Chapter 13 - File IO and Isolated Storage

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“The System.IO namespace allows you to interact with a machine’s file and directory structure. Over the course of this chapter, you will learn how to programmatically create (and destroy) a directory system as well as move data into and out of various streams (file based, string based, memory based, etc.). The latter part of this chapter examines the role of isolated storage, which allows you to persist per-user data into a safe sandbox, regardless of the security settings of a target machine. To understand certain aspects of the System.IO.IsolatedStorage API, you will also receive an overview of Code Access Security (CAS).…”...

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Nội dung Text: Chapter 13 - File IO and Isolated Storage

Chapter 13. File I/O and Isolated Storage Hoang Anh Viet VietHA@it-hut.edu.vn Hanoi University of Technology 1
Objectives “The System.IO namespace allows you to interact with a machine’s file and directory structure. Over the course of this chapter, you will learn how to programmatically create (and destroy) a directory system as well as move data into and out of various streams (file based, string based, memory based, etc.). The latter part of this chapter examines the role of isolated storage, which allows you to persist per-user data into a safe sandbox, regardless of the security settings of a target machine. To understand certain aspects of the System.IO.IsolatedStorage API, you will also receive an overview of Code Access Security (CAS).…” Microsoft 2
Roadmap 1. Exploring the System.IO namespace 2. Working with File System 3. The Abstract System Class 4. Working with StreamWriters and StreamReaders 5. Working with StringWriters and StringReaders 6. Working with BinaryWriters and BinaryReaders 7. Programmatically “Watching” Files 8. Performing Asynchronous File I/O 9. An Overview of Isolated Storage 10. Introducing Object Serialization Microsoft 3
13.1 Exploring the System.IO namespace System.IO namespace is the region of the base class  libraries devoted to file-based (and memory-based) input and output (I/O) services System.IO defines a set of classes, interfaces,  enumerations, structures, and delegates, most of which are contained in mscorlib.dll Microsoft 4
Figure 13.1 Key Members of the System.IO Namespace Nonabstract I/O Class Description Type BinaryReader, BinaryWriter These types allow you to store and retrieve primitive data types (integers, Booleans, strings, and whatnot) as a binary value. BufferedStream This type provides temporary storage for a stream of bytes that may be committed to storage at a later time Directory, DirectoryInfo These types are used to manipulate a machine’s directory structure. The Directory type exposes functionality using static members. The DirectoryInfo type exposes similar functionality from a valid object reference. DriveInfo This type provides detailed information regarding the drives used by a given machine. File, FileInfo These types are used to manipulate a machine’s set of files. The File type exposes functionality using static members. The FileInfo type exposes similar functionality from a valid object reference. FileStream This type allows for random file access (e.g., seeking capabilities)with data represented as a stream of bytes. Microsoft 5
Figure 13.1 Key Members of the System.IO Namespace (Cont.) Nonabstract I/O Class Description Type FileSystemWatcher This type allows you to monitor the modification of external files in a specified directory. MemoryStream This type provides random access to streamed data stored in memory rather than a physical file. Path This type performs operations on System.String types that contain file or directory path information in a platform-neutral manner. StreamWriter, StreamReader These types are used to store (and retrieve) textual information to (or from) a file. These types do not support random file access. StringWriter, StringReader Like the StreamReader/StreamWriter types, these classes also work with textual information. However, the underlying storage is a string buffer rather than a physical file. Microsoft 6
Roadmap 1. Exploring the System.IO namespace 2. Working with File System 3. The Abstract System Class 4. Working with StreamWriters and StreamReaders 5. Working with StringWriters and StringReaders 6. Working with BinaryWriters and BinaryReaders 7. Programmatically “Watching” Files 8. Performing Asynchronous File I/O 9. An Overview of Isolated Storage 10. Introducing Object Serialization Microsoft 7
13.2 Working with File System System.IO namespace include a few basic classes for  retrieving file system information Directory and File: These classes provide static methods that  allow you to retrieve information about any files and directories that are visible from your server DriveInfo, DirectoryInfo, FileInfo: These classes use similar  instance methods and properties to retrieve the same information ...  The simplest level of file access:  Involves retrieving information about existing files and directories  Performing typical file system operations such as copying files  and creating directories Microsoft 8
Classes File and Directory Figure 13.2 The File- and Directory-centric types Microsoft 9
Figure 13.3 Directory Methods Method Description CreateDirectory() Creates a new directory. If you specify a directory inside another nonexistent directory, ASP.NET will thoughtfully create all the required directories. Delete() Deletes the corresponding empty directory. To delete a directory along with its contents (subdirectories and files), add the optional second parameter of true. Exists() Returns true or false to indicate whether the specified directory exists. GetCreationTime(), Returns a DateTime object that represents the time the directory GetLastAccessTime(), was created, accessed, or written to. Each “Get” method has and GetLastWriteTime() a corresponding “Set” method, which isn’t shown in this table. GetDirectories() and GetFiles() Returns an array of strings, one for each subdirectory or file in the specified directory. These methods can accept a second parameter that specifies a search expression (such as ASP*.*). GetLogicalDrives() Returns an array of strings, one for each drive that’s defined on the current computer. Drive letters are in this format: c:\. Microsoft 10
Figure 13.3 Directory Methods (Cont.) Method Description GetParent() Parses the supplied directory string and tells you what the parent directory is. You could do this on your own by searching for the \ character (or, more generically, the Path.DirectorySeparatorChar), but this function makes life a little easier. GetCurrentDirectory() Allows you to set and retrieve the current directory, which is useful if and SetCurrentDirectory() you need to use relative paths instead of full paths. Generally, you shouldn’t rely on these functions—use full paths instead. Move() Accepts two parameters: the source path and the destination path. The directory and all its contents can be moved to any path, as long as it’s located on the same drive. GetAccessControl() and Returns or sets a System.Security.AccessControl.Directory- SetAccessControl() Security object. You can use this object to examine the Windows access control lists (ACLs) that are applied on this directory and even change them programmatically. Microsoft 11
Figure 13.4 File Methods Method Description Copy() Accepts two parameters: the fully qualified source filenameand the fully qualified destination filename. To allow overwriting, use the version that takes a Boolean third parameter and set it to true. Delete() Deletes the specified file but doesn’t throw an exception if the file can’t be found. Exists() Indicates true or false whether a specified file exists. GetAttributes() Retrieves or sets an enumerated value that can include any and SetAttributes() combination of the values from the FileAttributes enumeration. GetCreationTime(), Returns a DateTime object that represents the time the file GetLastAccessTime(), wascreated, accessed, or last written to. Each “Get” method has a and GetLastWriteTime() corresponding “Set” method, which isn’t shown in this table. Move() Accepts two parameters: the fully qualified source filename and the fully qualified destination filename. You can move a file across drives and even rename it while you move it (or rename it without moving it). Microsoft 12
Figure 13.4 File Methods (Cont.) Method Description Create() and CreateText() Creates the specified file and returns a FileStream object that you can use to write to it. CreateText() performs the same task but returns a StreamWriter object that wraps the stream. Open(), OpenText(), Opens a file (provided it exists). OpenText() and OpenRead() open a OpenRead(), and OpenWrite() file in read-only mode, returning a FileStream or StreamReader. OpenWrite() opens a file in write-only mode, returning a FileStream. ReadAllText(), Reads the entire file and returns its contents as a single string, an ReadAllLines(), array of strings (one for each line), or an array of bytes. Use this and ReadAllBytes() method only for very small files. For larger files, use streams to read one chunk at a time and reduce the memory overhead. WriteAllText(), WriteAllLines(), Writes an entire file in one shot using a supplied string, array of strings and WriteAllBytes() (one for each line), or array of bytes. If the file already exists, it is overwritten. GetAccessControl() Returns or sets a System.Security.AccessControl.FileSecurity and SetAccessControl() object. You can use this object to examine the Windows ACLs that are applied on this directory and even change them programmatically. Microsoft 13
Example 13.1 Displaying the name of each file in the current directory 14
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Example 16
The DirectoryInfo and FileInfo Classes Mirror the functionality in the Directory and File  classes Share a common set of properties and methods  because they derive from the common FileSystemInfo base class Microsoft 17
Figure 13.5 DirectoryInfo and FileInfo Members Method Description Attributes Allows you to retrieve or set attributes using a combination of values fromthe FileAttributes enumeration. CreationTime, Allows you to set or retrieve the creation time, last access time, and last write LastAccessTime, time using a DateTime object. and LastWriteTime Exists Returns true or false depending on whether the file or directory exists. In other words, you can create FileInfo and DirectoryInfo objects that don’t actually correspond to current physical directories, although you obviously won’t be able to use properties such as CreationTime and methods such as MoveTo(). FullName, Name, Returns a string that represents the fully qualified name, the directory or and Extension filename (with extension), or the extension on its own, depending on which property you use. Delete() Removes the file or directory, if it exists. When deleting a directory, it must be empty, or you must specify an optional parameter set to true. Refresh() Updates the object so it’s synchronized with any file system changes that have happened in the meantime (for example, if an attribute was changed manually using Windows Explorer). Create() Creates the specified directory or file. MoveTo() Copies the directory and its contents or the file. For a DirectoryInfo object, you need to specify the new path; for a FileInfo object, you specify a path and filename. Microsoft 18
Figure 13.6 Unique DirectoryInfo Members Method Description Parent and Root Returns a DirectoryInfo object that represents the parent or root directory. CreateSubdirectory( Creates a directory with the specified name in the directory represented ) by the DirectoryInfo object. It also returns a new DirectoryInfo object that represents the subdirectory. GetDirectories() Returns an array of DirectoryInfo objects that represent all the subdirectories contained in this directory. GetFiles() Returns an array of FileInfo objects that represent all the files contained in this directory. Microsoft 19
Figure 13.7 Unique FileInfo Members Method Description Directory Returns a DirectoryInfo object that represents the parent directory. DirectoryName Returns a string that identifies the name of the parent directory. Length Returns a long (64-bit integer) with the file size in bytes. CopyTo() Copies a file to the new path and filename specified as a parameter. It also returns a new FileInfo object that represents the new (copied) file. You can supply an optional additional parameter of true to allow Overwriting. Create() and Creates the specified file and returns a FileStream object that you can CreateText() use to write to it. CreateText() performs the same task but returns a StreamWriter object that wraps the stream. Open(), Opens a file (provided it exists). OpenRead() and OpenText() open a file in OpenRead(), read-only mode, returning a FileStream or StreamReader. OpenWrite() OpenText(), and opens a file in write-only mode, returning a FileStream. OpenWrite() Microsoft 20