This chapter is from the book
This chapter is from the book
This chapter is from the book
Input and Output in .NET
To make a crude generalization, the input/output functions in the .NET Framework can be divided into two broad categories, irrespective of the data storage (disk, memory, etc.) that is being written to or read from.
Data can be treated as a stream of bytes or characters. For example, we could read 500 bytes from a file and write them to a memory buffer. Data can also be treated as a set of objects. Reading and writing the objects is referred to as deserializing and serializing 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.
Stream Classes
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 with the IO examples) illustrates how to use the Stream classes. If the file does not exist, 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 creates and writes the file, and the second time reads and displays the file.)
unsigned char data __gc[] =
new unsigned char __gc [10];
FileStream *fs = new FileStream(
"FileStreamTest.txt", FileMode::OpenOrCreate);
if (fs->Length == 0)
{
Console::WriteLine("Writing Data...");
for (short i = 0; i < 10; i++)
data[i] = (unsigned char)i;
fs->Write(data, 0, 10);
}
else
{
fs->Seek(-5, SeekOrigin::End);
int count = fs->Read(data, 0, 10);
for (int i = 0; i < count; i++)
{
Console::WriteLine(data[i]);
}
}
fs->Close();
Primitive Data Types 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, and Int32, in and out of a stream, you should 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 data type 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.)
FileStream *fs = new FileStream(
"BinaryTest.bin", FileMode::OpenOrCreate);
if (fs->Length == 0)
{
Console::WriteLine("Writing Data...");
BinaryWriter *w = new BinaryWriter(fs);
for (short i = 0; i < 10; i++)
w->Write(i);
w->Close();
}
else
{
BinaryReader *r = new BinaryReader(fs);
for (int i = 0; i < 10; i++)
Console::WriteLine(r->ReadInt16());
r->Close();
}
fs->Close();
TextReader and TextWriter
The TextReader and TextWriter 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, respectively. StringReader and StringWriter read and write data in a character string, which is stored in an underlying StringBuilder object. The StringWriter constructor can take a StringBuilder object. The StringBuilder class was discussed in Chapter 3.
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 can create a Stream object and pass it to the StreamReader or StreamWriter 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. Run the program twice, first to create the file, and then to read it.
FileStream *fs = new FileStream(
"TextTest.txt", FileMode::OpenOrCreate);
if (fs->Length == 0)
{
Console::WriteLine("Writing Data...");
StreamWriter *sw = new StreamWriter(fs);
sw->Write(100);
sw->WriteLine(" One Hundred");
sw->WriteLine("End of File");
sw->Close();
}
else
{
String *text;
StreamReader *sr = new StreamReader(fs);
text = sr->ReadLine();
while (text != 0)
{
Console::WriteLine(text);
text = sr->ReadLine();
}
sr->Close();
}
fs->Close();
File Manipulation
The framework has two classes named File and FileInfo 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 static 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 13.
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 also 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 file names needed for input arguments have to be full paths. Or, you might want to manipulate only parts of the path. The Path class has static methods that make this easier. The Path class has static 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 static 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 the 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. In this example, the static Delete method of the File class is used to remove a specified file. The static CreateText method then creates a new file and returns a StreamWriter instance that is used to write some text to the file. The stream is then closed, and 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 are 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.
File::Delete("file2.txt");
StreamWriter *sw =
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();
File::Move("file.txt", "file2.txt");
FileInfo *fileInfo = new FileInfo("file2.txt");
Console::WriteLine(
"File {0} is {1} bytes in length, created on {2}",
fileInfo->FullName,
__box(fileInfo->Length),
__box(fileInfo->CreationTime));
Console::WriteLine("");
StreamReader *sr = fileInfo->OpenText();
String *s = sr->ReadLine();
while (s != 0)
{
Console::WriteLine(s);
s = sr->ReadLine();
}
sr->Close();
Console::WriteLine("");