1. VB.NET Classes ISYS 546

2. Classes A class is program structure that defines a user-defined data type that are used to create objects. An instance of a class is called an object. Abstraction is the process to create a model of an object, for the purpose of determining the characteristics (properties) and behaviors (methods) of the object.

3. Advantages of Object-Oriented Programming Reusable objects –A class can be used in multiple projects. –New classes can be created by inheritance. Building-block concept

4. Adding a Class to a Project Project/Add Window Form/Class –*** MyClass is a VB keyword. Steps: –Adding properties Declare Public variables in the General Declaration section Property procedures: Set / Get –Adding methods Private variables and procedures can be created for internal use.

5. Anatomy of a Class Module Class Module Public Variables & Property Procedures Public Procedures & Functions Exposed Part Private Variables Private Procedures & Functions Hidden Part

6. Class Example Public Eid As String Public Ename As String Public salary As Double Public Function tax() As Double tax = salary * 0.1 End Function

7. Creating Property with Property Procedures Implementing a property with a public variable the property value cannot be validated by the class. We can create read-only, write-only, or write-once properties with property procedure. Steps: –Declaring a private class variable to hold the property value. –Writing a property procedure to provide the interface to the property value.

8. Property Procedure Code Example Public Class Emp2 Public SSN As String Public Ename As String Public DateHired As Date Private hiddenJobCode As Long Public Property JobCode() Set(ByVal Value) If Value 4 Then hiddenJobCode = 1 Else hiddenJobCode = Value End If End Set Get JobCode = hiddenJobCode End Get End Property End Class

9. How the Property Procedure Works? When the program sets the property, the property procedure is called and the code between the Set and End Set statements is executed. The value assigned to the property is passed in the Value argument and is assigned to the hidden private variable. When the program reads the property, the property procedure is called and the code between the Get and End Get statements is executed.

10. Implementing a Read-Only Property Declare the property procedure as ReadOnly with only the Get block. Ex. Create a YearsEmployed property from the DateHired property: Public ReadOnly Property YearsEmployed() As Long Get YearsEmployed = Now.Year - DateHired.Year End Get End Property –Note: It is similar to a calculated field in database.

11. Implementing a Write-Only Property Declare the property procedure as WriteOnly with only the Set block. Ex. Create a Password property: Private hiddenPassword as String Public WriteOnly Property Password() As String Set(ByVal Value As String) hiddenPassword=Value End Set End Property How to create a write-once property?

12. Modeling 1:M Relation with Classes Object-Oriented systems allow multi-value attribute. Employee –EID –Ename –Dependents Department –DID –Dname –Employees

13. ArrayList ArrayList is a data structure used to store a set of values. –Its capacity is automatically expanded as needed. –Values stored in an arraylist do not have to be the same data type. –Flexibility when inserting/deleting elements.

14. ArrayList Properties & Methods Define an arraylist: –Dim myArrayList As New ArrayList() Properties:Count, Item, etc. –myArrayList.Item(0) 0-based index Methods: –Clear, Add, Insert, Remove, RemoveAt, Contains, IndexOf, etc.

15. ArrayList Demo Dim testArrayList As New ArrayList() Dim Fruits() As String = {"Apple", "orange", "Banana"} Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim f2 As New Form2() testArrayList.Add("David") testArrayList.Add(20) testArrayList.Add(Fruits) testArrayList.Add(f2) TextBox1.Text = testArrayList.Item(0) TextBox2.Text = testArrayList.Item(1).ToString TextBox3.Text = testArrayList.Item(2)(1) TextBox4.Text = testArrayList.Item(3).Age End Sub

16. For Each Loop with ArrayList Dim testArrayList As New ArrayList() Dim f2 As New DataForm2() Dim Fruits() As String = {"Apple", "orange", "Banana"} testArrayList.Add("David") testArrayList.Add(20) testArrayList.Add(Fruits) testArrayList.Add(f2) Dim myObj As Object For Each myObj In testArrayList MessageBox.Show(myObj.GetType.ToString) Next

17. Implementing a 1:M Relationship With ArrayList Public did As String Public dname As String Public emps As New arrayList () Public eid As String Public ename As String Public salary As Double Class Department Class Employee

18. To Add a New Emp Dim department As New Dept() Private Sub DeptForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load department.did = "D1" department.dname = "MIS" TextBox1.Text = department.did TextBox2.Text = department.dname End Sub Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Dim newemp As New Emp() With newemp.Eid = TextBox3.Text.Ename = TextBox4.Text.salary = Val(TextBox5.Text) End With TextBox3.Text = vbNullString TextBox4.Text = vbNullString TextBox5.Text = vbNullString department.emps.Add(newemp) End Sub

19. To Find Total Salary and Number of Employees Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click Dim temp As Emp Dim totalsal As Double totalsal = 0 For Each temp In department.emps totalsal = totalsal + temp.salary Next temp TextBox7.Text = totalsal.ToString End Sub Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click TextBox6.Text = department.emps.Count.ToString End Sub

20. Database Access with Classes

21. Single-Record-Handling Classes –Retrieves a single record from the database and makes it available to your application in the form of an object. –The fields in the record are exposed as the object’s properties. –Any actions performed by the data (updates, calculations, etc.) are exposed as the object’s methods.

22. Single-Record-Handling Class Example Imports System.Data.OleDb Public Class Customer Public cid As String Public CName As String Public City As String Public Rating As String Private hiddenexist As Boolean Private cn As OleDb.OleDbConnection Public ReadOnly Property RecExist() Get RecExist = hiddenexist End Get End Property

23. Public Sub getData(ByVal SearchID As String) Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\sales2k.mdb" Dim objConn As New OleDbConnection(strConn) Dim strSQL As String = "select * from customer where cid = '" & SearchID & "'" Dim objComm As New OleDbCommand(strSQL, objConn) objConn.Open() Dim objDataReader As OleDbDataReader objDataReader = objComm.ExecuteReader() If objDataReader.Read() = False Then hiddenexist = False MsgBox("Record does not exist") ‘Better let user’s program to display msg Else hiddenexist = True cid = objDataReader("cid") CName = objDataReader("CName") City = objDataReader("City") Rating = objDataReader("Rating") End If objConn.Close() End Sub

24. Public Sub SaveNew() Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\sales2k.mdb" Dim objConn As New OleDbConnection(strConn) objConn.Open() Dim strSQLInsert As String strSQLInsert = "Insert into Customer values ('" strSQLInsert = strSQLInsert & cid & "','" & CName & "','" strSQLInsert = strSQLInsert & City & "','" & Rating & "')" Dim objCommInsert As New OleDbCommand(strSQLInsert, objConn) objCommInsert.ExecuteNonQuery() objConn.Close() End Sub

25. Using the SaveNew Method to Add A New Customer Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim newCust As New Customer() newCust.cid = TextBox1.Text newCust.CName = TextBox2.Text newCust.City = TextBox3.Text newCust.Rating = TextBox4.Text newCust.SaveNew() TextBox1.Text = "" TextBox2.Text = "" TextBox3.Text = "" TextBox4.Text = "" End Sub

26. Classes That Create Oledb Objects Example: Create a class that uses methods to return DataReader.

27. Imports System.Data.OleDb Public Class ReaderServer Public Function CustReader() As OleDbDataReader Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\sales2k.mdb" Dim objConn As New OleDbConnection(strConn) Dim strSQL As String = "select * from customer;" Dim objComm As New OleDbCommand(strSQL, objConn) objConn.Open() CustReader = objComm.ExecuteReader() End Function Public Function OrderReader() As OleDbDataReader Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\sales2k.mdb" Dim objConn As New OleDbConnection(strConn) Dim strSQL As String = "select * from orders;" Dim objComm As New OleDbCommand(strSQL, objConn) objConn.Open() OrderReader = objComm.ExecuteReader() End Function End Class

28. Using the Class Dim cReader As New ReaderServer() Private Sub UseReader_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim reader As OleDb.OleDbDataReader reader = cReader.CustReader() Do While reader.Read() = True ListBox1.Items.Add(reader("cid")) Loop End Sub Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged Dim reader As OleDb.OleDbDataReader reader = cReader.OrderReader Dim strResult As String Do While reader.Read() = True If reader("cid") = ListBox1.SelectedItem Then strResult = strResult + reader("oid") + " " + reader("odate") + vbCrLf End If Loop TextBox1.Text = strResult End Sub

29. Importing Data From Database to Class Example: Customer/Order class: Public Class CustOrders –Public cid As String –Public CName As String –Public City As String –Public Rating As String –Public orders As New arrayList() –Public Sub getData(ByVal SearchID As String) Public Class Order – Public OID As String – Public Odate As Date – Public SalesPerson As String –End Class Demo: VBClass/CustOrdForm.vb

30. Public Sub getOrders(ByVal SearchID As String) Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\sales2k.mdb" Dim objConn As New OleDbConnection(strConn) Dim strSQL As String = "select * from orders where cid = '" & SearchID & "'" Dim objComm As New OleDbCommand(strSQL, objConn) objConn.Open() Dim objDataReader As OleDbDataReader objDataReader = objComm.ExecuteReader() Do While objDataReader.Read() = True Dim ord As New Order() ord.OID = objDataReader("oid") ord.Odate = objDataReader("odate") ord.SalesPerson = objDataReader("salesPerson") orders.Add(ord) Loop If orders.Count = 0 Then MessageBox.Show("customer has no order") End If objConn.Close() End Sub

31. Use the CustReader class to create a CID listbox. Use the GetData method to initialize the CustOrd object. Use the GetOrder method to retrieve customer’s orders and add to the Orders collection of the CustOrd object.

32. Creating Listbox Dim custOrd As New CustOrders() Dim cReader As New ReaderServer() Private Sub CustOrdForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim reader As OleDb.OleDbDataReader reader = cReader.CustReader() Do While reader.Read() = True ListBox1.Items.Add(reader("cid")) Loop End Sub

33. Initializing CustOrd Object custOrd.getData(ListBox1.SelectedItem) TextBox1.Text = custOrd.City TextBox2.Text = custOrd.CName custOrd.getOrders(ListBox1.SelectedItem) Dim strResult As String Dim ordItem As Order For Each ordItem In custOrd.orders strResult = strResult + ordItem.OID + " " + ordItem.Odate + " " + ordItem.SalesPerson + vbCrLf Next TextBox3.Text = strResult TextBox4.Text = custOrd.orders.Count.ToString

34. Assemblies An assembly is a logical grouping of functionality into a physical file. One or many business logic components can be reside in an assembly. This collection of components is compiled into a.DLL file. We can import this.DLL component to any VB projects.

35. Steps to Create An Assembly Create a class library with classes. –You can also use existing classes by Project/Add Existing Item Select Build/Build Solution to compile the code. –When the class library is compiled successfully, an assembly is created and stored in the project’s Bin folder. Note: A ClassLibrary project does not reference System.Windows.Forms namespace.

36. Using the Assembly Reference the assembly: Project/Add Reference and use the Browse button to select the assembly. Import the assembly.

37. Code Using Assembly Imports MyAssembly Public Class UseAssembly Inherits System.Windows.Forms.Form Dim testAssembly As New MyAssembly.Customer() Private Sub UseAssembly_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load testAssembly.cid = "1" testAssembly.getData(testAssembly.cid) TextBox1.Text = testAssembly.CName TextBox2.Text = testAssembly.Rating End Sub End Class