diff --git a/shadowsocks-csharp/Encryption/EncryptorBase.cs b/shadowsocks-csharp/Encryption/EncryptorBase.cs
index 2569418..b746678 100644
--- a/shadowsocks-csharp/Encryption/EncryptorBase.cs
+++ b/shadowsocks-csharp/Encryption/EncryptorBase.cs
@@ -1,5 +1,4 @@
-using System.Security.Cryptography;
-using System.Text;
+using System.Text;
 
 namespace Shadowsocks.Encryption
 {
diff --git a/shadowsocks-csharp/Encryption/EncryptorFactory.cs b/shadowsocks-csharp/Encryption/EncryptorFactory.cs
index 7959487..f1091f9 100644
--- a/shadowsocks-csharp/Encryption/EncryptorFactory.cs
+++ b/shadowsocks-csharp/Encryption/EncryptorFactory.cs
@@ -1,7 +1,7 @@
-
-using System;
+using System;
 using System.Collections.Generic;
 using System.Reflection;
+
 namespace Shadowsocks.Encryption
 {
     public static class EncryptorFactory
diff --git a/shadowsocks-csharp/Encryption/IEncryptor.cs b/shadowsocks-csharp/Encryption/IEncryptor.cs
index b45c62c..a1ddc7f 100644
--- a/shadowsocks-csharp/Encryption/IEncryptor.cs
+++ b/shadowsocks-csharp/Encryption/IEncryptor.cs
@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Text;
 
 namespace Shadowsocks.Encryption
 {
diff --git a/shadowsocks-csharp/Encryption/MbedTLSEncryptor.cs b/shadowsocks-csharp/Encryption/MbedTLSEncryptor.cs
index 0d657f3..e9272c5 100644
--- a/shadowsocks-csharp/Encryption/MbedTLSEncryptor.cs
+++ b/shadowsocks-csharp/Encryption/MbedTLSEncryptor.cs
@@ -1,9 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
-using System.Security.Cryptography;
-using System.Text;
-using System.Threading;
 
 namespace Shadowsocks.Encryption
 {
diff --git a/shadowsocks-csharp/Encryption/SodiumEncryptor.cs b/shadowsocks-csharp/Encryption/SodiumEncryptor.cs
index 8052fda..ce8f55b 100755
--- a/shadowsocks-csharp/Encryption/SodiumEncryptor.cs
+++ b/shadowsocks-csharp/Encryption/SodiumEncryptor.cs
@@ -1,7 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Text;
-using System.Threading;
 
 namespace Shadowsocks.Encryption
 {
diff --git a/shadowsocks-csharp/Util/SocketUtil.cs b/shadowsocks-csharp/Util/SocketUtil.cs
index ad413d4..46e18aa 100644
--- a/shadowsocks-csharp/Util/SocketUtil.cs
+++ b/shadowsocks-csharp/Util/SocketUtil.cs
@@ -52,6 +52,7 @@ namespace Shadowsocks.Util
 
             if (endPoint is DnsEndPoint)
             {
+                // use dual-mode socket
                 var socket = new Socket(AddressFamily.InterNetworkV6, socketType, protocolType);
                 socket.SetSocketOption(SocketOptionLevel.IPv6, (SocketOptionName)27, false);