2006 Pearson Education, Inc. All rights reserved. 1 18 Files and Streams

1

2006 Pearson Education, Inc. All rights reserved.

1818

Files and Streams

2


I can only assume that a “Do Not File” document is filed in a “Do Not File” file.

— Senator Frank Church

Senate Intelligence

Subcommittee Hearing, 1975

Consciousness ... does not appear to itself chopped up in bits. ... A “river” or a “stream” are the metaphors by which it is most naturally described.

— William James

I read part of it all the way through.— Samuel Goldwyn

3


OBJECTIVESIn this chapter you will learn: To create, read, write and update files. The C# streams class hierarchy. To use classes File and Directory to obtain

information about files and directories on your computer.

To become familiar with sequential-access file processing.

To use classes FileStream, StreamReader and StreamWriter to read text from and write text to files.

To use classes FileStream and BinaryFormatter to read objects from and write objects to files.

4


18.1 Introduction

18.2 Data Hierarchy

18.3 Files and Streams

18.4 Classes File and Directory

18.5 Creating a Sequential-Access Text File

18.6 Reading Data from a Sequential-Access Text File

18.7 Serialization

18.8 Creating a Sequential-Access File Using Object Serialization

18.9 Reading and Deserializing Data from a Sequential-Access Text File

18.10 Wrap-Up

5


18.1 Introduction

• Persistent Data– Data maintained in files

• Create, upload and process data files in C#

• Overview of data hierarchy

• Overview of FCL’s file-processing classes

6


18.2 Data Hierarchy

• Data Hierarchy– All data in computers are combinations of 0s and 1s– The smallest data item is called a bit (Contains 0 or 1)– Bytes are composed of eight bits

• C# uses the Unicode® character set in which characters are composed of 2 bytes

– Characters are composed of bits– A field is a group of characters– A record/file is composed of several related fields– A group of related files often are stored in a database

• A collection of programs designed to create and manage databases is called a database management system (DBMS)

7


Fig. 18.1 | Data hierarchy.

8


18.3 Files and Streams

• Files and Streams– C# views each file as a sequential stream of bytes

• Each file ends either with:– An end-of-file marker – Specific byte number

• Recorded in a system-maintained administrative data structure

– An object is created and a stream is associated with the object when a file is opened

– The runtime environment creates three stream objects: • Console.In (Refers to the standard input stream object)

– Enables a program to input data from the keyboard• Console.Out (Refers to the standard output stream object)

– Enables a program to output data to the screen• Console.Error (Refers to the standard error stream object)

– Enables a program to output error messages to the screen

9


Fig. 18.2 | C#’s view of an n-byte file.

10


18.3 Files and Streams (Cont.)• There are many file-processing classes in the FCL (Framework Class

Library)– Class StreamReader

• For text input from a file• Inherits from abstract class TextReader

– Class StreamWriter• For text output to a file• Inherits from abstract class TextWriter

– Class FileStream • For both input from and output to a file• Can be used to write data to and read data from files • Inherits from abstract class Stream

– Class MemoryStream • Enables the transfer of data directly to and from memory

– Class BufferedStream • Uses buffering to transfer data to or from a stream

– Properties of Console.In and Console.Out are of type TextReader and TextWriter, respectively

11


18.3 Files and Streams (Cont.)

• Buffering– An I/O performance-enhancement technique

– Each output operation is directed to a buffer • Aka region in memory

– Output is performed more efficiently • Large physical output operation each time the buffer fills

– Less overhead

– Used to speed input operations • Reading more data than is required into a buffer

• Subsequent reads get data from memory rather than external device

12


18.4 Classes File and Directory

• Classes File and Directory – Enable programs to manipulate files and directories on disk

– Class File • Determine information about files and can be used to open files for

reading or writing

– Class Directory • Provides capabilities for manipulating directories

• CreateDirectory method

– Returns DirectoryInfo object

• Contain information about a directory

13


Fig. 18.3 | File class static methods (partial list). (Part 1 of 2.)

static Method Description

AppendText Returns a StreamWriter that appends text to an existing file or creates a file if one does not exist.

Copy Copies a file to a new file.

Create Creates a file and returns its associated FileStream.

CreateText Creates a text file and returns its associated StreamWriter.

Delete Deletes the specified file.

Exists Returns true if the specified file exists and false otherwise.

GetCreationTime Returns a DateTime object representing when the file was created.

GetLastAccessTime Returns a DateTime object representing when the file was last accessed.

14


Fig. 18.3 | File class static methods (partial list). (Part 2 of 2.)


GetLastWriteTime Returns a DateTime object representing when the file was last modified.

Move Moves the specified file to a specified location.

Open Returns a FileStream associated with the specified file and equipped with the specified read/write permissions.

OpenRead Returns a read-only FileStream associated with the specified file.

OpenText Returns a StreamReader associated with the specified file.

OpenWrite Returns a read/write FileStream associated with the specified file.

15


Fig. 18.4 | Directory class static methods.


CreateDirectory Creates a directory and returns its associated DirectoryInfo object.

Delete Deletes the specified directory.

Exists Returns true if the specified directory exists and false otherwise.

GetDirectories Returns a string array containing the names of the subdirectories in the specified directory.

GetFiles Returns a string array containing the names of the files in the specified directory.

GetCreationTime Returns a DateTime object representing when the directory was created.

GetLastAccessTime Returns a DateTime object representing when the directory was last accessed.

GetLastWriteTime Returns a DateTime object representing when items were last written to the directory.

Move Moves the specified directory to a specified location.

16


1 // Fig 18.5: FileTestForm.cs

2 // Using classes File and Directory.

3 using System;

4 using System.Windows.Forms;

5 using System.IO;

6

7 // displays contents of files and directories

8 public partial class FileTestForm : Form

9 {

10 // parameterless constructor

11 public FileTestForm()

12 {

13 InitializeComponent();

14 } // end constructor

15

16 // invoked when user presses key

17 private void inputTextBox_KeyDown( object sender, KeyEventArgs e )

18 {

19 // determine whether user pressed Enter key

20 if ( e.KeyCode == Keys.Enter )

21 {

22 string fileName; // name of file or directory

23

24 // get user-specified file or directory

25 fileName = inputTextBox.Text;

Outline

FileTestForm.cs

(1 of 5)

17


26

27 // determine whether fileName is a file

28 if ( File.Exists( fileName ) )

29 {

30 // get file's creation date,

31 // modification date, etc.

32 outputTextBox.Text = GetInformation( fileName );

33

34 // display file contents through StreamReader

35 try

36 {

37 // obtain reader and file contents

38 StreamReader stream = new StreamReader( fileName );

39 outputTextBox.Text += stream.ReadToEnd();

40 } // end try

41 // handle exception if StreamReader is unavailable

42 catch ( IOException )

43 {

44 MessageBox.Show( "Error reading from file", "File Error",

45 MessageBoxButtons.OK, MessageBoxIcon.Error );

46 } // end catch

47 } // end if

48 // determine whether fileName is a directory

49 else if ( Directory.Exists( fileName ) )

50 {

51 string[] directoryList; // array for directories

52

53 // get directory's creation date,

54 // modification date, etc.

55 outputTextBox.Text = GetInformation( fileName );

Outline

FileTestForm.cs

(2 of 5)

Checks to see if filename is an existing file

Instantiates a StreamReader for reading text from a file

Checks to see if filename is an existing directory

18


56

57 // obtain file/directory list of specified directory

58 directoryList = Directory.GetDirectories( fileName );

59

60 outputTextBox.Text += "\r\n\r\nDirectory contents:\r\n";

61

62 // output directoryList contents

63 for ( int i = 0; i < directoryList.Length; i++ )

64 outputTextBox.Text += directoryList[ i ] + "\r\n";

65 } // end else if

66 else

67 {

68 // notify user that neither file nor directory exists

69 MessageBox.Show( inputTextBox.Text +

70 " does not exist", "File Error",


72 } // end else

73 } // end if

74 } // end method inputTextBox_KeyDown

75

76 // get information on file or directory

77 private string GetInformation( string fileName )

78 {

79 string information;

80

81 // output that file or directory exists

82 information = fileName + " exists\r\n\r\n";

Outline

FileTestForm.cs

(3 of 5)Obtain a string array containing the names of the subdirectories in

the specified directory

If successful, output results

If not, notify user

19


83

84 // output when file or directory was created

85 information += "Created: " +

86 File.GetCreationTime( fileName ) + "\r\n";

87

88 // output when file or directory was last modified

89 information += "Last modified: " +

90 File.GetLastWriteTime( fileName ) + "\r\n";

91

92 // output when file or directory was last accessed

93 information += "Last accessed: " +

94 File.GetLastAccessTime( fileName ) + "\r\n" + "\r\n";

95

96 return information;

97 } // end method GetInformation

98 } // end class FileTestForm

Outline

FileTestForm.cs

(4 of 5)

(a) (b)

Returns a DateTime object representing when the file

was created


was last modified


was last accessed

20


Outline

FileTestForm.cs

(5 of 5)

(c) (d)

21


1 // Fig 18.6: FileSearchForm.cs

2 // Using regular expressions to determine file types.

3 using System;


5 using System.IO;

6 using System.Text.RegularExpressions;

7 using System.Collections.Specialized;

8

9 // uses regular expressions to determine file types

10 public partial class FileSearchForm : Form

11 {

12 string currentDirectory = Directory.GetCurrentDirectory();

13 string[] directoryList; // subdirectories

14 string[] fileArray;

15

16 // store extensions found and number found

17 NameValueCollection found = new NameValueCollection();

18


20 public FileSearchForm()

21 {



24

Outline

FileSearchForm.cs

(1 of 6)

Returns the current project’s directory

Create an object of NameValueCollection

22


25 // invoked when user types in text box

26 private void inputTextBox_KeyDown( object sender, KeyEventArgs e )

27 {

28 // determine whether user pressed Enter

29 if ( e.KeyCode == Keys.Enter )

30 searchButton_Click( sender, e );

31 } // end method inputTextBox_KeyDown

32

33 // invoked when user clicks "Search Directory" button

34 private void searchButton_Click( object sender, EventArgs e )

35 {

36 // check for user input; default is current directory

37 if ( inputTextBox.Text != "" )

38 {

39 // verify that user input is valid directory name

40 if ( Directory.Exists( inputTextBox.Text ) )

41 {

42 currentDirectory = inputTextBox.Text;

43

44 // reset input text box and update display

45 directoryLabel.Text = "Current Directory:" +

46 "\r\n" + currentDirectory;

47 } // end if

48 else

49 {

50 // show error if user does not specify valid directory

51 MessageBox.Show( "Invalid Directory", "Error",


53 } // end else

54 } // end if

Outline

FileSearchForm.cs

(2 of 6)

Determine if inputTextBox.Text is

an existing directory

23


55

56 // clear text boxes

57 inputTextBox.Text = "";

58 outputTextBox.Text = "";

59

60 SearchDirectory( currentDirectory ); // search directory

61

62 // summarize and print results

63 foreach ( string current in found )

64 {

65 outputTextBox.Text += "* Found " +

66 found[ current ] + " " + current + " files.\r\n";

67 } // end foreach

68

69 // clear output for new search

70 found.Clear();

71 } // end method searchButton_Click

72

73 // search directory using regular expression

74 private void SearchDirectory( string currentDirectory )

75 {

76 // for file name without directory path

77 try

78 {

79 string fileName = "";

80

81 // regular expression for extensions matching pattern

82 Regex regularExpression = new Regex(

83 @"[a-zA-Z0-9]+\.(?<extension>\w+)" );

Outline

FileSearchForm.cs

(3 of 6)

Use key string to access corresponding value

Reset for the next search

Matches any sequence of numbers or letters followed by a period

and one or more letters

24


84

85 // stores regular-expression match result

86 Match matchResult;

87

88 string fileExtension; // holds file extensions

89

90 // number of files with given extension in directory

91 int extensionCount;

92

93 // get directories

94 directoryList = Directory.GetDirectories( currentDirectory );

95

96 // get list of files in current directory

97 fileArray = Directory.GetFiles( currentDirectory );

98

99 // iterate through list of files

100 foreach ( string myFile in fileArray )

101 {

102 // remove directory path from file name

103 fileName = myFile.Substring( myFile.LastIndexOf( @"\" ) + 1 );

104

105 // obtain result for regular-expression search

106 matchResult = regularExpression.Match( fileName );

107

108 // check for match

109 if ( matchResult.Success )

110 fileExtension = matchResult.Result( "${extension}" );

111 else

112 fileExtension = "[no extension]";

113

Outline

FileSearchForm.cs

(4 of 6)Retrieve the names of all subdirectories that belong to

the current directory

Store the names of files in the current directory

Eliminate the directory path

Return the matches for the regular expression

25


114 // store value from container

115 if ( found[ fileExtension ] == null )

116 found.Add( fileExtension, "1" );

117 else

118 {

119 extensionCount = Int32.Parse( found[ fileExtension ] ) + 1;

120 found[ fileExtension ] = extensionCount.ToString();

121 } // end else

122

123 // search for backup( .bak ) files

124 if ( fileExtension == "bak" )

125 {

126 // prompt user to delete ( .bak ) file

127 DialogResult result =

128 MessageBox.Show( "Found backup file " +

129 fileName + ". Delete?", "Delete Backup",

130 MessageBoxButtons.YesNo, MessageBoxIcon.Question );

131

132 // delete file if user clicked 'yes'

133 if ( result == DialogResult.Yes )

134 {

135 File.Delete( myFile );

136 extensionCount = Int32.Parse( found[ "bak" ] ) - 1;

137 found[ "bak" ] = extensionCount.ToString();

138 } // end if

139 } // end if

140 } // end foreach

Outline

FileSearchForm.cs

(5 of 6)

Determine if this is the first occurrence of the filename extension

If not, increment the value associated with the extension

Delete the file and decrement the corresponding value

26


141

142 // recursive call to search files in subdirectory

143 foreach ( string myDirectory in directoryList )

144 SearchDirectory( myDirectory );

145 } // end try

146 // handle exception if files have unauthorized access

147 catch ( UnauthorizedAccessException )

148 {

149 MessageBox.Show( "Some files may not be visible" +

150 " due to permission settings", "Warning",

151 MessageBoxButtons.OK, MessageBoxIcon.Information );

152 } // end catch

153 } // end method SearchDirectory

154 } // end class FileSearchForm

Outline

FileSearchForm.cs

(6 of 6)

Notify user of any unauthorized file access

27


18.5 Creating a Sequential-Access Text File

• C# imposes no structure on files– The concept of a “record” does not exist in C# files

• Must structure files to meet the requirements of your applications

• Class FileStream – Objects can open files to perform text manipulation

• Class StreamWriter– Objects is constructed with a FileStream argument

• Specifies the file to output text

28


1 // Fig. 18.7: BankUIForm.cs

2 // A reusable Windows Form for the examples in this chapter.

3 using System;


5

6 public partial class BankUIForm : Form

7 {

8 protected int TextBoxCount = 4; // number of TextBoxes on Form

9

10 // enumeration constants specify TextBox indices

11 public enum TextBoxIndices

12 {

13 ACCOUNT,

14 FIRST,

15 LAST,

16 BALANCE

17 } // end enum

18


20 public BankUIForm()

21 {



Outline

BankUIForm.cs

(1 of 4)

Define enumeration for the textbox’s indices

29


24

25 // clear all TextBoxes

26 public void ClearTextBoxes()

27 {

28 // iterate through every Control on form

29 for ( int i = 0; i < Controls.Count; i++ )

30 {

31 Control myControl = Controls[ i ]; // get control

32

33 // determine whether Control is TextBox

34 if ( myControl is TextBox )

35 {

36 // clear Text property ( set to empty string )

37 myControl.Text = "";

38 } // end if

39 } // end for

40 } // end method ClearTextBoxes

Outline

BankUIForm.cs

(2 of 4)

Clear textbox

30


41

42 // set text box values to string array values

43 public void SetTextBoxValues( string[] values )

44 {

45 // determine whether string array has correct length

46 if ( values.Length != TextBoxCount )

47 {

48 // throw exception if not correct length

49 throw( new ArgumentException( "There must be " +

50 ( TextBoxCount + 1 ) + " strings in the array" ) );

51 } // end if

52 // set array values if array has correct length

53 else

54 {

55 // set array values to text box values

56 accountTextBox.Text = values[ ( int ) TextBoxIndices.ACCOUNT ];

57 firstNameTextBox.Text = values[ ( int ) TextBoxIndices.FIRST ];

58 lastNameTextBox.Text = values[ ( int ) TextBoxIndices.LAST ];

59 balanceTextBox.Text = values[ ( int ) TextBoxIndices.BALANCE ];

60 } // end else

61 } // end method SetTextBoxValues

Outline

BankUIForm.cs

(3 of 4)

Set textboxes to corresponding values

31


62

63 // return text box values as string array

64 public string[] GetTextBoxValues()

65 {

66 string[] values = new string[ TextBoxCount ];

67

68 // copy text box fields to string array

69 values[ ( int ) TextBoxIndices.ACCOUNT ] = accountTextBox.Text;

70 values[ ( int ) TextBoxIndices.FIRST ] = firstNameTextBox.Text;

71 values[ ( int ) TextBoxIndices.LAST ] = lastNameTextBox.Text;

72 values[ ( int ) TextBoxIndices.BALANCE ] = balanceTextBox.Text;

73

74 return values;

75 } // end method GetTextBoxValues

76 } // end class BankUIForm

Outline

BankUIForm.cs

(4 of 4)

Return textboxes’ values

32


1 // Fig. 18.8: Record.cs

2 // Serializable class that represents a data record.

3 using System;

4 using System.Collections.Generic;

5 using System.Text;

6

7 public class Record

8 {

9 private int account;

10 private string firstName;

11 private string lastName;

12 private decimal balance;

13

14 // parameterless constructor sets members to default values

15 public Record() : this( 0, "", "", 0.0M )

16 {


18

19 // overloaded constructor sets members to parameter values

20 public Record( int accountValue, string firstNameValue,

21 string lastNameValue, decimal balanceValue )

22 {

23 Account = accountValue;

24 FirstName = firstNameValue;

25 LastName = lastNameValue;

26 Balance = balanceValue;


Outline

Record.cs

(1 of 3)private instance values representing all the information for a record

Initializes the private instance variables’ value

33


28

29 // property that gets and sets Account

30 public int Account

31 {

32 get

33 {

34 return account;

35 } // end get

36 set

37 {

38 account = value;

39 } // end set

40 } // end property Account

41

42 // property that gets and sets FirstName

43 public string FirstName

44 {

45 get

46 {

47 return firstName;

48 } // end get

49 set

50 {

51 firstName = value;

52 } // end set

53 } // end property FirstName

Outline

Record.cs

(2 of 3)

Property for private instance variable account

Property for private instance variable firstName

34


54

55 // property that gets and sets LastName

56 public string LastName

57 {

58 get

59 {

60 return lastName;

61 } // end get

62 set

63 {

64 lastName = value;

65 } // end set

66 } // end property LastName

67

68 // property that gets and sets Balance

69 public decimal Balance

70 {

71 get

72 {

73 return balance;

74 } // end get

75 set

76 {

77 balance = value;

78 } // end set

79 } // end property Balance

80 } // end class Record

Outline

Record.cs

(3 of 3)

Property for private instance variable lastName

Property for private instance variable balance

35


1 // Fig. 18.9: CreateFileForm.cs

2 // Creating a sequential-access file.

3 using System;


5 using System.IO;

6 using BankLibrary;

7

8 public partial class CreateFileForm : BankUIForm

9 {

10 private StreamWriter fileWriter; // writes data to text file

11 private FileStream output; // maintains connection to file

12


14 public CreateFileForm()

15 {



18

19 // event handler for Save Button

20 private void saveButton_Click( object sender, EventArgs e )

21 {

22 // create dialog box enabling user to save file

23 SaveFileDialog fileChooser = new SaveFileDialog();

24 DialogResult result = fileChooser.ShowDialog();

25 string fileName; // name of file to save data

26

27 fileChooser.CheckFileExists = false; // allow user to create file

Outline

CreateFileForm.cs

(1 of 8)

Create a StreamWriter object for writing text to a file

Create a FileStream object for connecting to a file

Prompt user for location to save file

Do not notify user that file does not exist

36


28

29 // exit event handler if user clicked "Cancel"

30 if ( result == DialogResult.Cancel )

31 return;

32

33 fileName = fileChooser.FileName; // get specified file name

34

35 // show error if user specified invalid file

36 if ( fileName == "" || fileName == null )

37 MessageBox.Show( "Invalid File Name", "Error",


39 else

40 {

41 // save file via FileStream if user specified valid file

42 try

43 {

44 // open file with write access

45 output = new FileStream( fileName,

46 FileMode.OpenOrCreate, FileAccess.Write );

47

48 // sets file to where data is written

49 fileWriter = new StreamWriter( output );

50

51 // disable Save button and enable Enter button

52 saveButton.Enabled = false;

53 enterButton.Enabled = true;

54 } // end try

Outline

CreateFileForm.cs

(2 of 8)Obtain user-selected file

Open the file if the file exists or create the file if it does not exist

Indicates that the program can perform only write operations with this object

Instantiate the StreamWriter object to write text to a file

37


55 // handle exception if there is a problem opening the file


57 {

58 // notify user if file does not exist

59 MessageBox.Show( "Error opening file", "Error",


61 } // end catch

62 } // end else

63 } // end method saveButton_Click

64

65 // handler for enterButton Click

66 private void enterButton_Click( object sender, EventArgs e )

67 {

68 // store TextBox values string array

69 string[] values = GetTextBoxValues();

70

71 // Record containing TextBox values to serialize

72 Record record = new Record();

73

74 // determine whether TextBox account field is empty

75 if ( values[ ( int ) TextBoxIndices.ACCOUNT ] != "" )

76 {

77 // store TextBox values in Record and serialize Record

78 try

79 {

80 // get account number value from TextBox

81 int accountNumber = Int32.Parse(

82 values[ ( int ) TextBoxIndices.ACCOUNT ] );

Outline

CreateFileForm.cs

(3 of 8)

Create a Record object to hold textboxes’ data

38


83

84 // determine whether accountNumber is valid

85 if ( accountNumber > 0 )

86 {

87 // store TextBox fields in Record

88 record.Account = accountNumber;

89 record.FirstName = values[ ( int ) TextBoxIndices.FIRST ];

90 record.LastName = values[ ( int ) TextBoxIndices.LAST ];

91 record.Balance = Decimal.Parse(

92 values[ ( int ) TextBoxIndices.BALANCE ] );

93

94 // write Record to file, fields separated by commas

95 fileWriter.WriteLine(

96 record.Account + "," + record.FirstName + "," +

97 record.LastName + "," + record.Balance );

98 } // end if

99 else

100 {

101 // notify user if invalid account number

102 MessageBox.Show( "Invalid Account Number", "Error",


104 } // end else

105 } // end try

106 // notify user if error occurs in serialization


108 {

109 MessageBox.Show( "Error Writing to File", "Error",


111 } // end catch

Outline

CreateFileForm.cs

(4 of 8)

Store and write record information

39


112 // notify user if error occurs regarding parameter format

113 catch ( FormatException )

114 {

115 MessageBox.Show( "Invalid Format", "Error",


117 } // end catch

118 } // end if

119

120 ClearTextBoxes(); // clear TextBox values

121 } // end method enterButton_Click

122

123 // handler for exitButton Click

124 private void exitButton_Click( object sender, EventArgs e )

125 {

126 // determine whether file exists

127 if ( output != null )

128 {

129 try

130 {

131 fileWriter.Close(); // close StreamWriter

132 output.Close(); // close file

133 } // end try

134 // notify user of error closing file


136 {

137 MessageBox.Show( "Cannot close file", "Error",


139 } // end catch

140 } // end if

Outline

CreateFileForm.cs

(5 of 8)

Release the program’s resources

40


141

142 Application.Exit();

143 } // end method exitButton_Click

144 } // end class CreateFileForm

Outline

CreateFileForm.cs

(6 of 8)

(a)

BankUI graphical

User interface

SaveFileDialog

Files and directories

(b)

41


Outline

CreateFileForm.cs

(7 of 8)

(c)

(e)

(d)

(f)

42


Outline

CreateFileForm.cs

(8 of 8)

(g) (h)

43


Good Programming Practice 18.1

When opening files, use the FileAccess enumeration to control user access to these files.

44


Common Programming Error 18.1

Failure to open a file before attempting to reference it in a program is a logic error.

45


Performance Tip 18.1

Close each file explicitly when the program no longer needs to reference the file. This can reduce resource usage in programs that continue executing long after they finish using a specific file. The practice of explicitly closing files also improves program clarity.

46


Performance Tip 18.2

Releasing resources explicitly when they are no longer needed makes them immediately available for reuse by other programs, thus improving resource utilization.

47


Fig. 18.10 | Sample data for the program of Fig. 18.9.

Account Number First Name Last Name Balance

100 Nancy Brown -25.54

200 Stacey Dunn 314.33

300 Doug Barker 0.00

400 Dave Smith 258.34

500 Sam Stone 34.98

48


18.6 Reading Data from a Sequential-Access Text File

• Programs normally start from the beginning of the file to retrieve data sequentially from a file

– Sometimes it is necessary to process a file sequentially several times

• FileStream object – Can reposition its file-position pointer to any position in the file

• Pointer contains the next byte to be read from or written

• Its file-position pointer is set to byte position 0 when file is opened

• Method Seek

– Reset the file-position pointer

• Specify the number of bytes it should be offset from the file’s beginning, end or current position

- The part of the file you want to be offset from is chosen using constants from the SeekOrigin enumeration

49


1 // Fig. 18.11: ReadSequentialAccessFileForm.cs

2 // Reading a sequential-access file.

3 using System;


5 using System.IO;


7

8 public partial class ReadSequentialAccessFileForm : BankUIForm

9 {

10 private FileStream input; // maintains connection to a file

11 private StreamReader fileReader; // reads data from a text file

12

13 // paramterless constructor

14 public ReadSequentialAccessFileForm()

15 {



18

19 // invoked when user clicks the Open button

20 private void openButton_Click( object sender, EventArgs e )

21 {

22 // create dialog box enabling user to open file

23 OpenFileDialog fileChooser = new OpenFileDialog();


25 string fileName; // name of file containing data

26

27 // exit event handler if user clicked Cancel


29 return;

Outline

ReadSequentialAccessFileForm.cs

(1 of 6)

Create a StreamReader object for reading text from a file


Prompt user for a file to read

Check to see if user pressed “Cancel”

50


30


32 ClearTextBoxes();

33





38 else

39 {

40 // create FileStream to obtain read access to file

41 input = new FileStream( fileName, FileMode.Open,

42 FileAccess.Read );

43

44 // set file from where data is read

45 fileReader = new StreamReader( input );

46

47 openButton.Enabled = false; // disable Open File button

48 nextButton.Enabled = true; // enable next record button

49 } // end else

50 } // end method openButton_Click

Outline


(2 of 6)Open file if exist or throw exception

Indicates that the program can perform only read operations with this object

Instantiate the StreamReader object to read text from a file

51


51

52 // invoked when user clicks Next button

53 private void nextButton_Click( object sender, EventArgs e )

54 {

55 try

56 {

57 // get next record available in file

58 string inputRecord = fileReader.ReadLine();

59 string[] inputFields; // will store individual pieces of data

60

61 if ( inputRecord != null )

62 {

63 inputFields = inputRecord.Split( ',' );

64

65 Record record = new Record(

66 Convert.ToInt32( inputFields[ 0 ] ), inputFields[ 1 ],

67 inputFields[ 2 ], Convert.ToDecimal( inputFields[ 3 ] ) );

68

69 // copy string array values to TextBox values

70 SetTextBoxValues( inputFields );

71 } // end if

72 else

73 {

74 fileReader.Close(); // close StreamReader

75 input.Close(); // close FileStream if no Records in file

76 openButton.Enabled = true; // enable Open File button

77 nextButton.Enabled = false; // disable Next Record button


Outline


(3 of 6)

Read line of text from file

Separate stream of characters that was read from the file into strings and store results as a Record object


52


79

80 // notify user if no Records in file

81 MessageBox.Show( "No more records in file", "",


83 } // end else

84 } // end try


86 {

87 MessageBox.Show( "Error Reading from File", "Error",


89 } // end catch

90 } // end method nextButton_Click

91 } // end class readSequentialAccessFileForm

Outline


(4 of 6)

(a)

53


Outline


(5 of 6)

(b)

(c) (d)

54


Outline


(6 of 6)

(e)

(g) (h)

(f)

55


Error-Prevention Tip 18.1

Open a file with the FileAccess.Read file-open mode if the contents of the file should not be modified. This prevents unintentional modification of the contents.

56


1 // Fig. 18.12: CreditInquiryForm.cs

2 // Read a file sequentially and display contents based on

3 // account type specified by user ( credit, debit or zero balances ).

4 using System;


6 using System.IO;


8

9 public partial class CreditInquiryForm : Form

10 {

11 private FileStream input; // maintains the connection to the file

12 private StreamReader fileReader; // reads data from text file

13

14 // name of file that stores credit, debit and zero balances

15 private string fileName;

16


18 public CreditInquiryForm()

19 {



22

23 // invoked when user clicks Open File button


25 {




Outline

CreditInquiryForm.cs

(1 of 8)

Create a StreamReader object for reading text from a file


Prompt user for the file to read

57


29



32 return;

33

34 fileName = fileChooser.FileName; // get name from user

35





40 else

41 {


43 input = new FileStream( fileName,

44 FileMode.Open, FileAccess.Read );

45

46 // set file from where data is read

47 fileReader = new StreamReader( input );

48

49 // enable all GUI buttons, except for Open File button

50 openButton.Enabled = false;

51 creditButton.Enabled = true;

52 debitButton.Enabled = true;

53 zeroButton.Enabled = true;

54 } // end else


Outline


(2 of 8)Open file if exist or throw exception

Indicates that the program can perform only read operations with this object

Instantiate the StreamReader object to read text from a file

58


56

57 // invoked when user clicks credit balances,

58 // debit balances or zero balances button

59 private void getBalances_Click( object sender, System.EventArgs e )

60 {

61 // convert sender explicitly to object of type button

62 Button senderButton = ( Button )sender;

63

64 // get text from clicked Button, which stores account type

65 string accountType = senderButton.Text;

66

67 // read and display file information

68 try

69 {

70 // go back to the beginning of the file

71 input.Seek( 0, SeekOrigin.Begin );

72

73 displayTextBox.Text = "The accounts are:\r\n";

74

75 // traverse file until end of file

76 while ( true )

77 {

78 string[] inputFields; // will store individual pieces of data

79 Record record; // store each Record as file is read

80 decimal balance; // store each Record's balance

81

82 // get next Record available in file

83 string inputRecord = fileReader.ReadLine();

Outline


(3 of 8)

Reset file-position point back to the beginning of the file

Read line of text from file

59


84

85 // when at the end of file, exit method

86 if ( inputRecord == null )

87 return;

88

89 inputFields = inputRecord.Split( ',' ); // parse input

90

91 // create Record from input

92 record = new Record(

93 Convert.ToInt32( inputFields[ 0 ] ), inputFields[ 1 ],

94 inputFields[ 2 ], Convert.ToDecimal( inputFields[ 3 ] ) );

95

96 // store record's last field in balance

97 balance = record.Balance;

98

99 // determine whether to display balance

100 if ( ShouldDisplay( balance, accountType ) )

101 {

102 // display record

103 string output = record.Account + "\t" +

104 record.FirstName + "\t" + record.LastName + "\t";

105

106 // display balance with correct monetary format

107 output += string.Format( "{0:F}", balance ) + "\r\n";

108

109 displayTextBox.Text += output; // copy output to screen

110 } // end if

111 } // end while

112 } // end try

113 // handle exception when file cannot be read

Outline


(4 of 8)

Separate stream of characters that was read from the file into strings and store results as a Record object

60



115 {

116 MessageBox.Show( "Cannot Read File", "Error",


118 } // end catch

119 } // end method getBalances_Click

120

121 // determine whether to display given record

122 private bool ShouldDisplay( decimal balance, string accountType )

123 {

124 if ( balance > 0 )

125 {

126 // display credit balances

127 if ( accountType == "Credit Balances" )

128 return true;

129 } // end if

130 else if ( balance < 0 )

131 {

132 // display debit balances

133 if ( accountType == "Debit Balances" )

134 return true;

135 } // end else if

136 else // balance == 0

137 {

138 // display zero balances

139 if ( accountType == "Zero Balances" )

140 return true;

141 } // end else

Outline


(5 of 8)

61


142

143 return false;

144 } // end method ShouldDisplay

145

146 // invoked when user clicks Done button

147 private void doneButton_Click( object sender, EventArgs e )

148 {


150 if ( input != null )

151 {

152 // close file and StreamReader

153 try

154 {

155 input.Close();

156 fileReader.Close();

157 } // end try

158 // handle exception if FileStream does not exist

159 catch( IOException )

160 {




164 } // end catch

165 } // end if

166


168 } // end method doneButton_Click

169 } // end class CreditInquiryForm

Outline


(6 of 8)


62


Outline


(7 of 8)

(a)

(b)

63


Outline


(8 of 8)

(c)

(d)

(e)

64


18.7 Serialization• Object serialization

– A serialized object is as a sequence of bytes that includes the object’s: • Data• Type • The types of data stored in the object

– Serialization can transmit objects between applications over a network– Serialized object can be read from the file

• Object Deserialization– The type information and bytes that represent the object and its data can be used to

recreate the object in memory

• Class BinaryFormatter – Enables entire objects to be written to or read from a stream– Method Serialize

• Writes an object’s representation to a file– Method Deserialize

• Reads this representation from a file and reconstructs the original object– Both methods throw a SerializationException– Both methods require a Stream object as a parameter

• So that the BinaryFormatter can access the correct stream

65


18.8 Creating a Sequential-Access File Using Object Serialization

• Serializable objects– Must include [Serializable] attribute or implement

interface ISerializable• Indicates to the CLR that objects of class can be serialized• Classes that write to or read from a stream must include this in their

declarations

– Must ensure that every instance variable of the class is also serializable

• Check class declaration to ensure that the type is serializable for variables of reference types

• All simple-type variables and strings are serializable• By default, array objects are serializable

– If the array contains references to other objects, those objects may or may not be serializable

66


1 // Fig. 18.13: RecordSerializable.cs

2 // Serializable class that represents a data record.

3 using System;

4

5 [ Serializable ]

6 public class RecordSerializable

7 {

8 private int account;

9 private string firstName;

10 private string lastName;

11 private decimal balance;

12

13 // parameterless constructor sets members to default values

14 public RecordSerializable()

15 : this( 0, "", "", 0.0M )

16 {


18

19 // overloaded constructor sets members to parameter values

20 public RecordSerializable( int accountValue, string firstNameValue,

21 string lastNameValue, decimal balanceValue )

22 {

23 Account = accountValue;

24 FirstName = firstNameValue;

25 LastName = lastNameValue;

26 Balance = balanceValue;


Outline

RecordSerializable.cs

(1 of 3)

Indicate to the CLR that objects of this class can be serialized

private instance values representing all the information for a record

Initializes the private instance variables’ value

67


28

29 // property that gets and sets Account

30 public int Account

31 {

32 get

33 {

34 return account;

35 } // end get

36 set

37 {

38 account = value;

39 } // end set

40 } // end property Account

41

42 // property that gets and sets FirstName

43 public string FirstName

44 {

45 get

46 {

47 return firstName;

48 } // end get

49 set

50 {

51 firstName = value;

52 } // end set

53 } // end property FirstName

Outline


(2 of 3)

Property for private instance variable account

Property for private instance variable firstName

68


54

55 // property that gets and sets LastName

56 public string LastName

57 {

58 get

59 {

60 return lastName;

61 } // end get

62 set

63 {

64 lastName = value;

65 } // end set

66 } // end property LastName

67

68 // property that gets and sets Balance

69 public decimal Balance

70 {

71 get

72 {

73 return balance;

74 } // end get

75 set

76 {

77 balance = value;

78 } // end set

79 } // end property Balance

80 } // end class RecordSerializable

Outline


(3 of 3)

Property for private instance variable lastName

Property for private instance variable balance

69


Common Programming Error 18.2

It is a logic error to open an existing file for output when the user wishes to preserve the file. The original file’s contents will be lost.

70


1 // Fig 18.14: CreateFileForm.cs

2 // Creating a sequential-access file using serialization.

3 using System;


5 using System.IO;

6 using System.Runtime.Serialization.Formatters.Binary;

7 using System.Runtime.Serialization;


9

10 public partial class CreateFileForm : BankUIForm

11 {

12 // object for serializing Records in binary format

13 private BinaryFormatter formatter = new BinaryFormatter();

14 private FileStream output; // stream for writing to a file

15


17 public CreateFileForm()

18 {



21

22 // handler for saveButton_Click

23 private void saveButton_Click( object sender, EventArgs e )

24 {

25 // create dialog box enabling user to save file

26 SaveFileDialog fileChooser = new SaveFileDialog();


28 string fileName; // name of file to save data

29

30 fileChooser.CheckFileExists = false; // allow user to create file

Outline

CreateFileForm.cs

(1 of 8)

Using specialize namespaces for serialization

Create a BinaryFormatter object for writing serialized object


Prompt user for location to save file

71


31

32 // exit event handler if user clicked "Cancel"


34 return;

35


37





42 else

43 {

44 // save file via FileStream if user specified valid file

45 try

46 {

47 // open file with write access

48 output = new FileStream( fileName,

49 FileMode.OpenOrCreate, FileAccess.Write );

50

51 // disable Save button and enable Enter button

52 saveButton.Enabled = false;

53 enterButton.Enabled = true;

54 } // end try

Outline

CreateFileForm.cs

(2 of 8)

Open the file if the file exists or create the file if it does not exist

Indicates that the program can perform only write operations with this object

Notify user of any problems with the appropriate message

Determine if user pressed “Cancel”

72


55 // handle exception if there is a problem opening the file


57 {

58 // notify user if file does not exist

59 MessageBox.Show( "Error opening file", "Error",


61 } // end catch

62 } // end else

63 } // end method saveButton_Click

64

65 // handler for enterButton Click

66 private void enterButton_Click( object sender, EventArgs e )

67 {

68 // store TextBox values string array

69 string[] values = GetTextBoxValues();

70

71 // Record containing TextBox values to serialize

72 RecordSerializable record = new RecordSerializable();

73

74 // determine whether TextBox account field is empty

75 if ( values[ ( int ) TextBoxIndices.ACCOUNT ] != "" )

76 {

77 // store TextBox values in Record and serialize Record

78 try

79 {

80 // get account number value from TextBox

81 int accountNumber = Int32.Parse(

82 values[ ( int ) TextBoxIndices.ACCOUNT ] );

Outline

CreateFileForm.cs

(3 of 8)

73


83

84 // determine whether accountNumber is valid

85 if ( accountNumber > 0 )

86 {

87 // store TextBox fields in Record

88 record.Account = accountNumber;

89 record.FirstName = values[ ( int ) TextBoxIndices.FIRST ];

90 record.LastName = values[ ( int ) TextBoxIndices.LAST ];

91 record.Balance = Decimal.Parse( values[

92 ( int ) TextBoxIndices.BALANCE ] );

93

94 // write Record to FileStream ( serialize object )

95 formatter.Serialize( output, record );

96 } // end if

97 else

98 {

99 // notify user if invalid account number

100 MessageBox.Show( "Invalid Account Number", "Error",


102 } // end else

103 } // end try

104 // notify user if error occurs in serialization

105 catch ( SerializationException )

106 {

107 MessageBox.Show( "Error Writing to File", "Error",


109 } // end catch

Outline

CreateFileForm.cs

(4 of 8)

Write the RecordSerializable object to the output file

Catch SerializationException if an exception is thrown during

serialization

74


110 // notify user if error occurs regarding parameter format

111 catch ( FormatException )

112 {

113 MessageBox.Show( "Invalid Format", "Error",


115 } // end catch

116 } // end if

117

118 ClearTextBoxes(); // clear TextBox values

119 } // end method enterButton_Click

120

121 // handler for exitButton Click

122 private void exitButton_Click( object sender, EventArgs e )

123 {


125 if ( output != null )

126 {

127 // close file

128 try

129 {

130 output.Close();

131 } // end try



134 {



137 } // end catch

138 } // end if

Outline

CreateFileForm.cs

(5 of 8)

75


139


141 } // end method exitButton_Click

142 } // end class CreateFileForm

Outline

CreateFileForm.cs

(6 of 8)

(a)

BankUI graphical user interface

76


Outline

CreateFileForm.cs

(7 of 8)

(b)

SaveFileDialog

Files and directories

(c) (d)

77


Outline

CreateFileForm.cs

(8 of 8)

(d)(g)

(e) (f)

78


18.9 Reading and Deserializing Data from a Sequential-Access Text File

•BinaryFormatter’s Method Deserialize – Reads a serialized object from a stream and reforms the

object in memory– Returns a reference of type object

• Must be cast to the appropriate type to manipulate the object

79


1 // Fig. 18.15: ReadSequentialAccessFileForm.cs

2 // Reading a sequential-access file using deserialization.

3 using System;


5 using System.IO;

6 using System.Runtime.Serialization.Formatters.Binary;

7 using System.Runtime.Serialization;


9

10 public partial class ReadSequentialAccessFileForm : BankUIForm

11 {

12 // object for deserializing Record in binary format

13 private BinaryFormatter reader = new BinaryFormatter();

14 private FileStream input; // stream for reading from a file

15


17 public ReadSequentialAccessFileForm()

18 {



21

22 // invoked when user clicks Open button


24 {




28 string fileName; // name of file containing data

Outline

ReadSequential AccessFileForm.cs

(1 of 6)Used to read serialized objects

80


29



32 return;

33



36





41 else

42 {


44 input = new FileStream(

45 fileName, FileMode.Open, FileAccess.Read );

46

47 openButton.Enabled = false; // disable Open File button

48 nextButton.Enabled = true; // enable Next Record button

49 } // end else


Outline


(2 of 6)

81


51

52 // invoked when user clicks Next button

53 private void nextButton_Click( object sender, EventArgs e )

54 {

55 // deserialize Record and store data in TextBoxes

56 try

57 {

58 // get next RecordSerializable available in file

59 RecordSerializable record =

60 ( RecordSerializable ) reader.Deserialize( input );

61

62 // store Record values in temporary string array

63 string[] values = new string[] {

64 record.Account.ToString(),

65 record.FirstName.ToString(),

66 record.LastName.ToString(),

67 record.Balance.ToString()

68 };

69 // copy string array values to TextBox values

70 SetTextBoxValues( values );

71 } // end try

72 // handle exception when there are no Records in file

73 catch( SerializationException )

74 {

75 input.Close(); // close FileStream if no Records in file

76 openButton.Enabled = true; // enable Open File button

77 nextButton.Enabled = false; // disable Next Record button

78


Outline


(3 of 6)

Use method Deserialize to read serialized data

Catch SerializationException if an exception is thrown during

serialization

82


80

81 // notify user if no Records in file

82 MessageBox.Show( "No more records in file", "",


84 } // end catch

85 } // end method nextButton_Click

86 } // end class readSequentialAccessFileForm

Outline


(4 of 6)

83


Outline


(5 of 6)

84


Outline


(6 of 6)

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2006 Pearson Education, Inc. All rights reserved. 1 18 Files and Streams