Delegates with Example

In this article I will to share with you, what is Delegates with a small example?

Well we can define Delegates as a type safe “Function Pointer” which holds references to static or instance methods. Delegates are used to call methods dynamically at runtime.

Example:-

Public Class MyAccount

    '   Delegates Declaration

    Public Delegate Function CalVariables(ByVal x As Double, ByVal y As Double) As Double

    '   Shared Function in class which can be call without creating object of this class.

    '   This Class use Delegates as Parameter.

    Public Shared Function Calculate(ByVal c As CalVariables, ByVal x As Double, ByVal y As Double, ByVal z As Double)

        Return c(x, y) + z

    End Function

End Class

Public Class Form1

    '''

    ''' On Button click, selected value of combo box is taken in Switch case.

    ''' If the user had selected Add then the delegates function will add x and y.

    ''' If the user had selected subtract then the delegate function will subtract x and y.

    ''' And so on...

    '''

    '''

    '''

    '''

    Private Sub BtnShowDetails_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnShowDetails.Click

        Select Case Convert.ToString(cmbCar.SelectedItem.ToString())

            Case "Add"

                Dim AddValue As New MyAccount.CalVariables(AddressOf AddValues)

                '   MyAccount.Calculate will call  Calculate in MyAccount class first.

                '   Then c(x, y) will call the AddValues function.

                MessageBox.Show(Convert.ToString(MyAccount.Calculate(AddValue, 5, 5, 5)))

            Case "Subtract"

                Dim subtractValue As New MyAccount.CalVariables(AddressOf subtractValues)

                '   MyAccount.Calculate will call  Calculate in MyAccount class first.

                '   Then c(x, y) will call the subtractValues function.

                MessageBox.Show(Convert.ToString(MyAccount.Calculate(subtractValue, 5, 5, 5)))

            Case "Multiply"

                Dim MultiplyValue As New MyAccount.CalVariables(AddressOf MultiplyValues)

                '   MyAccount.Calculate will call  Calculate in MyAccount class first.

                '   Then c(x, y) will call the MultiplyValues function.

                MessageBox.Show(Convert.ToString(MyAccount.Calculate(MultiplyValue, 5, 5, 5)))

            Case "Divide"

                Dim DivideValue As New MyAccount.CalVariables(AddressOf DivideValues)

                '   MyAccount.Calculate will call  Calculate in MyAccount class first.

                '   Then c(x, y) will call the DivideValues function.

                MessageBox.Show(Convert.ToString(MyAccount.Calculate(DivideValue, 5, 5, 5)))

        End Select

    End Sub

    ' Function To Add x and y

    Private Function AddValues(ByVal x As Double, ByVal y As Double) As Double

        Return x + y

    End Function

    ' Function To Subtract x and y

    Private Function subtractValues(ByVal x As Double, ByVal y As Double) As Double

        Return x - y

    End Function

    ' Function To Multiply x and y

    Private Function MultiplyValues(ByVal x As Double, ByVal y As Double) As Double

        Return x * y

    End Function

    ' Function To Divide x and y

    Private Function DivideValues(ByVal x As Double, ByVal y As Double) As Double

        Return x / y

    End Function

End Class

In MyAccount Class I have declared Delegate.

Public Delegate Function CalVariables(ByVal x As Double, ByVal y As Double) As Double

CalVariables is a shared function where CalVariables Delegates is used as Parameter.

In Form1 on Button click, selected value of combo box is taken in Switch case.

If the user had selected Add then,

Our delegates is addressed to AddValues Function.

Dim AddValue As New MyAccount.CalVariables(AddressOf AddValues)

MessageBox.Show(Convert.ToString(MyAccount.Calculate(AddValue, 5, 5, 5)))

Will be executed.

On calling MyAccount.Calculate(AddValue, 5, 5, 5)), Calculate method is called and

From there on c(x, y) + z; it calls to AddValues function.

This is a small basic example of .NET delegates as a powerful mechanism to perform dynamic method invocation.