Walkthrough: Persisting an Object in Visual Studio (Visual Basic)

Although you can set an object's properties to default values at design time, any values entered at run time are lost when the object is destroyed. You can use serialization to persist an object's data between instances, which enables you to store values and retrieve them the next time that the object is instantiated.

Note

In Visual Basic, to store simple data, such as a name or number, you can use the My.Settings object. For more information, see My.Settings Object.

In this walkthrough, you will create a simple Loan object and persist its data to a file. You will then retrieve the data from the file when you re-create the object.

Important

This example creates a new file, if the file does not already exist. If an application must create a file, that application must Create permission for the folder. Permissions are set by using access control lists. If the file already exists, the application needs only Write permission, a lesser permission. Where possible, it is more secure to create the file during deployment, and only grant Read permissions to a single file (instead of Create permissions for a folder). Also, it is more secure to write data to user folders than to the root folder or the Program Files folder.

Important

This example stores data in a binary. These formats should not be used for sensitive data, such as passwords or credit-card information.

Note

The dialog boxes and menu commands you see might differ from those described in Help depending on your active settings or edition. To change your settings, click Import and Export Settings on the Tools menu. For more information, see Personalize the Visual Studio IDE.

Creating the Loan Object

The first step is to create a Loan class and a test application that uses the class.

To create the Loan class

  1. Create a new Class Library project and name it "LoanClass". For more information, see Creating Solutions and Projects.

  2. In Solution Explorer, open the shortcut menu for the Class1 file and choose Rename. Rename the file to Loan and press ENTER. Renaming the file will also rename the class to Loan.

  3. Add the following public members to the class:

    Public Class Loan  
    Implements System.ComponentModel.INotifyPropertyChanged  

    Public Property LoanAmount As Double  
    Public Property InterestRate As Double  
    Public Property Term As Integer  

    Private p_Customer As String  
    Public Property Customer As String  
        Get  
            Return p_Customer  
        End Get  
        Set(ByVal value As String)  
            p_Customer = value  
            RaiseEvent PropertyChanged(Me,  
              New System.ComponentModel.PropertyChangedEventArgs("Customer"))  
        End Set  
    End Property  

    Event PropertyChanged As System.ComponentModel.PropertyChangedEventHandler _  
      Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged  

    Public Sub New(ByVal loanAmount As Double,  
                   ByVal interestRate As Double,  
                   ByVal term As Integer,  
                   ByVal customer As String)  

        Me.LoanAmount = loanAmount  
        Me.InterestRate = interestRate  
        Me.Term = term  
        p_Customer = customer  
    End Sub  
End Class

You will also have to create a simple application that uses the Loan class.

To create a test application

  1. To add a Windows Forms Application project to your solution, on the File menu, choose Add,New Project.

  2. In the Add New Project dialog box, choose Windows Forms Application, and enter LoanApp as the name of the project, and then click OK to close the dialog box.

  3. In Solution Explorer, choose the LoanApp project.

  4. On the Project menu, choose Set as StartUp Project.

  5. On the Project menu, choose Add Reference.

  6. In the Add Reference dialog box, choose the Projects tab and then choose the LoanClass project.

  7. Click OK to close the dialog box.

  8. In the designer, add four TextBox controls to the form.

  9. In the Code Editor, add the following code:

    Private WithEvents TestLoan As New LoanClass.Loan(10000.0, 0.075, 36, "Neil Black")  

Private Sub Form1_Load() Handles MyBase.Load  
    TextBox1.Text = TestLoan.LoanAmount.ToString  
    TextBox2.Text = TestLoan.InterestRate.ToString  
    TextBox3.Text = TestLoan.Term.ToString  
    TextBox4.Text = TestLoan.Customer  
End Sub

  10. Add an event handler for the PropertyChanged event to the form by using the following code:

    Public Sub CustomerPropertyChanged(  
      ByVal sender As Object,  
      ByVal e As System.ComponentModel.PropertyChangedEventArgs  
    ) Handles TestLoan.PropertyChanged  

    MsgBox(e.PropertyName & " has been changed.")  
End Sub

At this point, you can build and run the application. Note that the default values from the Loan class appear in the text boxes. Try to change the interest-rate value from 7.5 to 7.1, and then close the application and run it again—the value reverts to the default of 7.5.

In the real world, interest rates change periodically, but not necessarily every time that the application is run. Rather than making the user update the interest rate every time that the application runs, it is better to preserve the most recent interest rate between instances of the application. In the next step, you will do just that by adding serialization to the Loan class.

Using Serialization to Persist the Object

In order to persist the values for the Loan class, you must first mark the class with the Serializable attribute.

To mark a class as serializable

  • Change the class declaration for the Loan class as follows:

    <Serializable()>  
Public Class Loan

The Serializable attribute tells the compiler that everything in the class can be persisted to a file. Because the PropertyChanged event is handled by a Windows Form object, it cannot be serialized. The NonSerialized attribute can be used to mark class members that should not be persisted.

To prevent a member from being serialized

  • Change the declaration for the PropertyChanged event as follows:

    <NonSerialized()>  
Event PropertyChanged As System.ComponentModel.PropertyChangedEventHandler _  
  Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged

The next step is to add the serialization code to the LoanApp application. In order to serialize the class and write it to a file, you will use the System.IO and System.Xml.Serialization namespaces. To avoid typing the fully qualified names, you can add references to the necessary class libraries.

To add references to namespaces

  • Add the following statements to the top of the Form1 class:

    Imports System.IO  
Imports System.Runtime.Serialization.Formatters.Binary

    In this case, you are using a binary formatter to save the object in a binary format.

The next step is to add code to deserialize the object from the file when the object is created.

To deserialize an object

  1. Add a constant to the class for the serialized data's file name.

    Const FileName As String = "..\..\SavedLoan.bin"

  2. Modify the code in the Form1_Load event procedure as follows:

    Private WithEvents TestLoan As New LoanClass.Loan(10000.0, 0.075, 36, "Neil Black")  

Private Sub Form1_Load() Handles MyBase.Load  
    If File.Exists(FileName) Then  
        Dim TestFileStream As Stream = File.OpenRead(FileName)  
        Dim deserializer As New BinaryFormatter  
        TestLoan = CType(deserializer.Deserialize(TestFileStream), LoanClass.Loan)  
        TestFileStream.Close()  
    End If  

    AddHandler TestLoan.PropertyChanged, AddressOf Me.CustomerPropertyChanged  

    TextBox1.Text = TestLoan.LoanAmount.ToString  
    TextBox2.Text = TestLoan.InterestRate.ToString  
    TextBox3.Text = TestLoan.Term.ToString  
    TextBox4.Text = TestLoan.Customer  
End Sub

    Note that you first must check that the file exists. If it exists, create a Stream class to read the binary file and a BinaryFormatter class to translate the file. You also need to convert from the stream type to the Loan object type.

Next you must add code to save the data entered in the text boxes to the Loan class, and then you must serialize the class to a file.

To save the data and serialize the class

  • Add the following code to the Form1_FormClosing event procedure:

    Private Sub Form1_FormClosing() Handles MyBase.FormClosing  
    TestLoan.LoanAmount = CDbl(TextBox1.Text)  
    TestLoan.InterestRate = CDbl(TextBox2.Text)  
    TestLoan.Term = CInt(TextBox3.Text)  
    TestLoan.Customer = TextBox4.Text  

    Dim TestFileStream As Stream = File.Create(FileName)  
    Dim serializer As New BinaryFormatter  
    serializer.Serialize(TestFileStream, TestLoan)  
    TestFileStream.Close()  
End Sub

At this point, you can again build and run the application. Initially, the default values appear in the text boxes. Try to change the values and enter a name in the fourth text box. Close the application and then run it again. Note that the new values now appear in the text boxes.

See also