Chapter 9 Files, Printing, and Structure
Topics
•Using Files
•The OpenFileDialog, SaveFileDialog, FontDialog, and
ColorDialog Controls
•The PrintDocument Control
•Structures
The Life Span of Data
• Thus far, all of our data has been stored in controls
and variables existing in RAM
• This data disappears once the program stops running
• If data is stored in a file on a computer disk, it can be
retrieved and used at a later time
Three Steps in Using a File
1. The file must be opened
If it does not yet exist, it will be created
2. Data is read from or written to the file
3. The program closes the file
Reading and Writing to a File
• Data must be retrieved from disk and put in memory
for an application to work with it
• Data is transferred from disk to memory by:
– Reading it from an input file
– Placing it in variables or control properties
• Data is transferred from memory to disk by:
– Writing it to an output file
– Getting it from variables or control properties
• Data is frequently placed in the text property of a
control
File Types/Access Methods
• Text file type
– Character based text
– Contents can be viewed by Notepad
• Binary file type
– Pure binary form
– Contents cannot be viewed with a text editor
• Access Methods
– Sequential access – a continuous stream of data written and
read as a whole from beginning to end
– Random access – access in any order with data written to or
read from specific places in the file
– Like the difference between a cassette tape and a CD
Creating Files with StreamWriter Objects
• Add Imports System.IO before class declared
– Makes StreamWriter classes available in code
• A StreamWriter object is used to create a sequential
text file in the following way:
– Declare an object variable of type StreamWriter
Dim phoneFile As StreamWriter
– Call CreateText method passing the filename
phoneFile = File.CreateText("phonelist.txt")
– This Method returns a StreamWriter object
– Object is assigned to a StreamWriter variable
• Variable phoneFile now defines a stream of data that
can be written to phonelist.txt
Appending Text with StreamWriter
• A StreamWriter object is used to append data to a
sequential text file in the following way:
– Declare an object variable of type StreamWriter
Dim phoneFile As StreamWriter
– Call AppendText method passing the filename
phoneFile = File.AppendText("phonelist.txt")
– This Method AppendText returns a StreamWriter object
– Object is assigned to a StreamWriter variable
• Variable phoneFile now defines a stream of data that
can be added to the end of phonelist.txt
File Paths
• Filename can include the file path
– Can be a complete file path with drive letter
"C:\WordProc\memo.txt"
– Refer to a file in the default drive root directory
"\pricelist.txt"
– Or include no path information at all
"mytext.txt"
• If no path information specified, the bin folder
of the current project is used
Writing Data to a File
• The WriteLine method of a StreamWriter object
actually writes data to the file
ObjectVar.WriteLine(Data)
– Streamwriter object identified by ObjectVar
– The method’s Data argument consists of constants or
variables with data to be written
• WriteLine appends an invisible newline character to
the end of the data
• Omit argument to write a blank line to a file
ObjectVar.WriteLine()
Writing Data to a File Example
Dim studentFile As StreamWriter
studentFile = File.CreateText("StudentData.txt")
studentFile.WriteLine("Jim")
studentFile.WriteLine(95)
The
studentFile.WriteLine("Karen")
Resulting
Jim
studentFile.WriteLine(98)
File,
95
studentFile.WriteLine("Bob")
StudentData.txt
Karen
studentFile.WriteLine(82)
98
studentFile.Close()
Bob
82
The StreamWriter Write Method
ObjectVar.Write(Data)
• The Write method writes an item of data
without writing a newline character
• Usually need to provide some sort of
delineation or delimiter between data items
– A blank space could be used
– Comma is a more common delimiter
Closing a StreamWriter Object
• Should close files when finished with them
– Avoids losing data
– Data is initially written to a buffer
– Writes unsaved data from the buffer to the file
• The Close method of a StreamWriter object clears the
buffer and closes the file
ObjectVar.Close()
– Streamwriter object identified by ObjectVar
• Tutorial 9-1 provides an example of an application
that writes data to a file
Appending to a File
• If opening an existing file with CreateText
– Existing contents are deleted
– New text overwrites the old text
• If opening an existing file with AppendText
– Existing contents are retained
– New text adds on to the end of the old text
• If adding a new friend to friendFile, use:
friendFile = File.AppendText("MyFriends.txt")
Appending a File Example
'Declare an object variable
Dim friendFile as StreamWriter
friendFile
'Open the file
“After”
friendFile = File.AppendText("MyFriends.txt")
'Write the data
friendFile.WriteLine("Bill Johnson")
Jim Weaver
friendFile.WriteLine(30)
30
friendFile.WriteLine("36 Oak Street")
P.O. Box 124
'Close the file
Mary Duncan
Jim Weaver
24
friendFile.Close()
30
friendFile
“Before”
P.O. Box 124
Mary Duncan
24
47 Elm Street
Karen Warren
28
24 Love Lane
47 Elm Street
Karen Warren
28
24 Love Lane
Bill Johnson
30
36 Oak Street
StreamReader Objects
• Use StreamReader objects to read from a file
• Define and open similar to StreamWriter:
Dim ObjectVar As StreamReader
ObjectVar = File.OpenText(Filename)
• Sample code:
Dim phoneFile As StreamReader
phoneFile = File.OpenText("phonelist.txt")
• Variable phoneFile now defines a stream of data that
can be read from phonelist.txt
• Must have Imports System.IO before class
declaration as was done with StreamWriter
Reading Data from a File
• The ReadLine method of a StreamReader object
actually reads data from the file
dataVar = ObjectVar.ReadLine()
– Streamwriter object identified by ObjectVar
– The result of the method, the data read from the file, is
assigned to string variable dataVar
• Sample code:
Dim custFile As StreamReader
custFile = File.OpenText("customer.txt")
custName = custFile.ReadLine()
• custName holds the data read from the file
• StreamReader also has a Close method
Determining Whether a File Exists
• The File.OpenText method issues a runtime error if
the file does not exist
• Avoid this by using the File.Exists method
– Format is File.Exists(filename)
– Returns a boolean result that can be tested:
If System.IO.File.Exists(filename) Then
' Open the file.
inputFile = System.IO.File.OpenText(filename)
Else
MessageBox.Show(filename & " does not exist.")
End If
• Tutorial 9-2 shows how to read text file data
Detecting End of File
• The Peek method tests if you’ve reached end
of file (no more characters to read)
– Format is objectvar.Peek
– If no more characters, the value -1 is returned
Dim scoresFile As StreamReader
Dim strInput As String
scoresFile = File.OpenText("Scores.txt")
Do Until scoresFile.Peek() = -1
strInput = scoresFile.ReadLine()
lstResults.Items.Add(input)
Loop
scoresFile.Close()
• Tutorial 9-3 demonstrates the Peek method
Detecting End of File (Cont’d)
• The EndofStream method also tests if you’ve
reached end of file (no more characters to
read)
– Format is objectvar.EndOfStream
– If no more characters, the value True is returned
Dim scoresFile As StreamReader
Dim strInput As String
scoresFile = File.OpenText("Scores.txt")
Do Until scoresFile.EndOfStream()
strInput = scoresFile.ReadLine()
lstResults.Items.Add(input)
Loop
scoresFile.Close()
ReadToEnd Method
• ReadToEnd method returns the rest of the file
from the current read position to end of file
• Functions differently from ReadLine method
– ReadToEnd method ignores line delimiters
• The statement
input = textFile.ReadToEnd
reads the file contents and stores it in strInput
Dim textFile As StreamReader
Dim strInput As String
textFile = File.OpenText("names.txt")
strInput = textFile.ReadToEnd
textFile.Close()
Write Then Read Entire Array
Dim intValues(9) As Integer
-----------------------------------------------Dim outputFile as StreamWriter
outputFile = File.CreateText("values.txt")
For intCount = 0 To intValues.Length – 1
outputFile.WriteLine(intValues(intCount))
Next
outputFile.Close()
-----------------------------------------------Dim inputFile as StreamReader
inputFile = File.OpenText("values.txt")
For intCount = 0 To intValues.Length – 1
intValues(intCount) = CInt(inputFile.ReadLine)
Next
inputFile.Close()
OpenFileDialog and SaveFileDialog
• Windows has a standard method of allowing a
user to choose a file to open or save
– These methods let users browse for a file
• The OpenFileDialog and SaveFileDialog
controls provide this capability in VB
• To use the OpenFileDialog control
– Double click on this tool in the Toolbox
– Appears in component tray
– Use ofd as standard prefix when naming
• SaveFileDialog is used in a similar way
Displaying an Open Dialog Box
• Display control with the ShowDialog method
ControlName.ShowDialog()
• Method returns a value indicating which
dialog box button the user selects, either
– DialogResult.OK, or
– DialogResult.Cancel
• For example:
If ofdOpenfile.Showdialog() = DialogResult.OK Then
MessageBox.Show(ofdOpenFile.FileName)
Else
MessageBox.Show("You selected no file")
End If
Dialog Box Filter Property
• FileDialog controls have a Filter property
–
–
–
–
Limits files shown to specific file extensions
Specify filter description shown to user first
Then specify the filter itself
Pipe symbol (|) used as a delimiter
• Following Filter property lets user choose:
– Text files (*.txt), displays all .txt files
– All files (*.*), displays all file extensions
ofdOpenFile.Filter = "Text files (*.txt)|*.txt|" & _
"All files (*.*)|*.*"
Open Dialog Box Example
• InitialDirectory property
• Title property
• Filter property
SaveFileDialog Control
• SaveFileDialog uses the same methods:
– ShowDialog()
• The same properties:
–
–
–
–
Filter
InitialDirectory
Title
Filename
• And the same result constants:
– DialogResult.OK
– DialogResult.Cancel
• Tutorial 9-4 uses these controls in a text editor
ColorDialog Control
• Displays a typical Windows color dialog box
– Provides users the ability to choose a color
ColorDialog Control
• To use the ColorDialog control
– Double click the tool in the Toolbox
– Appears in component tray
– Use cd as standard prefix when naming
• The following code sets the text in control
lblMessage to the color selected by the user
cdColor.ShowDialog()
If cdColor.ShowDialog() = DialogResult.OK Then
lblMessage.ForeColor = cdColor.Color
End If
FontDialog Control
• Displays a Windows font selection dialog box
– Allows users to choose font, font size, etc.
FontDialog Control
• To use the FontDialog control
– Double click the tool in the Toolbox
– Appears in component tray
– Use fd as standard prefix when naming
• The following code sets the text in control
lblMessage to the font selected by the user
fdFont.ShowDialog()
If fdFont.ShowDialog() = DialogResult.OK Then
lblMessage.Font = fdFont.Font
End If
PrintDocument Control
• Allows you to send output to the printer
• To use the PrintDocument control
– Double click the tool in the Toolbox
– Appears in component tray
– Use pd as standard prefix when naming
• PrintDocument control has a Print method
– This method starts the printing process
– Format is:
PrintDocumentControl.Print()
– This triggers a PrintPage event
PrintPage Event Handler
• The code in the PrintPage event handler
performs the actual printing
–
–
–
–
Double click PrintDocument control in tray
This creates the PrintPage event handler
Insert your print code inside the event handler
Basic format of event handler shown below:
Private Sub pdPrint_PrintPage(ByVal sender As System.Object, _
ByVal e As System.Drawing.Printing.PrintPageEventArgs) _
Handles pdPrint.PrintPage
'Your print code inserted here
End Sub
DrawString Method
• The DrawString method is used inside the
PrintPage event to:
–
–
–
–
–
Specify data to send to the printer in string
Set font, font size, and font style
Determine horizontal position (HPos) of text
Determine vertical position (VPos) of text
Brushes.Black specifies output in black
• DrawString method is formatted as follows:
e.Graphics.DrawString(String, _
New Font(FontName, Size, Style), _
Brushes.Black, HPos, VPos)
Specifying Fonts, Sizes, Styles
• Fonts are specified with the string which names the
font to be used
– "Times New Roman"
– “Arial" , etc.
• Sizes are specified with a number
– 10, 12, etc.
• Print effects are specified with provided constants
– FontStyle.Regular
– FontStyle.Bold
– FontStyle.Underline
Sample PrintPage Event Procedure
Private Sub pdPrint_PrintPage(ByVal sender As System.Object, _
ByVal e As System.Drawing.Printing.PrintPageEventArgs) _
Handles pdPrint.PrintPage
Dim inputFile As StreamReader
Dim intX As Integer = 10
'Horizontal Position
Dim intY As Integer = 10
'Vertical Position
inputFile = File.OpenText(strFilename)
Do While inputFile.Peek <> -1
e.Graphics.DrawString(inputFile.ReadLine, _
New Font("Courier", 10, FontStyle.Regular), _
Brushes.Black, intX, intY)
intY += 12
'Increment Vert Pos
Loop
inputFile.Close()
End Sub
• Tutorial 9-5 adds a print feature to Tutorial 9-4
Printing Column Based Reports
• Business reports typically contain a:
– Report header printed at the top of the page
– Report body with the data, usually in columns
– Optional footer, often totalling certain columns
• Report header usually has column headings
• Monospaced font used for column reports
– Each character takes same amount of space
– This allows columns to be aligned
• String.Format used to align data along column
boundaries
String.Format Example
String.Format("{0, 7}{1, -10}{2, 7}", 50, "Arg1", 6)
Specifies
the argument
number
Argument 0
Argument 1
Specifies field width for arg
negative - left justified
positive - right justified
Results in the following output:
50Arg 1
7 spaces
10 spaces
Left Justified
6
7 spaces
Argument 2
Other OpenFileDialog Properties
• InitialDirectory property specifies folder to use
– Default if not specified is the current folder
– To set dialog box initial directory to C:\Data:
ofdOpenFile.InitialDirectory = "C:\Data"
• Title property specifies the text on the title bar
– Default title is Open if not specified
ofdOpenFile.Title = "Select a File to Open"
• Filename property returns file selected from
dialog box by user, in this case to selectedFile
selectedFile = ofdOpenFile.Filename
Open Dialog Box Example
•
•
•
•
User may choose to display .txt files or all files
Files from Data folder of hard drive are shown
Dialog box title shows Select a File to Open
Variable inputFile holds file selected by user
' Configure the Open dialog box and display it.
With ofdOpenFile
.Filter = "Text files (*.txt)|*.txt|" & _
"All files (*.*)|*.*"
.InitialDirectory = "C:\Data"
.Title = "Select a File to Open"
If .ShowDialog() = DialogResult.OK Then
inputFile = System.IO.File.OpenText(.Filename)
End If
End With
Structures
A Visual Basic programmer may also create new data types, called
user-defined data types (UDT).
Two kinds of UDTs: structure and class.
This chapter discusses the use of structure and Chapter 12 the use
of class.
Structures vs. Arrays
• Arrays:
– Multiple fields in one array
– All of the same data type
– Distinguished by a numerical index
• Structures
– Multiple fields in one structure
– Can be of differing data types
– Distinguished by a field name
Syntax for Declaring a Structure
[AccessSpecifier] Structure StructureName
FieldDeclarations
End Structure
• StructureName is a name that identifies the
structure itself
• FieldDeclarations are the declarations
of the individual fields within the structure
Structure Declaration Example
• Following declares a structure with six fields
intended to record employee payroll data
• Structure name is EmpPayData
Structure EmpPayData
Dim intEmpNumber As Integer
Dim strFirstName As String
Dim strLastName As String
Dim sngHours As Single
Dim decPayRate As Decimal
Dim decGrossPay As Decimal
End Structure
Creating and Initializing a Structure
• Using the EmpPayData structure just defined
– Define variable deptHead of type EmpPayData
– deptHead contains the six fields in the structure
– Access each field using varName.fieldName
Dim deptHead As EmpPayData
deptHead.intEmpNumber = 1101
deptHead.strFirstName = "Joanne"
deptHead.strLastName = "Smith"
deptHead.sngHours = 40
deptHead.decPayRate = 25
deptHead.decGrossPay = CDec(deptHead.sngHours) * _
deptHead.decPayRate
Passing Structure Variables
to Procedures and Functions
• Structures can be passed to procedures and functions
like any other variable
• The data type to use in the specification is the name of
the structure
Sub CalcPay(ByRef employee As EmpPaydata)
' This procedure accepts an EmpPayData variable
' as its argument. The employee’s gross pay
' is calculated and stored in the grossPay
' field.
With employee
.decGrossPay = .sngHours * .decPayRate
End With
End Sub
Structures Containing Arrays
• Structures can contain dynamic arrays
• Must ReDim after declaring structure variable
Structure StudentRecord
Dim strName As String
Dim sngTestScores() As Single
End Structure
Dim student As StudentRecord
ReDim student.sngTestScores(4)
student.strName = "Mary McBride"
student.sngTestScores(0) = 89
Student.sngTestScores(1) = 92
Student.sngTestScores(2) = 84
Student.sngTestScores(3) = 96
Student.sngTestScores(4) = 91
Arrays Containing Structures
• Can declare an array of structures
• Example below declares employees as an array of
type EmpPayData with 10 elements
• Can refer to each field using the format
arrayName(index).fieldName
Dim employees(9) As EmpPayData
' Refer to the empNumber of the first employee
employees(0).empNumber = 1101
• Tutorial 9-6 examines an application with a structure