1. Chapter 11 Saving Data and Objects In Files
Programming In
Visual Basic .NET

2. Data Files To save data from one run of the application to the next
Use when there is only a small amount of data that would not warrant the use of a database
Use for Windows applications, the default security policy for the Internet and Intranets does not allow access to disk files
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3. Data File Terminology File ==> Entire collection of data
Records ==> Rows, one per entity
Fields ==> Columns, data elements within row
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4. Sample Data File Records Fields Last Name First Name Phone
Maxwell Harry
Helm Jennifer
Colton Craig
Records
Fields
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5. File Handling Using Streams
A Stream is designed to transfer a series of bytes from one location to another
Streams are objects that have methods and properties
Found in the System.IO.namespace
File handling projects must contain an Imports statement before the statement declaring the form's class
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6. File I/O Reading and writing data in a disk file Writing = Output
Reading = Input
Write Output
Read Input
Form
Data
File
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7. Writing Data in a File User inputs data into text boxes
Declare a new StreamWriter object
Also declares the name of the data file
Use StreamWriter's WriteLine method
Copies data to a buffer in memory
Call StreamWriter's Close method
Transfers data from buffer to the file and releases system resources used by the stream
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8. Writing Data in a File (continued)
Declaring a new StreamWriter object opens the file
If the file does not exist, a new one is created
Declare the StreamWriter object either in the declarations section of your program or in a procedure
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9. Instantiating a StreamWriter Object
General Form
Default location for file is the bin directory beneath the folder for the current project
Can specify the complete path of the file
Dim ObjectName As New StreamWriter("FileName")
Dim ObjectName As New StreamWriter("FileName", BooleanAppend)
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10. Declaring a StreamWriter Object - Examples
Dim phoneStreamWriter As New StreamWriter("Phone.txt")
Dim namesStreamWriter As New StreamWriter("C:\MyFiles\Names.txt")
Dim logFileStreamWriter As New StreamWriter("C:\MyFiles\LogFile.txt", True)
True = Append data to this existing file
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11. Write and WriteLine Methods
Write Method
Places items consecutively in the file with no delimiter (separator)
WriteLine Method
Places an enter (carriage return) between items
Used in this chapter
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12. The WriteLine Method - General Form
DataToWrite argument may be string or numeric
Converts any numeric data to string and writes string data in the file
ObjectName.WriteLine(DataToWrite)
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13. The WriteLine Method - Examples
phoneStreamWriter.WriteLine(nameTextBox.Text)
phoneStreamWriter.WriteLine(phoneTextBox.Text)
namesStreamWriter.WriteLine("Sammy")
bankBalanceStreamWriter.WriteLine(balanceDecimal.ToString( ))
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14. Closing a File Use StreamWriter's Close method
Finishes writing all data from the stream's buffer to the disk and releases system resources
Commonly coded in form’s closing event procedure
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15. Closing a File - Example
Private Sub exitButton_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles exitButton.Click
' Close the file and the form.
phoneStreamWriter.Close( )
Me.Close( )
End Sub
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16. Viewing the Contents of a File
Text editor, such as Notepad
Visual Studio's IDE
In the Solution Explorer 1. Select the Project name 2. Click Show All Files button to display the bin folder 3. Select the file to display in the editor window
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17. Viewing the Contents of a File (continued)
Select project name
Show All Files
button
Contents of
Data File
bin folder
New File
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18. Reading Data from a File
Declare an object of the StreamReader class
Also declares the name of the data file
Opens the file
Use StreamReader's ReadLine method
May need a loop to retrieve multiple records
Call StreamReader's Close method
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19. Reading Data from a File (continued)
The file must exist in the specified location or an exception occurs
Declare the StreamReader object in a procedure and enclose it in a Try/Catch block
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20. Instantiating a StreamReader Object
General Form
Example
Dim ObjectName As New StreamReader("FileName")
Try
Dim namesStreamReader As New StreamReader("C:\MyFiles\Names.txt")
Catch
MessageBox.Show("File does not exist")
End Try
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21. Using the ReadLine Method
Use to read previously saved data
Each time it executes, it reads the next line of data
Assign the value from the read to the desired location, such as a label, text box, or string variable
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22. Using the ReadLine Method -Example
nameLabel.Text = phoneStreamReader.ReadLine( )
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23. Checking for the End of the File
Use StreamReader's Peek method
Peek looks at the next element without reading it
If you Peek beyond the last element the value returned is negative 1 (-1)
Code an If statement to execute Peek and compare for <> -1 before reading
Read elements in the exact same order as they were written to obtain valid data
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24. Using the File Common Dialog Box
The previous data file examples included hard-coded file names (and paths)
May allow the user to browse and enter the file name at run time
Use the OpenFileDialog common dialog component to display the dialog box
Then use the object's FileName property to open the selected file
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25. OpenFileDialog Component Properties
Name – Can use default name
CheckFileExists
CheckPathExists
FileName (will be set by user's selection at run time)
Filter
InitialDirectory (set this property in code)
Title
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26. Displaying the Open File Dialog Box
Add OpenFileDialog component to form
At design time set initial properties for Name, CheckFileExists, CheckPathExists, Filter and Title
In code set InitialDirectory property to Application.StartUpPath
Display dialog box using ShowDialog method and retrieve FileName property
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27. Open File to Read Dialog Box
Files of Type
Determined by Filter Property
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28. Open File to Read Dialog Box - Code Example
Private Sub fileOpenMenuItem_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles fileOpenMenuItem.Click
' Open the file.
Dim responseDialogResult As DialogResult
' Begin in the project folder.
OpenDialog1.InitialDirectory = Application.StartupPath
' Display the File Open dialog box.
responseDialogResult = OpenDialog1.ShowDialog( )
' Make sure that the user didn’t click the Cancel button.
If responseDialogResult <> DialogResult.Cancel Then
' Open the output file.
phoneStreamWriter = New StreamWriter(OpenDialog1.FileName)
End If
End Sub
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29. Checking for Successful File Open
If the StreamWriter or StreamReader were not instantiated, the file did not open
Use the Nothing keyword to check for instantiation
An object that has not been instantiated has a value of Nothing
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30. Using Nothing Keyword ' Is the file already open?
If Not phoneStreamWriter Is Nothing Then
phoneStreamWriter.Close( )
End If
Must use Keyword Is rather than equal sign
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31. Saving the Contents of a List Box
Do not assign values at design time, when program begins, open data file and read items into Items collection of List Box
If user makes changes to list, ask whether to save list when program ends, include a menu option to save list
If file of list elements does not exist when program begins, allow user to create new list by adding items
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32. Loading the List Box Read the file into the list in Form_Load
Loop through the file until all elements are read
Use the Items.Add method to add the data elements to the list
Dim coffeeFlavor As String
' Read all elements into the list.
Do Until flavorsStreamReader.Peek = -1
coffeeFlavorString = flavorStreamReader.ReadLine( )
coffeeComboBox.Items.Add(coffeeFlavorString)
Loop
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33. Checking for Existence of the File
When the StreamReader object is created, the constructor makes sure the file exists
If it does not exist, give the user options
Try to locate the data file again
Exit the program
Begin with an empty list, add items, and create a new file
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34. Saving the File Provide a menu option for the user to save the file
Open a StreamWriter object
Loop through the Items collection of the list box, saving each element with a WriteLine method
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35. Saving the File - Example
Private Sub fileSaveMenuItem_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles fileSaveMenuItem.Click
' Save the list box contents in a file.
Dim indexInteger As Integer
Dim numberItemsInteger As Integer
' Open the file.
Dim flavorsStreamWriter As New StreamWriter("Coffees.txt")
' Save the items in the file.
numberItemsInteger = coffeeComboBox.Items.Count - 1
For indexInteger = 0 To numberItemsInteger
flavorsStreamWriter.WriteLine(coffeeComboBox.Items(indexInteger))
Next indexInteger
flavorsStreamWriter.Close( ) ' Close the file.
isDirtyBoolean = False ' Reset changed status.
End Sub
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36. Querying the User to Save
Good idea to ask users if they want to save any changes made before program ends
Use a module-level Boolean variable, isDirtyBoolean, to keep track of changes
In procedures that allow changes, set variable to True
After saving file set variable to False
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37. The Form_Closing Procedure
Form’s Closing event procedure is best location to ask user to save the file
Closing event executes before the form closes no matter how user exits program or even Windows
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38. Serialization
Save an object and the current value of all of its properties using Serialization
Serialization refers to a series or stream of bits
Object's state is converted to a series of bits that can be saved and later used to recreate the object
Deserialization is reading the saved data back and recreating the object
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39. Serialization (continued)
To save an object
Class must be declared as Serializable
Must have a Formatter
Two types of Formatters
Binary, stores data in a binary form
SOAP (Simple Object Access Protocol), stores data in an XML format
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40. Serialization (continued)
Make any class module serializable allowing public properties of an object to be saved
Declare the class as Serializable
Declare a Formatter object
Declare a FileStream object that includes the name of the file
Use the Formatter object's Serialize method to save the object's properties
Close the FileStream
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41. Making a Class Serializable
Modify the class header to include the Serializable attribute
Add < Serializable ( ) > in front of the class header
Example
< Serializable ( ) > Public Class BookSale
' Body of the class.
End Class
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42. Adding a Formatter Object
Write the code to store (serialize) the object in your form's code, not in the serialize class
Declare a new formatter object using either
BinaryFormatter class
SoapFormatter class
A formatter object requires an Imports statement for the formatter class
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43. Adding a Formatter Object – Code Example
Imports System.IO
Imports System.Runtime.Serialization
Imports System.Runtime.Serialization.Formatters.Binary
Dim FormatObject As SoapFormatter = New SoapFormatter
Dim FormatObject As BinaryFormatter = New BinaryFormatter
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44. Using a FileStream
Serialization uses the FileStream class rather than StreamReader and StreamWriter classes
Declare a FileStream including
Name and Path for file
File Mode
Create ==> Open the file for output
Open ==> Retrieve the file
OpenOrCreate (module level, read and write)
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45. Declaring a FileStream Object
Constructor
Example
Dim ObjectName As FileStream = New FileStream("file name", _ FileMode.Open|Create)
Dim booksFileStream As FileStream = New FileStream _ ("Books.txt", FileMode.Create)
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46. Saving an Object
Use the Serialize method of the Formatter to save an object
Serialize method writes data to buffer
Include Close method to complete the save operation to disk
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47. Serialize Method General Form Example
FormatterObject.Serialize(StreamObject, ObjectToSave)
bookBinaryFormatter.Serialize(booksFileStream, aBookSale)
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48. Recreating an Object
Read an object back in with the Deserialize method of the formatter
Create a FileStream object in the Open mode
Declare a Formatter object
Use the Formatter's Deserialize method, converting the input to the desired type
Transfer fields from the object to the screen
Close the stream
Convert input data to correct type
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