Working with the Windows File System and Essential Data Structures

1
Chapter 5

• Working with the Windows File System and
  Essential Data Structures

2
Objectives (1)

• Understand the members of the System.IO namespace
• Understand the organization of the Windows file
  system and how to manage directories and files
• Configure the OpenFileDialog and SaveFileDialog
  controls as a means for the user to select files
  to open and save
• Read and write sequential files using the
  StreamReader and StreamWriter classes

3
Objectives (2)

• Understand the basics of collections and work
  with enumerators
• Create and manage lists of data with the
  ArrayList class
• Create and manage sorted lists of data with the
  SortedList class
• Create first-in first-out lists, called queues
• Understand last-in first-out lists, called stacks
• Read and write random files

4
Introduction to the System.IO Namespace

• The System.IO namespace supplies classes to
  manage directories and files, and to read and
  write files
• System.IO.Directory class has methods to create,
  move, rename, and delete directories
• System.IO.File class works with files
• System.IO.StreamReader and System.IO.StreamWriter
  read and write sequential files, respectively

5
File System Organization (1)

• The Windows file system is organized
  hierarchically
• Physical or logical drives (A, B, C, D, etc.)
  appear at the base of the hierarchy
• Drive names will vary based on configuration
• Backward slash separates each directory,
  subdirectory, and file
• Example to create an absolute reference to
  Temp.doc in the folder named C\Windows\Temp
• C\Windows\Temp\Temp.doc

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File System Organization (2)

• Relative references begin with
• .\ to reference the current directory
• ..\ to reference the parent directory
• Example
• ..\Chapter.05\Complete\Temp.doc

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Current Working Directory

• One directory is considered the current working
  directory
• Relative directory references are made based on
  the current working directory
• Current working directory is ignored for absolute
  directory references

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Working with the Windows File System

• System.IO.Directory has methods to manage the
  file system
• CreateDirectory creates a directory or
  subdirectory
• Delete deletes a directory or subdirectory
• Exists tests whether a directory exists
• GetCurrentDirectory gets current working
  directory
• GetLogicalDrives returns an array of strings
  containing logical disk drives
• Move method moves an existing directory to
  another directory
• SetCurrentDirectory sets the current working
  directory

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Methods of the Directory Class (1)

• Create an absolute and relative directory
• Dim dinfoCurrent As DirectoryInfo
• dinfoCurrent Directory.CreateDirectory("C\Demo"
  )
• dinfoCurrent Directory.CreateDirectory("Demo")
• Delete a directory
• Directory.Delete("C\Demo")
• Directory.Delete("C\Demo", True)
• Test whether a directory exists
• Dim pblnExists As Boolean
• pblnExists Directory.Exists("C\Demo")

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Methods of the Directory Class (2)

• Get the current working directory
• Dim pstrDirectory As String
• pstrDirectory Directory.GetCurrentDirectory()
• Move a Directory
• Directory.Move("C\Demo", "C\Backup\Demo")
• Directory.Move("C\Demo", "C\Demo1")

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The File Class

• Operates similarly to the Directory class
• Has static methods to create, move, and delete
  files
• Copy method copies source file to destination
  file
• Create method creates a new file
• Delete method deletes a file
• Exists method returns a Boolean value indicating
  whether the file exists
• GetAttributes and SetAttributes get and set file
  attributes, respectively
• GetCreationTime, GetLastAccessTime, and
  GetLastWriteTime get file modification information

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The File Class (Examples)

• Create a new file
• File.Create("C\Demo\DemoFile.txt")
• Copy a file
• File.Copy("C\Demo\DemoFile.txt", _
• "C\Demo\DemoFile1.txt")
• Move a file
• File.Move("C\Demo\DemoFile.txt", _
• "C\Demo\DemoFile1.txt")
• Delete a file
• File.Delete("DemoFile1.txt")

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File Attributes

• The FileAttributes enumeration contains file
  attributes
• Bitwise flags define each attribute
• Archive flag marks files for backup or removal
• Compressed flag is set if file is compressed
• Directory flag indicates that file is a directory
• ReadOnly flag indicates that file is marked as
  read-only
• System files marked with the System attribute

14
Bitwise Operations
15
OpenFileDialog and SaveFileDialog Controls

• Derived from the FileDialog control
• Supplies the means for the user to select a file
  to open or save
• OpenFileDialog
• System.Windows.Forms.OpenFileDialog
• SaveFileDialog
• System.Windows.Forms.SaveFileDialog

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OpenFileDialog and SaveFileDialog Properties (1)

• AddExtension property defines whether or not the
  system adds the current file extension if omitted
  by the user
• The Boolean CheckFileExists and CheckPathExists
  properties require the user to select a file and
  directory that already exist, respectively
• FileName contains the path and file name selected
  by the user
• FileNames contains an array of strings pertaining
  to the path and name of the user selected files

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OpenFileDialog and SaveFileDialog Properties (2)

• FilterIndex and Filter properties define possible
  file extensions selectable by the user and the
  current file extension
• InitialDirectory contains a string that defines
  the directory that VB .NET will initially search
  for files
• MultiSelect defines whether the user can select
  multiple files from the dialog boxes
• If the RestoreDirectory property is set to True,
  the current working directory is restored to its
  original state after the dialog box is closed

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OpenFileDialog and SaveFileDialog Methods

• OpenFileDialog and SaveFileDialog support the
  following public methods
• ShowDialog displays the OpenFileDialog or
  SaveFileDialog box
• This method returns a value of type DialogResult
  to indicate whether the user wants to continue
  the operation
• Reset sets all the properties to their default
  values
• FileOK event fires when the user clicks the Open
  or Save button in the OpenFileDialog or
  SaveFileDialog box, respectively

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OpenFileDialog (Illustration)
InitialDirectory property appears in Look in list
box
Title property appears in title bar
Filters appear in the Files of type list box
The FileName property appears in the File name
list box
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The Filter Property

• Most common file types have standard file
  extensions
• Word documents have a suffix of ".doc" for
  example
• OpenFileDialog typically restricts files to those
  having a particular extension via the Filter
  property
• Filter property stores a String containing
  description value pairs
• Description and value separated by a vertical bar
• Do NOT embed spaces between description value
  pairs

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The Filter Property (Example)

• Filter to select .txt and all files

ofdDemo.Filter "Text Files (.txt).txt" _
"All Files (.)."
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Reading and Writing Files

• Two types of files
• Sequential files contain records of varying
  lengths
• A carriage return separates each record
• A delimiter separates fields
• Comma is the common delimiter character
• Sequential files read from beginning to end
• Random files, also known as direct access files,
  are typically binary files
• Records in a random file have a fixed size
• Possible to read a specific record directly

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The StreamReader Class

• System.IO.StreamReader provides methods to open
  sequential files for reading
• Close method closes the file associated with the
  StreamReader instance
• You should explicitly close all files after
  reading is complete
• Read method reads one or more characters into a
  buffer as Integer values
• ReadLine method reads a line (record) into a
  string
• A carriage return terminates each line
• ReadToEnd method reads from the current position
  in the file to the end of the file

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StreamReader Class (Examples)

• Open a file by creating an instance of the
  StreamReader class
• Dim pstrFileName As String "C\Demo.txt"
• Dim psrdCurrent As New System.IO.StreamReader _
  (pstrFileName)
• Calling the methods of the StreamReader class
  allows you to read the file
• Example read file from beginning to end
• txtFile.Text psrdCurrent.ReadToEnd()

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Logic to Read a Sequential File
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The Split Method

• When reading a file one record at a time, record
  is read into a string
• String must be split into fields
• Split method applies to the String class
• Split method returns an array of strings

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Operation of the Split Method
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The StreamWriter Class

• System.IO.StreamWriter provides methods to write
  to files
• Close method closes the open file
• You should explicitly close all files after
  writing
• Write method writes a character or characters to
  the file
• WriteLine method writes a character or characters
  to the file
• Character stored in NewLine property written to
  the end of the line
• NewLine property defines the character marking
  the end of the line

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The StreamWriter Class

• Prepare to write to a file by creating an
  instance of the StreamWriter class
• Dim pstrFileName As String "C\Demo.txt"
• Dim psrdCurrent As New System.IO.StreamWriter _
• (pstrFileName)

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An Introduction to Collections

• A collection is nothing more than multiple
  objects, typically of the same type, organized in
  some logical way
• Collections discussed in this chapter
• ArrayList class is similar to an array
• SortedList class maintains elements in sorted
  order
• Queue class implements a first-in first-out list
• Stack class works the opposite of a queue
  (last-in first-out list)
• There are many more collections than those
  described in the preceding list

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Working with Collections

• Common members
• Count property returns the number of elements in
  the collection
• Count property is 1-based
• Clear method removes all of the elements from the
  collection
• ToArray method converts the elements in a
  collection into an array
• ToString method returns a String representing the
  collection objects

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The ArrayList Class

• System.Collections.ArrayList
• The ArrayList class is used to maintain a list of
  objects
• ArrayList class has additional properties and
  methods, making the ArrayList more robust than
  the Array class

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ArrayList Properties

• Capacity property defines the number of elements
  that the ArrayList can contain
• Capacity property is updated automatically
• Capacity property grows by increasingly larger
  blocks as elements are added
• Count property contains the number of elements
  actually stored in the ArrayList
• Count is always less than Capacity
• Count property is 1-based

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ArrayList Methods (1)

• Add method adds an element to the end of the
  ArrayList
• AddRange method adds multiple elements to the end
  of the ArrayList
• BinarySearch method uses a binary search
  algorithm to locate an element within a sorted
  ArrayList
• Remove method removes the first occurrence of a
  specific element from the ArrayList

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ArrayList Methods (1)

• RemoveAt method removes an element from the
  ArrayList
• Method accepts one argument the 0-based index of
  the element to remove
• Reverse method reverses the order of the elements
  stored in the ArrayList
• Sort method sorts the elements in all or part of
  the ArrayList

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Chapter ArrayList Implementation
37
SortedList Class

• System.Collections.SortedList
• SortedList class is similar to an ArrayList in
  that it stores a list of objects
• SortedList class internally maintains two arrays
• First array stores the keys used to locate the
  values associated with those keys
• Second array stores the values that are
  associated with the keys
• Values can be any object type

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Implementation of a SortedList
39
SortedList Methods and Properties (1)

• Add method adds an element to the SortedList
• One argument contains the key
• Another contains the index value associated with
  the key
• GetByIndex method gets the value associated with
  the associated index
• SetByIndex method stores a value based on a
  specific index
• Item method retrieves a value from the list given
  a specific key

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SortedList Methods and Properties (2)

• Remove method removes an item from the SortedList
• Method accepts one argument the key to remove
• RemoveAt method removes an item from the
  SortedList
• Accepts one argument the index of the key to
  remove
• Values property contains a collection
• Each element in the collection represents an
  object in the collection

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SortedList Examples (1)

• Locating an item in a SortedList
• Public Function FindRequest(ByVal ID As String)_
  
• As CallRecord
• Return CType(slRecords.Item(ID),
  CallRecord)
• End Function
• Adding an item to a SortedList
• Dim cr As New CallRecord()
• slRecords.Add(cr.ID, cr)

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SortedList Examples (2)

• Modifying a SortedList
• slRecords.SetByIndex(slRecords.IndexOfKey(cr.ID),
  _ cr)
• Removing a SortedList
• slRecords.RemoveAt(slRecords.IndexOfKey(cr.ID))

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Queue Class

• System.Collections.Queue
• A queue stores a list of objects much like an
  ArrayList stores a list of objects
• Queues are first-in first-out data structures
• The first item added to a queue is the first item
  removed from the queue

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Operation of a Queue
45
Queue Methods and Properties

• Dequeue method returns the element at the front
  of the queue and removes that element from the
  queue
• Enqueue method adds an element to the back of the
  queue
• Peek method returns the element at the front of
  the queue but does not remove the element from
  the queue
• Count property gets the number of elements stored
  in the queue

46
Stack Class

• System.Collections.Stack
• A stack is a last-in first-out list
• The item most recently added to the stack is the
  first item removed from the stack

47
Operation of a Stack
48
Stack Methods and Properties

• Push method adds an item to the stack
• Pop method returns the most recently added item
  from stack
• The item returned is also removed from the stack
• Peek method returns the object at the top of the
  stack without removing it
• Count property contains the number of elements in
  the stack

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Opening, Reading, and Writing Random Files

• Characteristics
• To open a random file for reading and writing,
  call the FileOpen function
• Records in a random file have a fixed length
• While a random file is open, any record can be
  read or written
• FileGet function reads a record
• FilePut function writes a record
• Seek method locates a record

50
Random Files and Strings

• Strings in random files must have the same size
• Fixed length strings
• Use attribute to declare a fixed length string

Private Structure Employeef Public EmployeeID
As Integer ltVBFixedString(20)gt Public
FirstName As String ltVBFixedString(20)gt
Public LastName As String Public
AccruedVacation As Integer Public AccruedSick
As Integer End Structure
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Opening Random Files

• Before reading or writing the records contained
  in a random file, it must be opened
• FileOpen function opens a file for random access
• First argument contains unique file number
• Second argument contains file name
• Third argument contains access mode
• RecordLength argument defines the length of each
  record

52
Opening Random Files (Example)

• Open a random file
• File name stored in ofdRandom.FileName

FileOpen(1, ofdRandom.FileName, OpenMode.Random,
_ OpenAccess.Default, , Len(EmpCurrent))
53
Reading andWriting Records in Random Files

• Read and write records after defining the
  structure of each record in the random access
  file
• FilePut function writes a record to a random file
• Filenumber argument contains a valid file number
• File number obtained when the file was opened
  with the FileOpen function
• Value argument contains the data to be written
• Recordnumber (optional) argument contains the
  record number to write.
• Argument is one-based