| Programming languages have undergone an evolution in how they deal with the important topic of input/output (I/O). Early languages, such as FORTRAN, COBOL, and the original BASIC, had I/O statements built into the language. Later languages have tended not to have I/O built into the language, but instead rely on a standard library for performing I/O, such as the <stdio.h> library in C. The library in languages like C works directly with files. Still later languages, such as C++ and Java, introduced a further abstraction called a stream . A stream serves as an intermediary between the program and the file. Read and write operations are done to the stream, which is tied to a file. This architecture is very flexible, because the same kind of read and write operations can apply not only to a file, but to other kinds of I/O, such as network sockets. This added flexibility introduces a slight additional complexity in writing programs, because you have to deal not only with files but also with streams, and there exists a considerable variety of stream classes. But the added complexity is well worth the effort, and C# strikes a nice balance, with classes that make performing common operations quite simple. Data can be treated as a stream of bytes or characters . We can read 500 bytes from a file to a memory buffer. Data can also be treated as a set of objects. Writing and reading the objects is referred to as serializing and deserializing the objects. We can serialize (write) the list of Customer objects to disk. We can then deserialize (read) the list of Customer objects back into memory. The System.IO namespace has several classes for reading and writing to various types of storage while treating the data as bytes or characters. Serialization functionality can be found in various places in the .NET Framework. The System.Runtime.Serialization namespace handles serialization of the Common Type System. The System.Xml.Serialization namespace handles XML serialization. Streams Stream is an abstract class that is the basis for reading from and writing bytes to some storage such as a file. It supports both synchronous and asynchronous reading and writing. Asynchronous methods are discussed later in this chapter. The Stream class has the typical methods that you would expect: Read , Write , Seek , Flush , and Close . The FileStream class is derived from Stream to represent the reading and writing of files as a series of bytes. The FileStream constructor builds the actual stream instance. The overridden Stream methods implement the reading and writing to the file. Other classes derived from Stream include MemoryStream , BufferedStream , and NetworkStream (in System.Net.Sockets ). The FileStream example (in the FileIO directory, as are all the I/O examples) illustrates how to use the Stream classes. If the file does not exist, then a new file is created and the numbers from 0 to 9 are written to the file. If the file already exists, the code starts reading 5 bytes from the end of the file and then writes them out. (You should run the example twice, the first time to write the data and the second time to read it.)  Dim data() As Byte = New Byte(9) {} Dim fs As System.IO.FileStream = _ New System.IO.FileStream(_ "FileStreamTest.txt", FileMode.OpenOrCreate) If fs.Length = 0 Then Console.WriteLine("Writing Data...") Dim i As Short For i = 0 To 9 data(i) = i Next fs.Write(data, 0, 10) Else fs.Seek(-5, SeekOrigin.End) Dim count As Integer = fs.Read(data, 0, 10) Dim i As Integer For i = 0 To count - 1 Console.WriteLine(data(i)) Next End If fs.Close() Primitive Datatypes and Streams The stream-derived classes work well if you are reading and writing bytes of data as a block. If you need to read and write the primitive common types, such as Boolean , String , Int32 , and so on, in and out of a stream, use the BinaryReader and the BinaryWriter classes. The Binary example in the FileIO directory shows how to use these classes. You create the appropriate stream ( FileStream in the example) and pass it to the BinaryReader or BinaryWriter constructor. You can then use one of the overloaded Read or Write methods to read or write a datatype to or from the stream. (Again, you should run the example twice. The first time creates and writes the file, and the second time reads the file.) Dim fs As FileStream = New FileStream(_ "BinaryTest.bin", FileMode.OpenOrCreate) If fs.Length = 0 Then Console.WriteLine("Writing Data...") Dim w As BinaryWriter = New BinaryWriter(fs) Dim i As Int16 For i = 0 To 9 w.Write(i) Next w.Close() Else Dim r As BinaryReader = New BinaryReader(fs) Dim i As Integer For i = 0 To 9 Console.WriteLine(r.ReadInt16()) Next r.Close() End If fs.Close() TextReader and TextWriter The TextReader and TextWrite r abstract classes treat the data as a sequential stream of characters (i.e., as text). TextReader has methods such as Close , Peek , Read , ReadBlock , ReadLine , and ReadToEnd . TextWriter has methods such as Close , Flush , Write , and WriteLine . The overloaded Read methods read characters from the stream. The overloaded Write and WriteLine methods write various types to the stream. If an object is written to the stream, the object's ToString method is used. StringReader and StringWriter are derived from TextReader and TextWriter . These classes read and write characters from a string. The StringWriter's constructor uses a StringBuilder object. The StringBuilder class was discussed in Chapter 4. StreamReader and StreamWriter are also derived from TextReader and TextWriter . They read and write text to and from a Stream object. As with the BinaryReader and BinaryWriter class, you create a stream and pass it to the constructor. Hence, these classes can use any Stream -derived class data storage. The Text example in the FileIO directory uses the StreamWriter and StreamReader classes. Dim fs As FileStream = New FileStream(_ "TextTest.txt", FileMode.OpenOrCreate) If fs.Length = 0 Then Console.WriteLine("Writing Data...") Dim sw As StreamWriter = New StreamWriter(fs) sw.Write(100) sw.WriteLine(" One Hundred") sw.WriteLine("End of File") sw.Close() Else Dim text As String Dim sr As StreamReader = New StreamReader(fs) text = sr.ReadLine() While text <> Nothing Console.WriteLine(text) text = sr.ReadLine() End While sr.Close() End If fs.Close() File Manipulation The .NET Framework has two classes that are very useful for working with files: If you need to manipulate the file in addition to reading and writing to it, the File class provides the basic functionality. Since the File class just has shared members , you have to provide the name of the file as an argument. The FileInfo class has a constructor that creates an object that represents a file. You then use the methods to manipulate that particular file. The File class methods always perform a security check. If you are going to continually access a particular file, you may want to use the FileInfo class, because the security check is made only once in the constructor. Security is discussed in more detail in Chapter 16. File Class The File class has methods for creating and opening files that return FileStream , StreamWriter , or StreamReader objects that do the actual reading and writing. The overloaded Create methods return a FileStream object. The CreateText method returns a StreamWriter . The overloaded Open method can either create a new file or open an existing one for reading or writing, depending on the method parameters. The object returned is a FileStream object. The OpenText method returns a StreamReader . The OpenRead method returns a FileStream object. The OpenWrite method returns a FileStream . The File class also has methods for copying, deleting, and moving files. You can test for the existence of a file. File attributes such as the following can be read or modified: -
creation time -
last access time -
last write time -
archive, hidden, normal, system, or temporary -
compressed, encrypted -
read-only -
is the file a directory? Path Class Many of the filenames needed for input arguments have to be full paths. Or you might only want to manipulate parts of the path. The Path class has shared methods that make this easier. The Path class has shared fields that indicate various platform-specific aspects of pathnames, such as the separator characters for directories, paths, and volumes , and the illegal characters for pathnames. Its shared methods let you change the extension of a file or find the directory where temporary files reside. The GetFullPath method is particularly useful. You can pass it a relative path such as ".\foo.txt" and it will return the full path of the file. This is very useful for the File or security classes that require the full file path. FileInfo Class The FileInfo constructor creates an object that represents a disk file. The constructor takes one argument, a string representing the name of the file. The class has properties that represent file properties such as the creation time, full pathname, and size of the file. It has creation and open methods that are analogous to the File class methods, but operate on this file instance and therefore do not need a filename parameter. The FileInfo class also has methods to move and copy the file. File Example The File example in the FileIO directory illustrates the use of the File and FileInfo classes. The shared Delete method of the File class is used to remove a previous version of a file if it is present. The shared CreateText method creates a new file and returns a StreamWriter instance that is used to write some text to the file. The stream is then closed. The static Move method then renames the file. A FileInfo instance is constructed to represent this renamed file. The complete file name, size, and creation date for the file is written to the console. The file is opened as text and a StreamReader instance is used to read and write out the contents of the file. System.IO.File.Delete("file2.txt") Dim sw As StreamWriter = _ System.IO.File.CreateText("file.txt") sw.Write("The time has come the Walrus said, ") sw.WriteLine("to talk of many things") sw.Write("Of shoes, and ships, and sealing wax, ") sw.WriteLine("of cabbages and kings") sw.Write("And why the sea is boiling hot, ") sw.WriteLine("and whether pigs have wings.") sw.Close() System.IO.File.Move("file.txt", "file2.txt") Dim fileInfo As FileInfo = New FileInfo("file2.txt") Console.WriteLine(_ "File {0} is {1} bytes in length, created on {2}", _ fileInfo.FullName, _ fileInfo.Length, _ fileInfo.CreationTime) Console.WriteLine("") Dim sr As StreamReader = fileInfo.OpenText() Dim s As String = sr.ReadLine() While s <> Nothing Console.WriteLine(s) s = sr.ReadLine() End While sr.Close() Console.WriteLine("") | 
