Section 18.9. Creating a Sequential-Access File Using Object Serialization

18.9. Creating a Sequential-Access File Using Object Serialization

We begin by creating and writing serialized objects to a sequential-access file. In this section, we reuse much of the code from Section 18.5, so we focus only on the new features.

Defining the RecordSerializable Class

Let us begin by modifying our Record class (Fig. 18.8) so that objects of this class can be serialized. Class RecordSerializable (Fig. 18.13) is marked with the <Serializable()> attribute (line 5), which indicates to the CLR that objects of class RecordSerializable can be serialized. The classes for objects that we wish to write to or read from a stream must include this attribute in their declarations or must implement interface ISerializable (from namespace System.Runtime.Serialization). The remainder of class RecordSerializable is identical to class Record (Fig. 18.8).

Figure 18.13. RecordSerializable class for serializable objects.

  1   ' Fig. 18.13: RecordSerializable.vb  2   ' Serializable class that represents a data record.  3  4  < Serializable()>  5  Public Class RecordSerializable  6     Private accountValue As Integer  7     Private firstNameValue As String  8     Private lastNameValue As String  9     Private balanceValue As Decimal 10 11     ' default constructor sets members to default values 12     Public Sub New() 13        MyClass.New(0, "", "", 0D) 14     End Sub ' New 15 16     ' overloaded constructor sets members to parameter values 17     Public Sub New(ByVal account As Integer, ByVal firstName As String, _ 18        ByVal lastName As String, ByVal balance As Decimal) 19 20        account = account 21        firstNameValue = firstName 22        lastNameValue = lastName 23        balanceValue = balance 24     End Sub ' New 25 26     ' property Account 27     Public Property Account() As Integer 28        Get 29           Return accountValue 30        End Get 31        Set(ByVal value As Integer) 32           accountValue = value 33        End Set 34     End Property ' Account 35 36     ' property FirstName 37     Public Property FirstName() As String 38        Get 39           Return firstNameValue 40        End Get 41        Set(ByVal value As String) 42           firstNameValue = value 43        End Set 44     End Property ' FirstName 45 46     ' property LastName 47     Public Property LastName() As String 48        Get 49           Return lastNameValue 50        End Get 51        Set(ByVal value As String) 52           lastNameValue = value 53        End Set 54     End Property ' LastName 55 56     ' property Balance 57     Public Property Balance() As Decimal 58        Get 59           Return balanceValue 60        End Get 61        Set(ByVal value As Decimal) 62           balanceValue = value 63        End Set 64     End Property ' Balance 65  End Class ' RecordSerializable 

In a class that is marked with the <Serializable()> attribute or that implements interface ISerializable, you must ensure that every instance variable of the class is also serializable. All simple-type variables and Strings are serializable. For variables of reference types, you must check the class declaration (and possibly its base classes) to ensure that the type is serializable. By default, array objects are serializable. However, if the array contains references to other objects, those objects may or may not be serializable.

Using a Serialization Stream to Create an Output File

Now let's create a sequential-access file with serialization (Fig. 18.14). Line 10 creates a BinaryFormatter for writing serialized objects. Lines 3839 open the FileStream to which this program writes the serialized objects. The String argument fileName that is passed to the FileStream's constructor represents the name and path of the file to be opened. This specifies the file to which the serialized objects will be written.

Figure 18.14. Sequential file created using serialization.

  1  ' Fig 18.14: FrmCreateFile.vb  2  ' Creating a sequential-access file using serialization.  3  Imports System.IO  4  Imports System.Runtime.Serialization.Formatters.Binary  5  Imports System.Runtime.Serialization  6  Imports BankLibrary  7  8  Public Class FrmCreateFile  9     ' object for serializing Records in binary format 10     Private formatter As New BinaryFormatter() 11     Private output As FileStream ' stream for writing to a file 12 13     ' handler for btnSave_Click 14     Private Sub btnSave_Click(ByVal sender As System.Object, _ 15        ByVal e As System.EventArgs) Handles btnSave.Click 16        ' create dialog box enabling user to save file 17        Dim fileChooser As New SaveFileDialog() 18        Dim result As DialogResult = fileChooser.ShowDialog() 19        Dim fileName As String ' name of file to save data 20 21        fileChooser.CheckFileExists = False ' allow user to create file 22 23        ' exit event handler if user clicked "Cancel" 24        If result = Windows.Forms.DialogResult.Cancel Then 25           Return 26        End If 27 28        fileName = fileChooser.FileName ' get specified file name 29 30        ' show error if user specified invalid file 31        If fileName = "" Or fileName Is Nothing Then 32           MessageBox.Show("Invalid File Name", "Error", _ 33              MessageBoxButtons.OK, MessageBoxIcon.Error) 34        Else 35           ' save file via FileStream if user specified valid file 36           Try 37              ' open file with write access 38              output = New FileStream( _ 39                 fileName, FileMode.OpenOrCreate, FileAccess.Write) 40 41              ' disable Save button and enable Enter button 42              btnSave.Enabled = False 43              btnEnter.Enabled = True 44              ' handle exception if there is a problem opening the file 45           Catch ex As IOException 46              ' notify user if file does not exist 47              MessageBox.Show("Error opening file", "Error", _ 48                 MessageBoxButtons.OK, MessageBoxIcon.Error) 49           End Try 50        End If 51     End Sub ' btnSave_Click 52 53     ' handler for btnEnter_Click 54     Private Sub btnEnter_Click(ByVal sender As System.Object, _ 55        ByVal e As System.EventArgs) Handles btnEnter.Click 56        ' store TextBox values string array 57        Dim values As String() = GetTextBoxValues() 58 59        ' Record containing TextBox values to serialize 60        Dim record As New RecordSerializable() 61 62        ' determine whether TextBox account field is empty 63        If values(TextBoxIndices.ACCOUNT) <> "" Then 64           ' store TextBox values in Record and serialize Record 65           Try 66              ' get account number value from TextBox 67              Dim accountNumber As Integer = _ 68                 Int32.Parse(values(TextBoxIndices.ACCOUNT)) 69 70              ' determine whether accountNumber is valid 71              If accountNumber > 0 Then 72                 ' store TextBox fields in Record 73                 record.Account = accountNumber 74                 record.FirstName = _ 75                    values(TextBoxIndices.FIRST) 76                 record.LastName = _ 77                    values(TextBoxIndices.LAST) 78                 record.Balance = _ 79                    Decimal.Parse(values(TextBoxIndices.BALANCE)) 80 81                 ' write Record to FileStream ( serialize object ) 82                 formatter.Serialize(output, record) 83              Else 84                 ' notify user if invalid account number 85                 MessageBox.Show("Invalid Account Number", "Error" , _ 86                    MessageBoxButtons.OK, MessageBoxIcon.Error) 87              End If 88              ' notify user if error occurs in serialization 89           Catch ex As SerializationException 90              MessageBox.Show("Error Writing to File", "Error" , _ 91                 MessageBoxButtons.OK, MessageBoxIcon.Error) 92              ' notify user if error occurs regarding parameter format 93           Catch ex As FormatException 94              MessageBox.Show("Invalid Format", "Error", _ 95                 MessageBoxButtons.OK, MessageBoxIcon.Error) 96           End Try 97        End If 98 99        ClearTextBoxes() ' clear TextBox values 100    End Sub ' btnEnter_Click 101 102    ' handler for btnSave_Click 103    Private Sub btnExit_Click(ByVal sender As System.Object, _ 104       ByVal e As System.EventArgs) Handles btnExit.Click 105       ' determine whether file exists 106       If output IsNot Nothing Then 107          ' close file 108          Try 109             output.Close() 110             ' notify user of error closing file 111          Catch ex As IOException 112             MessageBox.Show("Cannot close file", "Error" , _ 113                MessageBoxButtons.OK, MessageBoxIcon.Error) 114          End Try 115       End If 116 117       Application.Exit() 118    End Sub ' btnEnter_Click 119 End Class ' FrmCreateFile 

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Common Programming Error 18.2

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

This program assumes that data is input correctly and in the proper record-number order. Event handler btnEnter_Click (lines 54100) performs the write operation. Line 60 creates a RecordSerializable object, which is assigned values in lines 7379. Line 82 calls method Serialize to write the RecordSerializable object to the output file. Method Serialize receives as its first argument the FileStream object into which the BinaryFormatter writes its second argument. Note that only one statement is required to write the entire object.

In the sample execution for the program in Fig. 18.14, we entered information for five accountsthe same information shown in Fig. 18.10. The program does not show how the data records actually appear in the file. Remember that we are now using binary files, which are not human readable. To verify that the file was created successfully, the next section presents a program to read the file's contents and deserialize the objects.