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Socket.ConnectAsync Method (SocketType, ProtocolType, SocketAsyncEventArgs)

Begins an asynchronous request for a remote host connection.

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

public static bool ConnectAsync(
	SocketType socketType,
	ProtocolType protocolType,
	SocketAsyncEventArgs e


Type: System.Net.Sockets.SocketType
One of the SocketType values.
Type: System.Net.Sockets.ProtocolType
One of the ProtocolType values.
Type: System.Net.Sockets.SocketAsyncEventArgs
The System.Net.Sockets.SocketAsyncEventArgs object to use for this asynchronous socket operation.

Return Value

Type: System.Boolean
Returns true if the I/O operation is pending. The SocketAsyncEventArgs.Completed event on the e parameter will be raised upon completion of the operation.
Returns false if the I/O operation completed synchronously. In this case, The SocketAsyncEventArgs.Completed event on the e parameter will not be raised and the e object passed as a parameter may be examined immediately after the method call returns to retrieve the result of the operation.


An argument is not valid. This exception occurs if multiple buffers are specified, the SocketAsyncEventArgs.BufferList property is not null.


The e parameter cannot be null and the SocketAsyncEventArgs.RemoteEndPoint cannot be null.


The Socket is listening or a socket operation was already in progress using the System.Net.Sockets.SocketAsyncEventArgs object specified in the e parameter.


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


Windows XP or later is required for this method. This exception also occurs if the local endpoint and the SocketAsyncEventArgs.RemoteEndPoint are not the same address family.


The Socket has been closed.


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

If you are using a connection-oriented protocol, the M:System.Net.Sockets.Socket.ConnectAsync(System.Net.Sockets.SocketType,System.Net.Sockets.ProtocolType,System.Net.Sockets.SocketAsyncEventArgs) method starts an asynchronous request for a connection to the remote host. If you are using a connectionless protocol, ConnectAsync establishes a default remote host specified by the socketType and protocolType parameters.

To be notified of completion, you must create a callback method that implements the EventHandler<SocketAsyncEventArgs> delegate and attach the callback to the SocketAsyncEventArgs.Completed event.

The caller must set the SocketAsyncEventArgs.RemoteEndPoint property to the IPEndPoint of the remote host to connect to.

The caller may set the SocketAsyncEventArgs.UserToken property to any user state object desired before calling the ConnectAsync method, so that the information will be retrievable in the callback method. If the callback needs more information than a single object, a small class can be created to hold the other required state information as members.

If you are using a connectionless protocol such as UDP, you do not have to call ConnectAsync before sending and receiving data. You can use SendToAsync and ReceiveFromAsync to communicate with a remote host. If you do call ConnectAsync, any datagrams that arrive from an address other than the specified default will be discarded. If you want to change the default remote host, call the ConnectAsync method again with the desired endpoint.

If you wish to set the default remote host to a broadcast address, you must first call SetSocketOption and set Broadcast to true. If this is not done, the ConnectAsync method will throw a SocketException.

The following properties and events on the System.Net.Sockets.SocketAsyncEventArgs object are required:

Optionally, a buffer may be provided which will atomically be sent on the socket after the ConnectAsync method succeeds. In this case, the SocketAsyncEventArgs.Buffer property needs to be set to the buffer containing the data to send and the SocketAsyncEventArgs.Count property needs to be set to the number of bytes of data to send from the buffer. Once a connection is established, this buffer of data is sent.

If you are using a connection-oriented protocol and do not call Bind before calling ConnectAsync, the underlying service provider will assign the most appropriate local network address and port number.

If you are using a connectionless protocol, the service provider will not assign a local network IP address and port number until you call the SendAsync or ReceiveAsync methods.

The ConnectAsync method throws NotSupportedException if the address family of the Socket and the SocketAsyncEventArgs.RemoteEndPoint are not the same address family.


If you receive a SocketException when calling this method, 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.

.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, 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.