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

Sends data asynchronously to a connected Socket.

Namespace:  System.Net.Sockets
Assembly:  System (in System.dll)

public function BeginSend(
	buffer : byte[], 
	offset : int, 
	size : int, 
	socketFlags : SocketFlags, 
	callback : AsyncCallback, 
	state : Object
) : IAsyncResult


Type: System.Byte[]

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

Type: System.Int32

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

Type: System.Int32

The number of bytes to send.

Type: System.Net.Sockets.SocketFlags

A bitwise combination of the SocketFlags values.

Type: System.AsyncCallback

The AsyncCallback delegate.

Type: System.Object

An object that contains state information for this request.

Return Value

Type: System.IAsyncResult
An IAsyncResult that references the asynchronous send.


buffer is a null reference (Nothing in Visual Basic).


An error occurred when attempting to access the socket. See remarks section below.


offset is less than 0.


offset is less 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.


The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

The BeginSend method starts an asynchronous send operation to the remote host established in the Connect, BeginConnect, Accept, or BeginAccept method. BeginSend will throw an exception if you do not first call Accept, BeginAccept, Connect, or BeginConnect. Calling the BeginSend method gives you the ability to send data within a separate execution thread.

You can create a callback method that implements the AsyncCallback delegate and pass its name to the BeginSend 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 or structure to hold the Socket and the other required information. Pass an instance of this class to the BeginSend method through the state parameter.

Your callback method should invoke the EndSend method. When your application calls BeginSend, the system will use a separate thread to execute the specified callback method, and will block on EndSend 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 BeginSend 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 on writing callback methods see Callback Sample.

Although intended for connection-oriented protocols, BeginSend also works for connectionless protocols, provided that you first call the Connect or BeginConnect method to establish a default remote host. If you are using a connectionless protocol and plan to send data to several different hosts, you should use BeginSendTo. It is okay to use BeginSendTo even after you have established a default remote host with Connect. You can also change the default remote host prior to calling BeginSend by making another call to Connect or BeginConnect. With connectionless protocols, 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 BeginSend 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 in the MSDN library for a detailed description of the error.


All I/O initiated by a given thread is canceled when that thread exits. A pending asynchronous operation can fail if the thread exits before the operation completes.


state is an instantiation of a user-defined class.


The successful completion of a send does not indicate that the data was successfully delivered. If no buffer space is available within the transport system to hold the data to be transmitted, send will block unless the socket has been placed in nonblocking mode.


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


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.

The following code example begins asynchronously sending data to a remote host.

No code example is currently available or this language may not be supported.
Socket* s = __try_cast<Socket*>(ar->AsyncState);
StateObject* so2 = new StateObject();
so2->workSocket = s;
Byte buff[] = Encoding::ASCII->GetBytes(S"This is a test");
s->BeginSend(buff, 0, buff->Length, SocketFlags::None,
    new AsyncCallback(0, &Async_Send_Receive::Send_Callback), so2);    

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0