1. 18
Files and Streams

2. 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
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

3. OBJECTIVES In this chapter you will learn:
To create, read, write and update files.
The .NET framework 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   Case Study: A Credit-Inquiry Program
18.8   Serialization
18.9   Creating a Sequential-Access File Using Object Serialization
18.10  Reading and Deserializing Data from a Sequential-Access Text File
18.11   Wrap-Up

5. 18.1 Introduction Persistent Data
Data maintained in files
Create, upload and process data files in Visual Basic
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
Visual Basic 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 | Data hierarchy.

8. 18.3 Files and Streams Files and Streams
Visual Basic 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 | Visual Basic’s view of an n-byte file.

10. 18.3 Files and Streams (Cont.)
There are many file-processing classes in the FCL
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
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 Shared methods (partial list). (Part 1 of 2.)

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

15. Fig. 18.4 | Directory class Shared methods.

16. Checks to see if filename is an existing file
Outline
FrmFileTest.vb
(1 of 4 )
Checks to see if filename is an existing file

17. Outline Instantiates a StreamReader for reading text from a file
FrmFileTest.vb
(2 of 4 )
Checks to see if filename is an existing directory
Obtain a String array containing the names of the subdirectories in the specified directory
If successful, output results

18. Outline (3 of 4 ) If not, notify user
FrmFileTest.vb
(3 of 4 )
If not, notify user
Returns a DateTime object representing when the file was created
Returns a DateTime object representing when the file was last modified
Returns a DateTime object representing when the file was last accessed

19. Outline
FrmFileTest.vb
(4 of 4 )

20. Outline Returns the current project’s directory (1 of 6 )
FrmFileSearch.vb
(1 of 6 )
Create an object of NameValueCollection

21. Determine if txtInput.Text is an existing directory
Outline
FrmFileSearch.vb
(2 of 6 )
Determine if txtInput.Text is an existing directory

22. Outline (3 of 6 ) Use key string to access corresponding value
FrmFileSearch.vb
(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

23. Outline
Retrieve the names of all subdirectories that belong to the current directory
FrmFileSearch.vb
(4 of 6 )
Store the names of files in the current directory
Eliminate the directory path
Return the matches for the regular expression

24. Determine if this is the first occurrence of the filename extension
Outline
FrmFileSearch.vb
(5 of 6 )
If not, increment the value associated with the extension
Delete the file and decrement the corresponding value

25. Notify user of any unauthorized file access
Outline
FrmFileSearch.vb
(6 of 6 )
Notify user of any unauthorized file access

26. 18.5 Creating a Sequential-Access Text File
Visual Basic 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

27. Define enumeration for the textbox’s indices
Outline
FrmBankUI.vb
(1 of 3 )
Define enumeration for the textbox’s indices
Clear textbox

28. Set textboxes to corresponding values
Outline
FrmBankUI.vb
(2 of 3 )
Set textboxes to corresponding values

29. Return textboxes’ values
Outline
FrmBankUI.vb
(3 of 3 )
Return textboxes’ values

30. Outline
Private instance values representing all the information for a record
Record.vb
(1 of 3 )
Initializes the Private instance variables’ value

31. Outline Property for Private instance variable accountValue (2 of 3 )
Record.vb
(2 of 3 )
Property for Private instance variable firstNameValue
Property for Private instance variable lastNameValue

32. Property for Private instance variable balanceValue
Outline
Property for Private instance variable balanceValue
Record.vb
(3 of 3 )

33. Outline Create a StreamWriter object for writing text to a file
FrmCreateFile.vb
(1 of 7 )
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

34. Outline Obtain user-selected file
Open the file if the file exists or create the file if it does not exist
FrmCreateFile.vb
(2 of 7 )
Indicates that the program can perform only write operations with this object
Instantiate the StreamWriter object to write text to a file

35. Outline (3 of 7 ) Create a Record object to hold textboxes’ data
FrmCreateFile.vb
(3 of 7 )
Create a Record object to hold textboxes’ data
Store record information

36. Write record information
Outline
Write record information
FrmCreateFile.vb
(4 of 7 )

37. Release the program’s resources
Outline
Release the program’s resources
FrmCreateFile.vb
(5 of 7 )

38. Outline
FrmCreateFile.vb
(6 of 7 )

39. Outline
FrmCreateFile.vb
(7 of 7 )

40. Common Programming Error 18.1
Failure to open a file before attempting to reference it in a program is a logic error.

41. 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.

42. 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.

43. 18.6 Reading Data from a Sequential-Access Text File
StreamReader method ReadLine
Reads a line of text from file
Returns Nothing is not more text in the file

44. Fig. 18.10 | Sample data for the program in Fig. 18.9.

45. Outline Create a FileStream object for connecting to a file (1 of 6 )
FrmRead SequentialAccess File.vb
(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”

46. Outline Open file if exist or throw exception
Indicates that the program can perform only read operations with this object
FrmRead SequentialAccess File.vb
(2 of 6 )
Instantiate the StreamReader object to read text from a file
Read line of text from file
Separate stream of characters that was read from the file into strings

47. Outline Store results as a Record object (3 of 6 )
FrmRead SequentialAccess File.vb
(3 of 6 )
Release the program’s resources

48. Outline
FrmRead SequentialAccess File.vb
(4 of 6 )

49. Outline
FrmRead SequentialAccess File.vb
(5 of 6 )

50. Outline
FrmRead SequentialAccess File.vb
(6 of 6 )

51. 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.

52. 18.7 Case Study: A Credit-Inquiry Program
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

53. Outline Create a FileStream object for connecting to a file (1 of 8 )
FrmCreditInquiry .vb
(1 of 8 )
Create a StreamReader object for reading text from a file
Prompt user for the file to read

54. Outline Open file if exist or throw exception (2 of 8 )
FrmCreditInquiry .vb
(2 of 8 )
Indicates that the program can perform only read operations with this object
Instantiate the StreamReader object to read text from a file
Event handler for three buttons’ Click events

55. Outline Reset file-position point back to the beginning of the file
FrmCreditInquiry .vb
(3 of 8 )
Read line of text from file

56. Outline
FrmCreditInquiry .vb
(4 of 8 )
Separate stream of characters that was read from the file into strings and store results as a Record object

57. Outline
FrmCreditInquiry .vb
(5 of 8 )

58. Release the program’s resources
Outline
FrmCreditInquiry .vb
(6 of 8 )
Release the program’s resources

59. Outline
FrmCreditInquiry .vb
(7 of 8 )

60. Outline
FrmCreditInquiry .vb
(8 of 8 )

61. 18.8 Serialization Object Deserialization 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

62. 18.9 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 primitive-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

63. Outline
Indicate to the CLR that objects of this class can be serialized
Record Serializable.vb
(1 of 3 )
Private instance values representing all the information for a record
Initializes the Private instance variables’ value

64. Outline Property for Private instance variable accountValue (2 of 3 )
Record Serializable.vb
(2 of 3 )
Property for Private instance variable firstNameValue

65. Outline Property for Private instance variable lastNameValue (3 of 3 )
Record Serializable.vb
(3 of 3 )
Property for Private instance variable balanceValue

66. 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.

67. Outline Using specialize namespaces for serialization
Create a BinaryFormatter object for writing serialized object
FrmCreateFile.vb
(1 of 7 )
Create a FileStream object for connecting to a file
Prompt user for location to save file
Determine if user pressed “Cancel”

68. Notify user of any problems with the appropriate message
Outline
Notify user of any problems with the appropriate message
FrmCreateFile.vb
(2 of 7 )
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

69. Outline
FrmCreateFile.vb
(3 of 7 )

70. Write the RecordSerializable object to the output file
Outline
Write the RecordSerializable object to the output file
FrmCreateFile.vb
(4 of 7 )
Catch SerializationException if an exception is thrown during serialization

71. Outline
FrmCreateFile.vb
(5 of 7 )

72. Outline
FrmCreateFile.vb
(6 of 7 )

73. Outline
FrmCreateFile.vb
(7 of 7 )

74. 18.10 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

75. Used to read serialized objects
Outline
FrmRead Sequential AccessFile.vb
(1 of 6 )
Used to read serialized objects

76. Use method Deserialize to read serialized data
Outline
FrmRead Sequential AccessFile.vb
(2 of 6 )
Use method Deserialize to read serialized data

77. Outline
FrmRead Sequential AccessFile.vb
(3 of 6 )
Catch SerializationException if an exception is thrown during serialization

78. Outline
FrmRead Sequential AccessFile.vb
(4 of 6 )

79. Outline
FrmRead Sequential AccessFile.vb
(5 of 6 )

80. Outline
FrmRead Sequential AccessFile.vb
(6 of 6 )