How to check if a socket is connected disconnected in C

Question

How can you check if a network socket  System Net Sockets Socket  is still connected if the other host doesn t send you a packet when it disconnects  e g  because it disconnected ungracefully

User · Answer

I made an extension method based on this MSDN article  This is how you can determine whether a socket is still connected   public static bool IsConnected this Socket client        bool blockingState   client Blocking       try               byte   tmp   new byte 1            client Blocking   false          client Send tmp  0  0           return true            catch  SocketException e                   10035    WSAEWOULDBLOCK         if  e NativeErrorCode Equals 10035                         return true                    else                       return false                      finally               client Blocking   blockingState

User · Answer

As Alexander Logger pointed out in zendars answer  you have to send something to be completely sure   In case your connected partner  does not read on this socket at  all  you can use the following  code   bool SocketConnected Socket s         Exit if socket is null   if  s    null      return false    bool part1   s Poll 1000  SelectMode SelectRead     bool part2    s Available    0     if  part1  amp  amp  part2      return false    else         try             int sentBytesCount   s Send new byte 1   1  0         return sentBytesCount    1            catch             return false                But even then it might take a few seconds until a broken network cable or something similar is detected

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public static class SocketExtensions       private const int BytesPerLong   4     32   8     private const int BitsPerByte   8       public static bool IsConnected this Socket socket                try                       return   socket Poll 1000  SelectMode SelectRead   amp  amp  socket Available    0                     catch  SocketException                        return false                             lt summary gt          Sets the keep-alive interval for the socket           lt  summary gt           lt param name  socket  gt The socket  lt  param gt           lt param name  time  gt Time between two keep alive  pings   lt  param gt           lt param name  interval  gt Time between two keep alive  pings  when first one fails  lt  param gt           lt returns gt If the keep alive infos were succefully modified  lt  returns gt      public static bool SetKeepAlive this Socket socket  ulong time  ulong interval                try                          Array to hold input values              var input   new                                  time    0    interval    0    0UL   1UL     on or off                 time                  interval                                Pack input into byte struct              byte   inValue   new byte 3   BytesPerLong               for  int i   0  i  lt  input Length  i                                  inValue i   BytesPerLong   3     byte  input i   gt  gt    BytesPerLong - 1    BitsPerByte   amp  0xff                   inValue i   BytesPerLong   2     byte  input i   gt  gt    BytesPerLong - 2    BitsPerByte   amp  0xff                   inValue i   BytesPerLong   1     byte  input i   gt  gt    BytesPerLong - 3    BitsPerByte   amp  0xff                   inValue i   BytesPerLong   0     byte  input i   gt  gt    BytesPerLong - 4    BitsPerByte   amp  0xff                                 Create bytestruct for result  bytes pending on server socket               byte   outValue   BitConverter GetBytes 0                   Write SIO VALS to Socket IOControl              socket SetSocketOption SocketOptionLevel Socket  SocketOptionName KeepAlive  true               socket IOControl IOControlCode KeepAliveValues  inValue  outValue                     catch  SocketException                        return false                     return true             Copy the SocketExtensions class to your project Call the SetKeepAlive on your socket - socket SetKeepAlive 1000  2   Add a timer to check the IsConnected function

User · Answer

The best way is simply to have your client send a PING every X seconds  and for the server to assume it is disconnected after not having received one for a while   I encountered the same issue as you when using sockets  and this was the only way I could do it  The socket connected property was never correct   In the end though  I switched to using WCF because it was far more reliable than sockets

User · Answer

Following the advice from NibblyPig and zendar  I came up with the code below  which works on every test I made  I ended up needing both the ping  and the poll  The ping will let me know if the cable has been disconnected  or the physical layer otherwise disrupted  router powered off  etc   But sometimes after reconnect I get a RST  the ping is ok  but the tcp state is not    region CHECKS THE SOCKET S HEALTH     if   tcpClient Client Connected                      Do a ping test to see if the server is reachable             try                               Ping pingTest   new Ping                   PingReply reply   pingTest Send ServeripAddress                   if  reply Status    IPStatus Success  ConnectionState   false                catch  PingException    ConnectionState   false                   See if the tcp state is ok             if   tcpClient Client Poll 5000  SelectMode SelectRead   amp  amp    tcpClient Client Available    0                                 ConnectionState   false                                    else   ConnectionState   false     endregion

User · Answer

The Socket Connected property will tell you whether a socket thinks it s connected  It actually reflects the status of the last send receive operation performed on the socket   If the socket has been closed by your own actions  disposing the socket  calling methods to disconnect   Socket Connected will return false  If the socket has been disconnected by other means  the property will return true until you next attempt to send or recieve information  at which point either a SocketException or ObjectDisposedException will be thrown   You can check the property after the exception has occurred  but it s not reliable before

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As Paul Turner  answered Socket Connected cannot be used in this situation  You need to poll connection every time to see if connection is still active  This is code I used   bool SocketConnected Socket s        bool part1   s Poll 1000  SelectMode SelectRead       bool part2    s Available    0       if  part1  amp  amp  part2          return false      else         return true      It works like this    s Poll returns true if    connection is closed  reset  terminated or pending  meaning no active connection  connection is active and there is data available for reading  s Available returns number of bytes available for reading if both are true     there is no data available to read so connection is not active

User · Answer

As zendar wrote  it is nice to use the Socket Poll and Socket Available  but you need to take into consideration that the socket might not have been initialized in the first place  This is the last  I believe  piece of information and it is supplied by the Socket Connected property  The revised version of the method would looks something like this    static bool IsSocketConnected Socket s                return    s Poll 1000  SelectMode SelectRead   amp  amp   s Available    0       s Connected       The long  but simpler-to-understand version           bool part1   s Poll 1000  SelectMode SelectRead           bool part2    s Available    0           if   part1  amp  amp  part2       s Connected              return false          else             return true

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The accepted answer doesn t seem to work if you unplug the network cable  Or the server crashes  Or your router crashes  Or if you forget to pay your internet bill  Set the TCP keep-alive options for better reliability   public static class SocketExtensions       public static void SetSocketKeepAliveValues this Socket instance  int KeepAliveTime  int KeepAliveInterval                  KeepAliveTime  default value is 2hr           KeepAliveInterval  default value is 1s and Detect 5 times            the native structure           struct tcp keepalive             ULONG onoff            ULONG keepalivetime            ULONG keepaliveinterval                        int size   Marshal SizeOf new uint             byte   inOptionValues   new byte size   3      4   3   12         bool OnOff   true           BitConverter GetBytes  uint  OnOff   1   0   CopyTo inOptionValues  0           BitConverter GetBytes  uint KeepAliveTime  CopyTo inOptionValues  size           BitConverter GetBytes  uint KeepAliveInterval  CopyTo inOptionValues  size   2            instance IOControl IOControlCode KeepAliveValues  inOptionValues  null                     Socket sock  sock SetSocketKeepAliveValues 2000  1000     The time value sets the timeout since data was last sent  Then it attempts to send and receive a keep-alive packet  If it fails it retries 10 times  number hardcoded since Vista AFAIK  in the interval specified before deciding the connection is dead   So the above values would result in 2 10 1   12 second detection  After that any read   wrtie   poll operations should fail on the socket

User · Answer

Just use the KeepAlive like  toster-cx says and then use the Socket Connected status to check if the Socket is still connected  Set your receive timeout at the same timeout of the keepalive  If you have more questions i am always happy to help

User · Answer

Use Socket Connected Property   --UPDATE-- As Paul Turner answered Socket Connected cannot be used in this situation  You need to poll connection every time to see if connection is still active  See 2

[c#] How to check if a socket is connected/disconnected in C#?

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