Difference between [...]Async and Begin[...] .net asynchronous APIs

Can anybody explain me what is the difference between the Begin[...开发者_JAVA百科]/End[...] asynchronous API pattern and the later [...]Async pattern in .NET 3.5?

• Why was the later created?
• Why would one prefer one pattern over another?

For example, Socket.BeginAccept() and Socket.AcceptAsync().

Note that most *Async methods (with corresponding *Completed events) are using the Event-Based Asynchronous Pattern. The older (but still perfectly valid) Begin* and End* is a pattern called the Asynchronous Programming Model. The Socket class is an exception to this rule; its *Async methods do not have any corresponding events; it's essentially just APM done in a way to avoid excessive memory allocations.

The biggest difference between APM and EBAP is the thread used for completion notification. APM will call back on a thread pool thread (unless the request completes synchronously). EBAP will use a cross-framework strategy to call back on a UI thread (if the operation was started from a UI thread).

However, both APM and EBAP are being replaced with a much more flexible approach based on the Task Parallel Library. Since the TPL can wrap APMs easily, older classes will likely not be updated directly; extension methods are used to provide Task equivalents for the old APM methods.

Update 2012-07-14: I was wrong when I stated "older classes will likely not be updated directly". For performance reasons, the BCL/TPL teams decided to review each BCL type and add TAP methods directly instead of using extension methods. These changes will be in .NET 4.5.

MSDN will answer that better than me:

http://msdn.microsoft.com/en-us/library/system.net.sockets.socketasynceventargs.aspx

The main feature of these enhancements is the avoidance of the repeated allocation and synchronization of objects during high-volume asynchronous socket I/O. The Begin/End design pattern currently implemented by the System.Net.Sockets.Socket class requires a System.IAsyncResult object be allocated for each asynchronous socket operation.