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Socket::BeginSendFile Method (String, array<Byte>, array<Byte>, TransmitFileOptions, AsyncCallback, Object)

Sends a file and buffers of data asynchronously to a connected Socket object.

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

[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
IAsyncResult^ BeginSendFile(
	String^ fileName, 
	array<unsigned char>^ preBuffer, 
	array<unsigned char>^ postBuffer, 
	TransmitFileOptions flags, 
	AsyncCallback^ callback, 
	Object^ state


Type: System::String
A string that contains the path and name of the file to be sent. This parameter can be nullptr.
Type: array<System::Byte>
A Byte array that contains data to be sent before the file is sent. This parameter can be nullptr.
Type: array<System::Byte>
A Byte array that contains data to be sent after the file is sent. This parameter can be nullptr.
Type: System.Net.Sockets::TransmitFileOptions
A bitwise combination of TransmitFileOptions values.
Type: System::AsyncCallback
An AsyncCallback delegate to be invoked when this operation completes. This parameter can be nullptr.
Type: System::Object
A user-defined object that contains state information for this request. This parameter can be nullptr.

Return Value

Type: System::IAsyncResult
An IAsyncResult object that represents the asynchronous operation.


The Socket object has been closed.


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


The operating system is not Windows NT or later.

- or -

The socket is not connected to a remote host.


The file fileName was not found.

This overload requires the name of the file you want to send and a bitwise combination of TransmitFileOptions values. The preBuffer parameter contains any data you want to precede the file. postBuffer contains data you want to follow the file. If fileName is in the local directory, it may be identified with just the name of the file; otherwise, the full path and name of the file must be specified. Wildcards ("..\\myfile.txt") and UNC share names ("\\\\shared directory\\myfile.txt") are supported. If the file is not found, the exception FileNotFoundException is thrown.

The flags parameter provides the Window Sockets service provider with additional information about the file transfer. For more information about how to use this parameter, see TransmitFileOptions.

This method uses the TransmitFile function found in the Windows Sockets 2 API. For more information about the TransmitFile function and its flags, see the Windows Sockets documentation in the MSDN Library.

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

To complete the operation, you can create a callback method that is invoked by the AsyncCallback delegate parameter. To do this, at the very minimum, the state parameter must contain the Socket object being used for communication. If your callback needs more information, you can create a class or structure to hold the Socket and the other required information. Pass an instance of this custom object to the BeginSendFile method through the state parameter.

Your callback method must invoke the EndSendFile method. When your application calls BeginSendFile, the system uses a separate thread to execute the specified callback method, and blocks on EndSendFile until the Socket sends the entire file or throws an exception. For additional information on writing callback methods see Callback Sample.

Although intended for connection-oriented protocols, BeginSendFile also works for connectionless protocols, provided that you first call the Connect or BeginConnect method to establish a default remote host. With connectionless protocols, you must also be sure that the size of your file does not exceed the maximum packet size of the underlying service provider. If it does, the datagram is not sent and BeginSendFile throws a SocketException exception.


If you receive a SocketException exception, 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.


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.

The following code example creates and connects a socket for asynchronous communication and begins sending the file "text.txt" asynchronously to the remote host. In this example, a preBuffer and a postBuffer of data is created to send with the file and the default TransmitFileOptions value is used. The callback delegate calls EndSendFile to complete the transmission.

   static void AsynchronousFileSendWithBuffers()
      // Send a file asynchronously to the remote device. Send a buffer before the file and a buffer afterwards.
      // Establish the remote endpoint for the socket.
      IPHostEntry^ ipHostInfo = Dns::GetHostEntry( Dns::GetHostName() );
      IPAddress^ ipAddress = ipHostInfo->AddressList[ 0 ];
      IPEndPoint^ remoteEP = gcnew IPEndPoint( ipAddress,11000 );

      // Create a TCP/IP socket.
      Socket^ client = gcnew Socket( AddressFamily::InterNetwork,SocketType::Stream,ProtocolType::Tcp );

      // Connect to the remote endpoint.
      client->BeginConnect( remoteEP, gcnew AsyncCallback( ConnectCallback ), client );

      // Wait for connect.

      // Send a file fileName to the remote device with preBuffer and postBuffer data.
      // Create the preBuffer data.
      String^ string1 = String::Format( "This is text data that precedes the file.{0}", Environment::NewLine );
      array<Byte>^preBuf = Encoding::ASCII->GetBytes( string1 );

      // Create the postBuffer data.
      String^ string2 = String::Format( "This is text data that will follow the file.{0}", Environment::NewLine );
      array<Byte>^postBuf = Encoding::ASCII->GetBytes( string2 );

      // There is a file test.txt in the root directory.
      String^ fileName = "C:\\test.txt";

      //Send file fileName with buffers and default flags to the remote device.
      Console::WriteLine( fileName );
      client->BeginSendFile( fileName, preBuf, postBuf, static_cast<TransmitFileOptions>(0), gcnew AsyncCallback( AsynchronousFileSendCallback ), client );

      // Release the socket.
      client->Shutdown( SocketShutdown::Both );

   static void AsynchronousFileSendCallback( IAsyncResult^ ar )
      // Retrieve the socket from the state object.
      Socket^ client = dynamic_cast<Socket^>(ar->AsyncState);

      // Complete sending the data to the remote device.
      client->EndSendFile( ar );

.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.