1.1 Introduction
This chapter lays the foundation for virtually every networking example contained in this book. Without a working knowledge of how .NET handles I/O, it may prove difficult to adapt the code examples in this book to your own needs.
I/O applies to network data transfers, as well as saving and loading to your computer’s hard disk Later chapters will describe how to perform network transfers; however, this chapter will be concerned with the underlying I/O operations that are common to both types of transfers. The first half of this chapter will demonstrate how to read and write to disk, using .NET streams.
The second half of this chapter develops the stream concept by demonstrating how to convert complex objects, such as database queries, into a format that can be written to a .NET stream.
1.2 Streams
In order to provide similar programmatic interfaces to the broad range of I/O devices with which a programmer has to contend, a stream-based architecture was developed in .NET. I/O devices can be anything from printers to hard disks to network interfaces.
Not all devices support the same functions. For example, it is possible to read only the second half of a 1-Mb file, but not possible to download only the second half of a Web page. Therefore, not all streams support the same methods.
Properties such as canRead(), canSeek(), and canWrite() indicate the capabilities of the stream when applied to a particular device.
The most important stream in the context of this book is the networkStream, but another important stream is ileStream, which is used extensively throughout this book to demonstrate file transfers over networks.
Streams can be used in two ways: asynchronously or synchronously. When using a stream synchronously, upon calling a method, the thread will halt until the operation is complete or fails. When using a stream asynchronously, the thread will return from the method call immediately, and whenever the operation is complete, a method will be called to signify the completion of the operation, or some other event, such as I/O failure.
It is not user friendly to have a program “hang” when it is waiting for an operation to complete. Therefore, synchronous method calls must be used in a separate thread.
Through the use of threads and synchronous method calls, computers achieve the illusion of being able to do several things at once. In reality, most computers have only one central processing unit (CPU), and the illusion is achieved by quickly switching between tasks every few milliseconds.
The following application illustrates the two techniques. The code in this book will tend toward using synchronous streams, but it is important to be able to recognize and understand asynchronous streams.