This is from an old VB text but coding conventions still apply

Sequential Access Files and Error Handling

Chapter Microsoft Visual Basic .NET: Reloaded

1

This is from an old VB text but coding conventions still apply

2Microsoft Visual Basic .NET: Reloaded

Objectives

• Declare StreamReader and StreamWriter variables

• Open a sequential access file

• Determine whether a sequential access file exists

• Write information to a sequential access file

• Align the text written to a sequential access file


Objectives (continued)

• Read information from a sequential access file

• Test for the end of a sequential access file

• Close a sequential access file

• Handle exceptions using a Try/Catch block

• Write records to a sequential access file

• Read records from a sequential access file


File Types

• Output files• Files to which output is written

• Store output produced by the application

• Input Files• Files that are read by the computer

• Application uses information stored in files

• Sequential• Often referred to as “text files”

• Binary and Random• Not covered in this text


Using Sequential Access Files


HOW TO…


Declaring StreamWriter and StreamReader Variables

• StreamWriter object

• Writes a sequence of characters to a sequential access file

• Referred to as a “stream of characters”

• Also referred to as a “stream”

• StreamReader object

• Reads a stream (sequence of characters) to a sequential access file


HOW TO…


Opening a Sequential Access file

• Declare variable of StreamWriter or StreamReader object datatype• Then use variable to refer to object and file in

the program

• OpenText method• Opens an existing file for input

• CreateText method• Creates a new empty file for output

• AppendText method• Appends data to the end of an existing

sequential access file


HOW TO…


Opening a Sequential Access file (continued)


Determining Whether a File Exists


Writing Information to a Sequential Access File

• Write method • Positions file pointer at end of last character it

writes to the file

• WriteLine method• Positions file pointer at beginning of next line in

the file by appending a line termination character (carriage return, line feed)

• Space function• Use to write a specific number of spaces to file


HOW TO…


Aligning Columns of Information

• PadLeft(length, [character])

• Pads at beginning of string with a specified character until string is a specified length

• Character is optional and default is a space

• PadRight(length, [character])

• Same as above only pads at end of string


HOW TO…


Reading Information from a Sequential Access file

• ReadLine method• Reads a line of text from sequential access file

• A line is a sequence of characters followed by the line termination character

• Peek method• Peeks into a file to see if file contains another

character to read

• Returns the number -1 if no more data in file

• Typically used within a loop to prevent a file read error


HOW TO…


Closing a Sequential Access File

• Close method• Closes file associate with stream variable


The Friends Application

• Allows user to write names of his or her friends to a sequential access file

• Allows names to be read from above file


The Friends Application (continued)






HOW TO…



• btnRead_Click event • Code causes name read from file

to be displayed in a message box


Using a Try/Catch Block

• Try statement• Use to catch (or trap) an exception

• Exception is an error that occurs while program is running

• Catch statement• Use to have computer take appropriate action• More than one catch can occur after a try

• Multiple catch allow for trapping of different types of errors

• Try/Catch block• Block of code that uses both Try and Catch

statements


HOW TO…


HOW TO…(continued)


Using a Try/Catch Block (continued)


Using a Try/Catch Block (continued)


Writing and Reading Records

• Field

• A single item of information about a person, place, or thing

• Examples: Name, salary, price, age

• Record

• One or more related fields that contain all the necessary data about a specific person, place, or thing


HOW TO…


HOW TO…


Programming Example - PAO Application

• Application allows user to enter the political party and age

• The input information is saved to a sequential access file

• In addition, the application calculates and displays the number of voters in each political party


TOE Chart


TOE Chart (continued)


User Interface


Objects, Properties, and Settings


Objects, Properties, and Settings (continued)


Tab Order


PseudocodebtnExit Click event procedure

close the applicationbtnWrite Click event procedure

open pao.txt file for appendif txtparty control contains D, R, or I

Write contents of txtParty control, a comma, and txtAge to pao.txt file

elseDisplay message prompting user to enter D,R, or I

End ifclose the pao.txt fileClear contents of txtParty and txtAge controlsSend focus to txtParty controlInclude a Try/Catch block to catch general errors displaying error description in a messagebox


Pseudocode (continued)

btnDisplay click event procedureopen pao.txt file for inputrepeat until no more characters to read

read record from fileif first character is letter D

add 1 to Democrat counterelse if first character is R

add 1 to Republican counterelse add 1 to Independent counterend if

end tryclose pao.txt file


Pseudocode (continued)

btnDisplay Click event (continued)

Display Democrat counter value in lblTotalDem

Display Republican counter value in lblTotalRep

Display Independent counter value in lblTotalInd

Include Try/Catch block with 2 Catch sections

The first catches FileNotFoundException and displays “Cannot locate the pao.txt file” in a messagebox

The second catches general errors and displays a description of the error in a messagebox


Code


Code (continued)


Summary• Information in a sequential access file is

accessed in consecutive order (from beginning to end)

• Use the StreamWriter object to write a sequence of characters (stream) to a sequential access file

• Use a StreamReader object to read a stream from a sequential access file

• Exists method returns a boolean value indicating whether file exists


Summary (continued)• Write method writes but does not write a line

termination characters• WriteLine method appends a line termination

character• ReadLine method reads a line of text • Peek method determines whether file has

more data to be read• Space function writes specific number of

spaces to file• PadLeft and PadRight pad characters to file to

align columns of information


Summary (continued)

• To prevent loss of data, call the Close method• Use a Try/Catch block to catch an exception

(error) and to take appropriate action when exception occurs

• Use a general Catch statement as the last Catch statement to handle unexpected errors

• Display exception using syntax variablename.Message where variablename is the name of variable in catch statement

• Sequential files are used to store fields and records

This is from an old VB text but coding conventions still apply

Documents

sequential access fileclose

sequential access filealign

sequential access filetest

sequential access filereferred

existing file

reloadedwriting information

specified character

reloadeddeclaring streamwriter