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Socket.BeginReceiveFrom Method

Begins to asynchronously receive data from a specified network device.

Namespace:  System.Net.Sockets
Assembly:  System (in System.dll)
[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public IAsyncResult BeginReceiveFrom(
	byte[] buffer,
	int offset,
	int size,
	SocketFlags socketFlags,
	ref EndPoint remoteEP,
	AsyncCallback callback,
	Object state
)

Parameters

buffer
Type: System.Byte[]

An array of type Byte that is the storage location for the received data.

offset
Type: System.Int32

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

size
Type: System.Int32

The number of bytes to receive.

socketFlags
Type: System.Net.Sockets.SocketFlags

A bitwise combination of the SocketFlags values.

remoteEP
Type: System.Net.EndPoint%

An EndPoint that represents the source of the data.

callback
Type: System.AsyncCallback

The AsyncCallback delegate.

state
Type: System.Object

An object that contains state information for this request.

Return Value

Type: System.IAsyncResult
An IAsyncResult that references the asynchronous read.
ExceptionCondition
ArgumentNullException

buffer is null.

-or-

remoteEP is null.

SocketException

An error occurred when attempting to access the socket. See the Remarks section for more information.

ArgumentOutOfRangeException

offset is less than 0.

-or-

offset is greater than the length of buffer.

-or-

size is less than 0.

-or-

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

ObjectDisposedException

The Socket has been closed.

SecurityException

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

NoteNote:

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 BeginReceiveFrom method starts asynchronously reading connectionless datagrams from a remote host. Calling the BeginReceiveFrom method gives you the ability to receive data within a separate execution thread.

You can create a callback method that implements the AsyncCallback delegate and pass its name to the BeginReceiveFrom 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 BeginReceiveFrom method through the state parameter.

Your callback method should invoke the EndReceiveFrom method. When your application calls BeginReceiveFrom, the system will use a separate thread to execute the specified callback method, and it will block on EndReceiveFrom until the Socket reads data or throws an exception. If you want the original thread to block after you call the BeginReceiveFrom method, use WaitHandle.WaitOne. 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.

NoteNote:

Before calling BeginReceiveFrom, you must explicitly bind the Socket to a local endpoint using the Bind method, or BeginReceiveFrom will throw a SocketException.

This method reads data into the buffer parameter, and captures the remote host endpoint from which the data is sent. For information on how to retrieve this endpoint, refer to EndReceiveFrom. This method is most useful if you intend to asynchronously receive connectionless datagrams from an unknown host or multiple hosts. In these cases, BeginReceiveFrom will read the first enqueued datagram received into the local network buffer. If the datagram you receive is larger than the size of buffer, the BeginReceiveFrom method will fill buffer with as much of the message as is possible, and throw a SocketException. If you are using an unreliable protocol, the excess data will be lost. If you are using a reliable protocol, the excess data will be retained by the service provider and you can retrieve it by calling the BeginReceiveFrom method with a large enough buffer.

To guarantee that the remote host endpoint is always returned, an application should explicitly bind the Socket to a local endpoint using the Bind method and then call the SetSocketOption method with the optionLevel parameter set to IP or IPv6 as appropriate, the optionName parameter set to PacketInformation, and the optionValue parameter to enable this option before calling the BeginReceiveFrom method. Otherwise, it is possible for the remote host endpoint to not be returned when the sender has sent a number of datagrams before the receiver has called the BeginReceiveFrom method.

Although BeginReceiveFrom is intended for connectionless protocols, you can use a connection-oriented protocol as well. If you choose to do so, you must first either establish a remote host connection by calling the Connect / BeginConnect method or accept an incoming connection request by calling the Accept or BeginAccept method. If you call the BeginReceiveFrom method before establishing or accepting a connection, you will get a SocketException. You can also establish a default remote host for a connectionless protocol prior to calling the BeginReceiveFrom method. In either of these cases, the BeginReceiveFrom method will ignore the remoteEP parameter and only receive data from the connected or default remote host.

With connection-oriented sockets, BeginReceiveFrom will read as much data as is available up to the number of bytes specified by the size parameter.

To cancel a pending BeginReceiveFrom, call the Close method.

NoteNote:

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.

NoteNote:

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

NoteNote:

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 asynchronously receives connectionless datagrams from a remote host.

	IPHostEntry lipa = Dns.Resolve("host.contoso.com");
	IPEndPoint lep = new IPEndPoint(lipa.AddressList[0], 11000);

       Socket s = new Socket(lep.Address.AddressFamily,
       	                           SocketType.Dgram,
                                         ProtocolType.Udp);

       IPEndPoint sender = new IPEndPoint(IPAddress.Any, 0);
       EndPoint tempRemoteEP = (EndPoint)sender;
       s.Connect(sender);

       try{
            while(true){
                 allDone.Reset();
                 StateObject so2 = new StateObject();
                 so2.workSocket = s;
                 Console.WriteLine("Attempting to Receive data from host.contoso.com");

                 s.BeginReceiveFrom(so2.buffer, 0, StateObject.BUFFER_SIZE,0, ref tempRemoteEP,
	                                   new AsyncCallback(Async_Send_Receive.ReceiveFrom_Callback), so2);	
                 allDone.WaitOne();
            }
       }
       catch (Exception e){
            Console.WriteLine(e.ToString());
       }

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
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