Programming C# 3.0, 5th Edition

Asynchronous I/O

All the programs you've looked at so far perform synchronous I/O, meaning that while your program is reading or writing, all other activity is stopped. It can take a long time (relatively speaking) to read data to or from the backing store, especially if the backing store is a slow disk or (horrors!) a source on the Internet.

With large files, or when reading or writing across the network, you'll want asynchronous I/O, which allows you to begin a read and then turn your attention to other matters while the CLR fulfills your request. The .NET Framework provides asynchronous I/O through the BeginRead( ) and BeginWrite( ) methods of Stream.

The sequence is to call BeginRead( ) on your file and then to go on to other, unrelated work while the read continues, possibly in another thread. When the read completes, you are notified via a callback method. You can then process the data that was read, kick off another read, and then go back to your other work.

In addition to the three parameters you've used in the binary read (the buffer, the offset, and how many bytes to read), BeginRead( ) asks for a delegate and a state object.

Tip

This is an instance of the more general async pattern seen throughout .NET (e.g., async stream I/O, async socket operations, async delegate invocation, etc.).

The delegate is an optional callback method, which, if provided, is called when the data is read. The state object is also optional. In this example, pass in null for the state object. The state ...