C# Files and Data Hierarchy

Introduction to Files in C#

• Files are used for long-term retention of large amounts of data, even after the program terminates.
• Data maintained in files is called persistent data.
• Files are stored on secondary storage devices such as magnetic disks, optical disks, flash memory, and magnetic tapes.

Data Hierarchy

• A bit is the smallest data item that computers support.
• Bytes are composed of eight bits.
• C# uses the Unicode character set, where characters are composed of 2 bytes.
• A field is a group of characters that conveys meaning.
• Records are collections of related fields that contain data about an entity.
• Files are a group of related records.

File Types

• Sequential Access Files: Each record is read in order based on its relative position.
• Random Access Files: Records are not in any sequence and can be accessed in any order.

Files and Streams

• C# views each file as a sequential stream of bytes.
• Each file ends either with an end-of-file marker or at a specific byte number that is recorded in a system-maintained administrative data structure.

Understanding Streams

• A stream functions as a pipeline or channel between an input device and an application, and potentially an output device.
• Streams can be read from and written to other streams, adding additional functionality (e.g., buffering, encoding, and compression).

Types of Streams in .NET Framework

• Base Streams: Read and write data from and to an external data storage mechanism.
• Pass-through Streams: Read from and write to other streams, adding additional functionality (e.g., buffering, encoding, and compression).

Main Operations with Streams

• Construct (Create): Connects to the data transfer/storage mechanism or to another stream.
• Reading: Retrieves data from the stream.
• Writing: Sends data to the stream.
• Positioning: Moves the current stream position (if positioning is supported).
• Closure: Completes the work on the stream and releases the used resources.

System.IO Classes

• BinaryReader: Reads primitive data types as binary values.
• BinaryWriter: Writes primitive types in binary to a stream.
• Directory: Exposes static methods for creating and moving through directories and subdirectories.
• DirectoryInfo: Exposes instance methods for creating and moving through directories and subdirectories.

Readers and Writers

• TextReader: Represents a reader that can read a sequential series of characters.
• TextWriter: Represents a writer that can write a sequential series of characters.
• StreamReader: Reads text data from a stream.
• StreamWriter: Writes text data to a stream.

File Streams

• File Streams are used to read and write data to and from files.
• StreamReader and StreamWriter classes are used to read and write text data to and from files.

Writing Text Files

• Use the StreamWriter class to write text data to a file.
• Use the Close() method to finish storing values.

StreamReader and StreamWriter Members

• StreamReader Members: Close(), DiscardBufferedData(), Dispose(), Peek(), Read(), ReadBlock(), ReadLine(), ReadToEnd().

• StreamWriter Members: AutoFlush (Property), Close(), Dispose(), Flush(), NewLine (Property), Write(), WriteLine().### File Streams

• The FileStream class is used to work with file streams, inherits from the Stream class, and supports all its methods and properties.

• It supports reading, writing, positioning (if the device where the file is located supports these operations).

• In its constructor, it is set: file name, way to open the file, access mode, and access for competing users.

Constructing a File Stream

• FileStream constructor: FileStream fs = new FileStream(string fileName, FileMode [, FileAccess [, FileShare]]);
• FileMode: file opening mode (Open, Append, Create, CreateNew, OpenOrCreate, Truncate)
• FileAccess: file access mode (Read, Write, ReadWrite)
• FileShare: access mode for other users while we keep the file open (None, Read, Write, ReadWrite)

BinaryReader and BinaryWriter Classes

• Used for writing and reading binary data, rather than character strings.
• Files created are readable by the computer but cannot be opened and read using Notepad.
• A program is needed to interpret the file contents.

BinaryWriter Class

• Instantiates objects of the FileStream and BinaryWriter classes.
• FileStream object is then sent as an argument to the BinaryWriter constructor.

BinaryReader Class

• Cannot simply open a binary file in Notepad and view its contents.
• Need to write program statements that use the BinaryReader class to retrieve the results.
• Constructor for the FileStream object includes values for two enumerated types: FileMode.Open and FileAccess.Read.

File and Directory Classes

• File class: contains methods to access information about files, contained in the System.IO namespace.
• Directory class: provides information about directories or folders.

File and FileInfo Classes

• Provide functionality to: Create file, Open a file, Copy file, Move (rename) a file, Delete file, Extract the time of last access and modification, Existence check.
• The File class provides these methods statically and FileInfo by instance.

Directory and DirectoryInfo Classes

• Provide functionality to: Create directories and subdirectories, Retrieve all files, Retrieve all subdirectories, Move (rename), Delete, Extract the parent directory, Existence check, Extract the full name.
• The Directory class provides these methods statically and DirectoryInfo by instance.

File Class Static Members and Their Descriptions

• AppendAllText(): Appends the specified string to the file, creating the file if it does not already exist.
• AppendText(): Creates a StreamWriter that appends UTF-8 encoded text to an existing file or creates a file if one does not exist.
• Copy(): Copies an existing file to a new file.
• Create(): Creates a file in the specified path and returns its associated StreamWriter.
• Delete(): Deletes the specified file.
• Exists(): Determines whether the specified file exists.

Directory Class Public Static Members and Their Descriptions

• CreateDirectory(): Creates all the directories in a specified path.
• Delete(): Deletes a specified directory.
• Exists(): Determines whether the given path refers to an existing directory.
• GetCreationTime(): Gets the creation date and time of a directory.
• GetCurrentDirectory(): Gets the current working directory of the application.

Utility Classes

• Utility classes allow you to manipulate files and directory structures.

• Aid in copying, moving, renaming, creating, opening, deleting, and appending files.

• Expose only static members.

• Objects are not instantiated from these classes.

• To invoke the method, the method name is preceded by the class name (as opposed to an object’s name).### Environment Special Folders

• Environment.GetFolderPath method is used to retrieve the path of special folders.

• Some special folders include DesktopDirectory, MyDocuments, Favorites, and MyMusic.

• The method returns a string representing the path of the specified special folder.

Understanding Serialization and Deserialization

• Serialization is the process of converting objects into streams of bytes.
• Deserialization is the process of converting streams of bytes back into objects.
• Disadvantages of writing to a text file include:
  • Data is easily readable and not secure.
  • Converting each field to text and combining fields with delimiters is cumbersome.

Object Serialization in C#

• Object serialization allows entire objects to be written to or read from a file.
• A serialized object is an object represented as a sequence of bytes that includes the object's data, its type, and the types of data stored in the object.

BinaryFormatter Class

• The BinaryFormatter class enables entire objects to be written to or read from a stream.
• The Serialize method writes an object's representation to a file.
• The Deserialize method reads the representation from a file and reconstructs the original object.
• Both methods throw a SerializationException if an error occurs during serialization or deserialization.

Example Serializable Class

• A class must be marked with the [Serializable] attribute to be serializable.
• In a serializable class, every instance variable must also be serializable.
• Example of a serializable class:
  • [Serializable] class Employee with properties EmpNum, Name, and Salary.

Serializable Class Requirements

• In a serializable class, every instance variable must also be serializable.
• By default, all C# simple data types are serializable, including strings.
• If a class contains fields that are more complex data types, they must be checked to ensure they are serializable.
• Namespaces required for serialization:
  • System.Runtime.Serialization.Formatters.Binary
  • System.Runtime.Serialization

Serialization Example

• Example of serializing an Employee object to a file and deserializing it back.
• Using FileStream to create and read from a file.
• Using BinaryFormatter to serialize and deserialize the object.