ASP.NET Visual Basic Lesson 26: Interfaces

	Interfaces

Interfaces Fundamentals

Introduction

An interface is a structural layout that resembles a class but has the following characteristics:

An interface presents a layout or design that classes and/or structures can follow to define their particular behaviors

Like a class or a structure, an interface has methods and/or properties as members
Unlike a class or a structure, an interface doesn't implement the methods and/or properties that are its members
A new class or a structure can be derived from an interface. Since an interface presents a layout or design that classes and/or structures can follow, and since an interface doesn't implement any of its methods or properties, any class or structure that derives from an interface must implement all the members of the interface. As a result, a class or structure that is based on an interface is said to "implement" the interface as opposed to derive from the interface

Creating an Interface

The basic formula to create an interface is:

[ access-modifier(s) ] [ Shadows ] Interface name
    members
End Interface

The access modifiers can be any of those we have seen in previous lessons (Public, Protected, Friend, Private, or Protected Friend). If you omit the access modifier, it is assumed to be Public. By tradition or good habits, the name of an interface starts with I. The section between the Interface name line the the End Interface line is the body of the interface. Here is an example of a starting interface:

Public Interface IPolygon

End Interface

You can create an interface in a Visual Basic file that has the .vb extension. If you are creating the interface in the file of a webpage, you should include it in a <script> section. This can be done as follows:

<script runat="server">
Interface IPolygon

End Interface
</script>

After creating an interface, you can implement it, which, as mentioned in our introduction, consists of creating a class or a structure from it. When implementing an interface, instead of the Inherits keyword, use Implements followed by the name of the interface. Here is an example of a new class named Triangle and that is based on the above IPolygon interface:

<script runat="server">
Interface IPolygon

End Interface

Public Class Triangle
    Implements IPolygon

End Class
</script>

As with other classes, once you have implemented the class, you can create objects from it and instantiate it using the New operator. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<script runat="server">
Public Interface IPolygon

End Interface

Public Class Triangle
    Implements IPolygon

End Class
</script>
<html>
<head runat="server">
<title>Geometry - Polygons</title>
</head>
<body>
<%
    Dim tri As New Triangle()
%>
</body>
</html>

The Members of an Interface

Introduction

As mentioned earlier, the purpose of an interface is to create a skeleton for classes. When creating an interface, in its body, create the necessary members. In the class or a structure that is based on the interface, you must implement the members. To do this, at the end of the first line that presents the member in the class, type the Implements keyword, followed by the name of the interface, followed by a period, and ending with the name of the member.

A member can start with the Shadows keyword if necessary.

Unlike classes and structures, the following keywords are not allowed on the members of an interface: Public, Private, Friend, Protected, Shared, Overrides, MustOverride, and Overridable.

Adding a Procedure to an Interface

An interface can contain one or more methods. A method that is a sub-procedure follows this formula:

Sub method-name()

When implementing the method in a class, use the following formula;

access-modifier Sub method-name() Implements interface-name.method-name

Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<script runat="server">
Public Interface IPolygon
    Sub Describe()
End Interface

Public Class Triangle
    Implements IPolygon

    Public Sub Describe() Implements IPolygon.Describe
        My.Response.Write("<p>A polygon is a geometric flat figure made of a fixed number of straight lines that close in a chain.</p>" &
                          "<p>A triangle is a polygon made of three non-intersecting lines.</p>")
    End Sub

End Class
</script>
<html>
<head runat="server">
<title>Geometry - Polygons</title>
</head>
<body>
<%
    Dim tri As New Triangle()

    tri.Describe()
%>
</body>
</html>

This would produce:

Adding a Procedure to an Interface

A Function as a Member of an Interface

To add a function to an interface, use this formula:

Function name() As data-type

When implementing the function in the body of the class, make sure you return the propriate type of value. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<script runat="server">
Public Interface IPolygon
    Sub Describe()
    Function Calculate(ByVal side As Double) As Double
End Interface

Public Class Triangle
    Implements IPolygon

    Public Sub Describe() Implements IPolygon.Describe
        My.Response.Write("<p>A polygon is a geometric flat figure made of a fixed number of straight lines that close in a chain.</p>" &
                          "<p>A triangle is a polygon made of three non-intersecting lines.</p>")
    End Sub

    Public Function Calculate(ByVal side As Double) As Double Implements IPolygon.Calculate
        Return side * 3.0
    End Function
End Class
</script>
<html>
<head runat="server">
<title>Geometry - Polygons</title>
</head>
<body>
<%
    Dim side = 48.17
    Dim tri As New Triangle()

    tri.Describe()
    Response.Write("<p>Perimeter: " & tri.Calculate(side) & "</p>")
%>
</body>
</html>

This would produce:

A Function as a Member of an Interface

A Property in an Interface

An interface can contain one or more properties. If a property is intended to be read-write, the formula to create it is:

Property property-name As data-type

When implementing the property in a class, end its first line with Implements, a period, and the name of the property. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<script runat="server">
Public Interface IPolygon
    Property Side As Double
End Interface

Public Class Triangle
    Implements IPolygon

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Edge = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
    End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property
End Class
</script>
<html>
<head runat="server">
<title>Geometry - Polygons: The Equilateral Triangle</title>
</head>
<body>
<h5>Geometry - Polygons: The Equilateral Triangle</h5>

<%
    Dim length As Double = 248.97
    Dim equilateral As Triangle

    equilateral = New Triangle(length)

    Response.Write("<p>Side: " & equilateral.Side & "</p>")
%>
</body>
</html>

This would produce:

A Property in an Interface

If the property is intended to be read-only, the formula to create it is:

ReadOnly Property name Get

Notice that you must start with the ReadOnly keyword and end with Get. When implementing the property, at the end of the first line, omit Get to use it in the body of the property. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<script runat="server">
Public Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Triangle
    Implements IPolygon

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Edge = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Area As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(3.0) / 4.0
        End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
    Dim equilateral As Triangle
    
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    equilateral = New Triangle(side)

    txtArea.Text = equilateral.Area
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  205px;
}
</style>

<html>
<head runat="server">
<title>Geometry - Polygons: The Equilateral Triangle</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Equilateral Triangle</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
    <td>&nbsp;</td>
    <td><asp:Button id="btnCalculate" runat="server"
                    Text="Calculate" Width="85px"
                    OnClick="BtnCalculateClick" />
    </td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Here is an example of running the program:

A Property in an Interface

If the property is intended to be write-only, the formula to create it is:

WriteOnly Property name Set

Other than that, you can create as many classes as you want and that implement an interface of your choice. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<script runat="server">
Public Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Hexagon
        Implements IPolygon

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
            Me.Edge = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
                Return Me.Edge
        End Get
        Set(ByVal value As Double)
                Me.Edge = value
        End Set
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(3.0) * 3.0 / 2.0
        End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.00
    Dim figure As Hexagon

    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    figure = New Hexagon(side)

    txtArea.Text = figure.Area
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  205px;
}
</style>

<html>
<head runat="server">
<title>Geometry - Polygons: The Hexagon</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Hexagon</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                      Text="Calculate" Width="85px"
                      OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Here is an example of running the program:

A Property in an Interface

You can also add new members to the classes, members that are not related to the interface. Here are examples:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Triangle
    Implements IPolygon
    
    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 3
        Me.Edge  = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Height As Double
        Get
            Return Me.Edge * Math.Sqrt(3.0) / 2.0
        End Get
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(3.0) / 4.0
        End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
    Dim equilateral As Triangle
    
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    equilateral = New Triangle(side)

    txtHeight.Text = equilateral.Height
    txtArea.Text = equilateral.Area
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometry - Polygons: The Equilateral Triangle</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Equilateral Triangle</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                                Text="Calculate" Width="85px"
                                OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Height:</td>
    <td><asp:TextBox id="txtHeight" runat="server" /></td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Here is an example of using the webpage:

A Property in an Interface

Primary Options on Using an Interface

Declaring a Variable of Interface Type

You cannot declare a variable of an interface and use it directly as you would a class. On the othe hand, you can declare a variable by using the name of the interface but not allocate memory for the variable. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface
</script>
<title>Geometry - Polygons</title>
</head>
<body>
<%
    Dim figure As IPolygon
%>
</body>
</html>

When allocating memory for the object using the New operator, you must use a class that implements that interface. After that, you can use the object. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Square
    Implements IPolygon
    
    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 4
        Me.Edge  = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
    End Get
        Set(ByVal value As Double)
            Me.Edge = value
    End Set
    End Property

    Public ReadOnly Property Diagonal As Double
        Get
            Return Me.Side * Math.Sqrt(2.00)
    End Get
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side
    End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
    Dim plate As IPolygon
    
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    plate = New Square(side)

    txtArea.Text = plate.Area
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometry - Polygons: The Square</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Square</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table style="width: 300px">
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                                Text="Calculate" Width="85px"
                                OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Here is an example of using the webpage:

Declaring a Variable of Interface Type

You can also declare the variable and allocate its memory on the same line. Here is an example:

<script runat="server">
. . .

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
        
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    Dim plate As IPolygon = New Square(side)

    txtArea.Text = plate.Area
End Sub
</script>

If you use any of these two techniques, you can access only the members of the interface. The non-interface members of the class would not be available. As a result, the following will produce an error:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Square
    Implements IPolygon
    
    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 4
        Me.Edge = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Diagonal As Double
        Get
            Return Me.Side * Math.Sqrt(2.00)
        End Get
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side
        End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
        
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    Dim plate As IPolygon = New Square(side)

    txtArea.Text = plate.Area
    ' Since the Diagonal is not a member of the interface, the variable 
    ' (declared from the interface) cannot access it
    txtDiagonal.Text = plate.Diagonal
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometry - Polygons: The Square</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Square</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                      Text="Calculate" Width="85px"
                      OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Diagonal:</td>
    <td><asp:TextBox id="txtDiagonal" runat="server" /></td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Passing an Interface As Argument

If you are defining a procedure, a function, or a method that receives an argument and the argument is an object of a class that either inherits from another class or implements an interface, in some cases, you can pass either its parent class or the interface it implements. In the body of the procedure, you can access the members of the interface. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Pentagon
    Implements IPolygon
    
    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 6
        Me.Edge  = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
Return Me.Side * Me.Side * Math.Sqrt(5.00 * (5.00 + (2.00 * Math.Sqrt(5.00))))/4.00
        End Get
    End Property
End Class

Sub Present(ByVal figure As IPolygon)
    txtArea.Text = figure.Area
End Sub

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
    Dim pol As IPolygon
    
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    pol = New Pentagon(side)
    Present(pol)
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometry - Polygons: The Pentagon</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Pentagon</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                                Text="Calculate" Width="85px"
                                OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Here is an example of using the webpage:

Passing an Interface As Argument

Keep in mind that you can access only the members of the interface.

Returning an Interface

A function or a method can return an object based on an interface. When creating the function or method, specify its return type as the desired interface. Here is an example:

<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Function Create() As IPolygon
 
End Function
</script>

Remember that you cannot simply instantiate an interface and directly use it as you would an object of a class. As a result, you cannot directly return an object based on an interface. Instead, you can declare a variable of a class that implements the interface and return that variable. In the code section where you need to use the object, you can get the value returned by the function and use it. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Class Pentagon
    Implements IPolygon
    
    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 6
        Me.Edge = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
    End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Diagonal As Double
        Get
            Return Me.Side * ((1.00 * Math.Sqrt(5.00)) / 2.00)
        End Get
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
Return Me.Side * Me.Side * Math.Sqrt(5.00 * (5.00 + (2.00 * Math.Sqrt(5.00)))) / 4.00
        End Get
    End Property
End Class

Sub Present(ByVal figure As IPolygon)
    txtArea.Text = figure.Area
End Sub

Function Create() As IPolygon
    Dim side As Double = 0.0
    Dim pol As IPolygon

    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    pol = New Pentagon(side)
    Return pol
End Function

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim shape As Pentagon
    
    shape = Create()
    Present(shape)
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometry - Polygons: The Pentagon</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Pentagon</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" />
        <asp:Button id="btnCalculate" runat="server"
                    Text="Calculate" Width="85px"
                    OnClick="BtnCalculateClick" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td>
      </td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

In reality, the function must return an object based on either the interface itself or of a class that implements the interface.

Options on Creating, Implementing, and Using Interfaces

Inheriting an Interface

An interface can be derived from another interface (but an interface cannot derive from a class). To create an interface based on another interface, use the Inherits keyword. Obviously the derived interface is supposed to add some behavior using methods and/or properties. Here is an example:

<script runat="server">
Public Interface IPolygon
    Property Side() As Double
    ReadOnly Property Area() As Double
End Interface

Public Interface IPolyhedron
    Inherits IPolygon

    ReadOnly Property Volume() As Double
End Interface
</script>

As you should know already that nothing is implemented in an interface, a member of the parent interface cannot be defined in a derived interface. Also, any class that needs the behavior(s) of the derived interface must implement all members of the derived interface and those of the parent interface(s). Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Public Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Public Interface IPolyhedron
    Inherits IPolygon

    ReadOnly Property Volume As Double
End Interface

Public Class Tetrahedron
    Implements IPolyhedron

    Protected Edge As Double
    Public ReadOnly Faces As Integer
    Public ReadOnly Edges As Integer

    Public Sub New(ByVal side As Double)
        Me.Faces = 4
        Me.Edges = 6
        Me.Edge = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    ReadOnly Property PyramidHeight As Double
        Get
            Return MyClass.Edge * Math.Sqrt(2.0 / 3.0)
        End Get
    End Property

    ReadOnly Property FaceArea As Double Implements IPolygon.Area
        Get
            Return MyClass.Edge * MyClass.Edge * Math.Sqrt(3.0) * 2.0
        End Get
    End Property

    ReadOnly Property TotalArea As Double
        Get
            Return Me.FaceArea * 4
        End Get
    End Property

    ReadOnly Property Volume As Double Implements IPolyhedron.Volume
        Get
            Return MyClass.Edge * MyClass.Edge * MyClass.Edge * Math.Sqrt(2.0) / 12.0
        End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
    Dim tetra As Tetrahedron

    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    tetra = New Tetrahedron(side)

    txtFaces.Text = tetra.Faces
    txtEdges.Text = tetra.Edges
    txtPyramidHeight.Text = tetra.PyramidHeight
    txtFaceArea.Text = tetra.FaceArea
    txtTotalArea.Text = tetra.TotalArea
    txtVolume.Text = tetra.Volume
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometric Volumes: The Tetrahedron</title>
</head>
<body>
<p id="main-title">Geometric Volumes: The Tetrahedron</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                                Text="Calculate" Width="85px"
                                OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Number of Faces:</td>
    <td><asp:TextBox id="txtFaces" Width="75px" runat="server" /></td>
  </tr>
  <tr>
    <td>Number of Edges:</td>
    <td><asp:TextBox id="txtEdges" Width="75px" runat="server" /></td>
  </tr>
  <tr>
    <td>Height of Pyramid:</td>
    <td><asp:TextBox id="txtPyramidHeight" runat="server" /></td>
  </tr>
  <tr>
    <td>Face Area:</td>
    <td><asp:TextBox id="txtFaceArea" runat="server" /></td>
  </tr>
  <tr>
    <td>Total Area:</td>
    <td><asp:TextBox id="txtTotalArea" runat="server" /></td>
  </tr>
  <tr>
    <td>Volume:</td>
    <td><asp:TextBox id="txtVolume" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

Here is an example of using the webpage:

Inheriting an Interface

In the same way, an interface can inherit from an interface that itself inherits from another interface, and from another, and so on.

Implementing Many Interfaces

You cannot create a class that inherits from many classes at the same time. Instead, you can create a class that implements more than one interface. To create a class based on more than one interface, after the Implements keyword, enter the name of each interface and separate them with commas. Here is an example:

<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Interface IPerimeter
    ReadOnly Property Perimeter As Double
End Interface

Public Class Triangle
    Implements IPolygon, IPerimeter
End Class
</script>

To make your code easy to read, you can write each interface on its own line. Here is an example:

<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Interface IPerimeter
    ReadOnly Property Perimeter As Double
End Interface

Public Class Triangle
    Implements IPolygon,
               IPerimeter
End Class
</script>

In the class, you must implement all members of the parent interfaces. If an inherited interface has (a) parent(s), you must also implement all members of the inherited ancestors (if any). Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side As Double
    ReadOnly Property Area As Double
End Interface

Interface IPerimeter
    ReadOnly Property Perimeter As Double
End Interface

Public Class Triangle
    Implements IPolygon,
               IPerimeter
    
    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 3
        Me.Edge  = side
    End Sub

    Public Property Side As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Height As Double
        Get
            Return Me.Edge * Math.Sqrt(3.0) / 2.0
        End Get
    End Property

    Public ReadOnly Property Perimeter As Double Implements IPerimeter.Perimeter
        Get
            Return Me.Edge * Me.Sides
        End Get
    End Property

    ReadOnly Property Area As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(3.0) / 4.0
        End Get
    End Property
End Class

Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side = 0.0
    Dim equilateral As Triangle
    
    If Not String.IsNullOrEmpty(txtSide.Text) Then
        side = CDbl(txtSide.Text)
    End If

    equilateral = New Triangle(side)

    txtHeight.Text = equilateral.Height
    txtPerimeter.Text = equilateral.Perimeter
    txtArea.Text = equilateral.Area
End Sub
</script>
<style>
#main-title
{
    font-size: 1.08em;
    font-weight: bold;
    text-align: center;
    font-family: Georgia, Garamond, 'Times New Roman', Times, serif;
}
#whole
{
    margin: auto;
    width:  305px;
}
</style>
<title>Geometry - Polygons: The Equilateral Triangle</title>
</head>
<body>
<p id="main-title">Geometry - Polygons: The Equilateral Triangle</p>
<form id="frmGeometry" runat="server">
<div id="whole">
 
<table>
  <tr>
    <td>Side:</td>
    <td><asp:TextBox id="txtSide" Width="75px" runat="server" /></td>
    </tr>
  <tr>
      <td>&nbsp;</td>
      <td><asp:Button id="btnCalculate" runat="server"
                                Text="Calculate" Width="85px"
                                OnClick="BtnCalculateClick" />
      </td>
  </tr>
  <tr>
    <td>Height:</td>
    <td><asp:TextBox id="txtHeight" runat="server" /></td>
  </tr>
  <tr>
    <td>Perimeter:</td>
    <td><asp:TextBox id="txtPerimeter" runat="server" /></td>
  </tr>
  <tr>
    <td>Area:</td>
    <td><asp:TextBox id="txtArea" runat="server" /></td>
  </tr>
</table>
</div>
</form>
</body>
</html>

In the above example, we created a class that implements only two interfaces. You can create a class that implements as many interfaces as you want. Also, the same interface can be implemented differently in different classes. Here are examples:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head runat="server">
<script runat="server">
Interface IPolygon
    Property Side() As Double
    ReadOnly Property Area() As Double
End Interface

Interface IPerimeter
    ReadOnly Property Perimeter() As Double
End Interface

Interface IPolyRadius
    ReadOnly Property InscribedRadius() As Double
    ReadOnly Property CircumscribedRadius() As Double
End Interface

Interface IPolyhedron
    Inherits IPolygon

    ReadOnly Property Volume() As Double
End Interface

Public Class Triangle
    Implements IPolygon,
               IPerimeter,
               IPolyRadius

    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 3
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Height() As Double
        Get
            Return Me.Edge * Math.Sqrt(3.0) / 2.0
        End Get
    End Property

    ' Also called Apothem
    ReadOnly Property InscribedRadius() As Double Implements IPolyRadius.InscribedRadius
        Get
            Return Me.Edge * Math.Sqrt(3.0) / 6.0
        End Get
    End Property

    ReadOnly Property CircumscribedRadius() As Double Implements IPolyRadius.CircumscribedRadius
        Get
            Return Me.Edge / Math.Sqrt(3.0)
        End Get
    End Property

    Public ReadOnly Property Perimeter() As Double Implements IPerimeter.Perimeter
        Get
            Return Me.Edge * Me.Sides
        End Get
    End Property

    ReadOnly Property Area() As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(3.0) / 4.0
        End Get
    End Property
End Class

Public Class Square
    Implements IPolygon,
               IPerimeter,
               IPolyRadius

    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 4
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Diagonal() As Double
        Get
            Return Me.Side * Math.Sqrt(2.0)
        End Get
    End Property

    ReadOnly Property InscribedRadius() As Double Implements IPolyRadius.InscribedRadius
        Get
            Return Me.Edge / 2.0
        End Get
    End Property

    ReadOnly Property CircumscribedRadius() As Double Implements IPolyRadius.CircumscribedRadius
        Get
            Return Me.Diagonal
        End Get
    End Property

    Public ReadOnly Property Perimeter() As Double Implements IPerimeter.Perimeter
        Get
            Return Me.Edge * Me.Sides
        End Get
    End Property

    ReadOnly Property Area() As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side
        End Get
    End Property
End Class

Public Class Pentagon
    Implements IPolygon,
               IPerimeter

    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 5
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Height() As Double
        Get
            Return Me.Edge * Math.Sqrt(5.0 + (2.0 * Math.Sqrt(5))) / 2.0
        End Get
    End Property

    Public ReadOnly Property Diagonal() As Double
        Get
            Return Me.Side * ((1 + Math.Sqrt(5.0)) / 2.0)
        End Get
    End Property

    Public ReadOnly Property Perimeter() As Double Implements IPerimeter.Perimeter
        Get
            Return Me.Edge * Me.Sides
        End Get
    End Property

    ReadOnly Property Area() As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(5.0 * (5 + (2 * Math.Sqrt(5.0)))) / 4.0
        End Get
    End Property
End Class

Public Class Hexagon
    Implements IPolygon,
               IPerimeter

    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 6
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Perimeter() As Double Implements IPerimeter.Perimeter
        Get
            Return Me.Edge * Me.Sides
        End Get
    End Property

    ReadOnly Property Area() As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * Math.Sqrt(3.0) * 3.0 / 2.0
        End Get
    End Property
End Class

Public Class Octagon
    Implements IPolygon,
               IPerimeter

    Public ReadOnly Sides

    Protected Edge As Double

    Public Sub New(ByVal side As Double)
        Me.Sides = 8
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    Public ReadOnly Property Perimeter() As Double Implements IPerimeter.Perimeter
        Get
            Return Me.Edge * Me.Sides
        End Get
    End Property

    ReadOnly Property Area() As Double Implements IPolygon.Area
        Get
            Return Me.Side * Me.Side * 2.0 * (1 + Math.Sqrt(2.0))
        End Get
    End Property
End Class

Public Class Tetrahedron
    Implements IPolyhedron,
               IPolyRadius

    Protected Edge As Double
    Public ReadOnly Faces As Integer
    Public ReadOnly Edges As Integer

    Public Sub New(ByVal side As Double)
        Me.Faces = 4
        Me.Edges = 6
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    ReadOnly Property PyramidHeight() As Double
        Get
            Return MyClass.Edge * Math.Sqrt(2.0 / 3.0)
        End Get
    End Property

    ReadOnly Property InscribedRadius() As Double Implements IPolyRadius.InscribedRadius
        Get
            Return MyClass.Edge / Math.Sqrt(24.0)
        End Get
    End Property

    ReadOnly Property CircumscribedRadius() As Double Implements IPolyRadius.CircumscribedRadius
        Get
            Return MyClass.Edge * Math.Sqrt(6.0) / 4.0
        End Get
    End Property

    ReadOnly Property FaceArea() As Double Implements IPolygon.Area
        Get
            Return MyClass.Edge * MyClass.Edge * Math.Sqrt(3.0) / 4.0
        End Get
    End Property

    ReadOnly Property SurfaceArea() As Double
        Get
            Return MyClass.Edge * MyClass.Edge * Math.Sqrt(3.0)
        End Get
    End Property

    ReadOnly Property Volume() As Double Implements IPolyhedron.Volume
        Get
            Return MyClass.Edge * MyClass.Edge * MyClass.Edge * Math.Sqrt(2.0) / 12.0
        End Get
    End Property
End Class

Public Class Octahedron
    Implements IPolyhedron,
               IPolyRadius

    Protected Edge As Double
    Public ReadOnly Faces As Integer
    Public ReadOnly Edges As Integer

    Public Sub New(ByVal side As Double)
        Me.Faces = 8
        Me.Edges = 12
        Me.Edge = side
    End Sub

    Public Property Side() As Double Implements IPolygon.Side
        Get
            Return Me.Edge
        End Get
        Set(ByVal value As Double)
            Me.Edge = value
        End Set
    End Property

    ReadOnly Property Midradius() As Double
        Get
            Return MyClass.Side / 2.0
        End Get
    End Property

    ReadOnly Property InscribedRadius() As Double Implements IPolyRadius.InscribedRadius
        Get
            Return MyClass.Side / Math.Sqrt(6.0) / 6.0
        End Get
    End Property

    ' CircumsphereRadius
    ReadOnly Property CircumscribedSphereRadius() As Double Implements IPolyRadius.CircumscribedRadius
        Get
            Return MyClass.Side * Math.Sqrt(2.0) / 2.0
        End Get
    End Property

    ReadOnly Property SurfaceArea() As Double Implements IPolygon.Area
        Get
            Return MyClass.Side * MyClass.Side * Math.Sqrt(3.0) * 2.0
        End Get
    End Property

    ReadOnly Property TotalArea() As Double
        Get
            Return MyClass.Side * MyClass.Side * MyClass.Side
        End Get
    End Property

    ReadOnly Property Volume() As Double Implements IPolyhedron.Volume
        Get
            Return Me.Side * Me.Side * Me.Side * Math.Sqrt(2.0) / 3.0
        End Get
    End Property
End Class
</script>
<title>Geometry - Polygonal Figures</title>
</head>
<body>

</body>
</html>

Built-In Interfaces

Introduction

The .NET Framework provides a large collection of interfaces that you can implement in your classes. Many of the classes available in the .NET Framework implement these interfaces. In some of your projects, you may have to implement an interface if its default behavior doesn't apply to your objects.

Cloning an Object

Copying an object consists of creating another sample of it and that contains the same values as the original. To make this operation available to your class, you can implement an interface named ICloneable. The ICloneable interface is defined in the System namespace of the mscorlib.dll library.

The ICloneable interface is equipped with one method named Clone. Its syntax is:

Function Clone As Object

To assist you with making a copy of a variable, the Object class is equipped with a method named MemberwiseClone. This means that all classes of the .NET Framework and any class you create in your project automatically inherits this method. The syntax of this method is:

Protected Function MemberwiseClone As Object

Therefore, when implementing the ICloneable interface, in your class, you can simply call the MemberwiseClone() method.

Comparing Two Objects

Comparing two objects consists of finding out which one comes first. The comparison is simple if you are dealing with values of primitive types. For example, it is easy to know that 2 is lower than 5, but it is not obvious to compare two objects created from a composite type, such as two students, two cars, or two food items.

To assist you with comparing two objects, the .NET Framework provides various comparable interfaces. One of these interfaces is named IComparable. The IComparable interface is a member of the System namespace. Obviously you must define what would be compared and how the comparison would be carried.

Most of the .NET Framework's classes that need to perform comparison already implement the IComparable interface or one of its equivalents.

Formatting a Value

The collection of techniques and formulas used by a language to display its values is referred to as a format provider. When you use a variable that uses a particular formula to display its value, to help you specify the right formula, the .NET Framework provides the IFormatProvider interface. IFormatProvider is defined in the System namespace.

There are two main ways you can use the IFormatProvider. You can create a class that implements it. The IFormatProvider is equipped with only one method: GetFormat.

In most cases, you will use classes that already implement the IFormatProvider interface. Those classes are equipped with an overridden version of the ToString() method that takes IFormatProvider as argument. Its syntax is:

Public Function ToString(provider As IFormatProvider) As String

This method requires that you build an IFormatProvider object and pass it as argument. An alternative is to pass a string. This is possible with another version of the ToString() method whose syntax is:

Public Function ToString(format As String) As String

This method takes a string as argument. The string can take a character as one of the following:

Character		Description
c	C	Currency values
d	D	Decimal numbers
e	E	Scientific numeric display such as 1.45e⁵
f	F	Fixed decimal numbers
d	D	General and most common type of numbers
n	N	Natural numbers
r	R	Roundtrip formatting
s	S	Hexadecimal formatting
p	P	Percentages


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