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Socket::BeginSend Method (array<Byte>, Int32, Int32, SocketFlags, SocketError%, AsyncCallback, Object)

Sends data asynchronously to a connected Socket.

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

[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
IAsyncResult^ BeginSend(
	array<unsigned char>^ buffer, 
	int offset, 
	int size, 
	SocketFlags socketFlags, 
	[OutAttribute] SocketError% errorCode, 
	AsyncCallback^ callback, 
	Object^ state


Type: array<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.Net.Sockets::SocketError%
A SocketError object that stores the socket error.
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 nullptr.


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 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 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.

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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