Socket.BeginSendTo(Byte[], Int32, Int32, SocketFlags, EndPoint, AsyncCallback, Object) Method


Sends data asynchronously to a specific remote host.

 IAsyncResult ^ BeginSendTo(cli::array <System::Byte> ^ buffer, int offset, int size, System::Net::Sockets::SocketFlags socket_flags, System::Net::EndPoint ^ remote_end, AsyncCallback ^ callback, System::Object ^ state);
public IAsyncResult BeginSendTo (byte[] buffer, int offset, int size, System.Net.Sockets.SocketFlags socket_flags, System.Net.EndPoint remote_end, AsyncCallback callback, object state);
member this.BeginSendTo : byte[] * int * int * System.Net.Sockets.SocketFlags * System.Net.EndPoint * AsyncCallback * obj -> IAsyncResult
Public Function BeginSendTo (buffer As Byte(), offset As Integer, size As Integer, socket_flags As SocketFlags, remote_end As EndPoint, callback As AsyncCallback, state As Object) As IAsyncResult



An array of type Byte that contains the data to send.


The zero-based position in buffer at which to begin sending data.


The number of bytes to send.


A bitwise combination of the SocketFlags values.


An EndPoint that represents the remote device.


The AsyncCallback delegate.


An object that contains state information for this request.


An IAsyncResult that references the asynchronous send.


buffer is null.


remoteEP is null.

An error occurred when attempting to access the socket.

offset is less than 0.


offset is greater than the length of buffer.


size is less than 0.


size is greater than the length of buffer minus the value of the offset parameter.

The Socket has been closed.

A caller higher in the call stack does not have permission for the requested operation.


The following code example asynchronously sends data to the specified remote host.

IPHostEntry^ lipa = Dns::Resolve( "" );
IPEndPoint^ lep = gcnew IPEndPoint( lipa->AddressList[ 0 ], 11000 );

Socket^ s = gcnew Socket( lep->Address->AddressFamily,
   ProtocolType::Tcp );
   while ( true )

      array<Byte>^ buff = Encoding::ASCII->GetBytes( "This is a test" );

      Console::WriteLine( "Sending Message Now.." );
      s->BeginSendTo( buff, 0, buff->Length, SocketFlags::None, lep,
         gcnew AsyncCallback( &Async_Send_Receive::Connect_Callback ), s );

catch ( Exception^ e ) 
   Console::WriteLine( e );
IPHostEntry lipa = Dns.Resolve("");
IPEndPoint lep = new IPEndPoint(lipa.AddressList[0], 11000);

   Socket s = new Socket(lep.Address.AddressFamily,

             byte[] buff = Encoding.ASCII.GetBytes("This is a test");
             Console.WriteLine("Sending Message Now..");
             s.BeginSendTo(buff, 0, buff.Length, 0, lep, new AsyncCallback(Async_Send_Receive.SendTo_Callback), s);

   catch (Exception e){
   Dim lipa As IPHostEntry = Dns.Resolve("")
   Dim lep As New IPEndPoint(lipa.AddressList(0), 11000)
   Dim s As New Socket(lep.Address.AddressFamily, SocketType.Stream, ProtocolType.Tcp)
      While True
         Dim buff As Byte() = Encoding.ASCII.GetBytes("This is a test")
         Console.WriteLine("Sending Message Now..")
         s.BeginSendTo(buff, 0, buff.Length, 0, lep, New AsyncCallback(AddressOf Async_Send_Receive.SendTo_Callback), s)
      End While
   Catch e As Exception
   End Try
End Sub


The BeginSendTo method starts an asynchronous send operation to the remote host specified in the remoteEP parameter. Calling the BeginSendTo method gives you the ability to send data within a separate execution thread. Although intended for connectionless protocols, BeginSendTo works with both connectionless and connection-oriented protocols.

You can create a callback method that implements the AsyncCallback delegate and pass its name to the BeginSendTo method. To do this, at the very minimum, your state parameter must contain the connected or default Socket being used for communication. If your callback needs more information, you can create a small class to hold the Socket, and the other required information. Pass an instance of this class to the BeginSendTo method through the state parameter.

Your callback method should invoke the EndSendTo method. When your application calls BeginSendTo, the system will use a separate thread to execute the specified callback method, and will block on EndSendTo until the Socket sends the number of bytes requested or throws an exception. If you want the original thread to block after you call the BeginSendTo method, use the WaitHandle.WaitOne method. Call the Set method on a T:System.Threading.ManualResetEvent in the callback method when you want the original thread to continue executing. For additional information about writing callback methods see Marshaling a Delegate as a Callback Method.

If you are using a connection-oriented protocol, you must first call the Connect, BeginConnect, Accept, or BeginAccept method, or BeginSendTo will throw a SocketException. BeginSendTo will ignore the remoteEP parameter and send data to the EndPoint established in the Connect, BeginConnect, Accept, or BeginAccept method.

If you are using a connectionless protocol, you do not need to establish a default remote host with the Connect or BeginConnect method prior to calling SendTo. You only need to do this if you intend to call the BeginSend method. If you do call the Connect or BeginConnect method prior to calling SendTo, the remoteEP parameter will override the specified default remote host for that send operation only. You are also not required to call the Bind method. In this case, the underlying service provider will assign the most appropriate local network address and port number. Use a port number of zero if you want the underlying service provider to select a free port. If you need to identify the assigned local network address and port number, you can use the LocalEndPoint property after the EndSendTo method successfully completes.

If you want to send data to a broadcast address, you must first call the SetSocketOption method and set the socket option to SocketOptionName.Broadcast. -You must also be sure that the size of your buffer does not exceed the maximum packet size of the underlying service provider. If it does, the datagram will not be sent and EndSendTo will throw a SocketException.

If you specify the DontRoute flag as the socketflags parameter, the data you are sending will not be routed.


If you receive a SocketException, use the SocketException.ErrorCode property to obtain the specific error code. After you have obtained this code, refer to the Windows Sockets version 2 API error code documentation for a detailed description of the error.


This member outputs trace information when you enable network tracing in your application. For more information, see Network Tracing in the .NET Framework.


The execution context (the security context, the impersonated user, and the calling context) is cached for the asynchronous Socket methods. After the first use of a particular context (a specific asynchronous Socket method, a specific Socket instance, and a specific callback), subsequent uses of that context will see a performance improvement.

Applies to

See also