharp/Server/WebSocketServiceHost.cs
index 5d1fb378..1db348ec 100644
--- a/websocket-sharp/Server/WebSocketServiceHost.cs
+++ b/websocket-sharp/Server/WebSocketServiceHost.cs
@@ -187,7 +187,7 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _sessions.State.CheckIfStartable ();
+        var msg = _sessions.State.CheckIfAvailable (true, false, false);
         if (msg != null) {
           _logger.Error (msg);
           return;
@@ -221,7 +221,9 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _sessions.State.CheckIfStartable () ?? value.CheckIfValidWaitTime ();
+        var msg = _sessions.State.CheckIfAvailable (true, false, false) ??
+                  value.CheckIfValidWaitTime ();
+
         if (msg != null) {
           _logger.Error (msg);
           return;
diff --git a/websocket-sharp/Server/WebSocketServiceManager.cs b/websocket-sharp/Server/WebSocketServiceManager.cs
index 1ed1e6fa..2e3a218b 100644
--- a/websocket-sharp/Server/WebSocketServiceManager.cs
+++ b/websocket-sharp/Server/WebSocketServiceManager.cs
@@ -385,7 +385,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void Broadcast (byte[] data)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -405,7 +405,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void Broadcast (string data)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -434,7 +434,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void BroadcastAsync (byte[] data, Action completed)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -462,7 +462,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void BroadcastAsync (string data, Action completed)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -494,7 +494,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void BroadcastAsync (Stream stream, int length, Action completed)
     {
-      var msg = _state.CheckIfStart () ??
+      var msg = _state.CheckIfAvailable (false, true, false) ??
                 stream.CheckIfCanRead () ??
                 (length < 1 ? "'length' is less than 1." : null);
 
@@ -538,7 +538,7 @@ namespace WebSocketSharp.Server
     /// </returns>
     public Dictionary<string, Dictionary<string, bool>> Broadping ()
     {
-      var msg = _state.CheckIfStart ();
+      var msg = _state.CheckIfAvailable (false, true, false);
       if (msg != null) {
         _logger.Error (msg);
         return null;
@@ -567,7 +567,9 @@ namespace WebSocketSharp.Server
         return Broadping ();
 
       byte[] data = null;
-      var msg = _state.CheckIfStart () ?? WebSocket.CheckPingParameter (message, out data);
+      var msg = _state.CheckIfAvailable (false, true, false) ??
+                WebSocket.CheckPingParameter (message, out data);
+
       if (msg != null) {
         _logger.Error (msg);
         return null;
@@ -592,7 +594,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public bool TryGetServiceHost (string path, out WebSocketServiceHost host)
     {
-      var msg = _state.CheckIfStart () ?? path.CheckIfValidServicePath ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? path.CheckIfValidServicePath ();
       if (msg != null) {
         _logger.Error (msg);
         host = null;
diff --git a/websocket-sharp/Server/WebSocketSessionManager.cs b/websocket-sharp/Server/WebSocketSessionManager.cs
index 051aa023..ddd0ca6f 100644
--- a/websocket-sharp/Server/WebSocketSessionManager.cs
+++ b/websocket-sharp/Server/WebSocketSessionManager.cs
@@ -393,7 +393,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void Broadcast (byte[] data)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -413,7 +413,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void Broadcast (string data)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -442,7 +442,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void BroadcastAsync (byte[] data, Action completed)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -470,7 +470,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void BroadcastAsync (string data, Action completed)
     {
-      var msg = _state.CheckIfStart () ?? data.CheckIfValidSendData ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? data.CheckIfValidSendData ();
       if (msg != null) {
         _logger.Error (msg);
         return;
@@ -502,7 +502,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public void BroadcastAsync (Stream stream, int length, Action completed)
     {
-      var msg = _state.CheckIfStart () ??
+      var msg = _state.CheckIfAvailable (false, true, false) ??
                 stream.CheckIfCanRead () ??
                 (length < 1 ? "'length' is less than 1." : null);
 
@@ -545,7 +545,7 @@ namespace WebSocketSharp.Server
     /// </returns>
     public Dictionary<string, bool> Broadping ()
     {
-      var msg = _state.CheckIfStart ();
+      var msg = _state.CheckIfAvailable (false, true, false);
       if (msg != null) {
         _logger.Error (msg);
         return null;
@@ -572,7 +572,9 @@ namespace WebSocketSharp.Server
         return Broadping ();
 
       byte[] data = null;
-      var msg = _state.CheckIfStart () ?? WebSocket.CheckPingParameter (message, out data);
+      var msg = _state.CheckIfAvailable (false, true, false) ??
+                WebSocket.CheckPingParameter (message, out data);
+
       if (msg != null) {
         _logger.Error (msg);
         return null;
@@ -831,7 +833,7 @@ namespace WebSocketSharp.Server
     /// </param>
     public bool TryGetSession (string id, out IWebSocketSession session)
     {
-      var msg = _state.CheckIfStart () ?? id.CheckIfValidSessionID ();
+      var msg = _state.CheckIfAvailable (false, true, false) ?? id.CheckIfValidSessionID ();
       if (msg != null) {
         _logger.Error (msg);
         session = null;