Error 27 Option Strict On disallows operands of type Object for operator '='. Use the 'Is' operator to test for object identity.

Error 27 Option Strict On disallows operands of type Object for operator '='. Use the 'Is' operator to test for object identity.

• hakan65

   

  Hi

   

  Thanks

   

   

  
  

• danny y

  Glad I coud help. . .

  Yeah it looks like alot of code. . . thats because it is vb  . On the other hand alot of it is IDE generated, too.

  What this is doing is making your form as "dumb" as possible. The Class file is the 'model'. The form's load method sets up the 'view'. the events specify the 'control.'

  The important thing to note is that the goal was not to replace the select/case statement.

  The select case statement produced a "smell" indicating that perhaps the model was not really representative of the problem domain.

  By designing a more robust class hierarchy, the need for the select/case was removed. At the same time we gained a more flexible model. 

  The elimination of the select/case is just a side effect!

  
  
  

• Easy_Care

  Hi Blair

  Thanks for the reply, I agree select case statements are bad practice a hang up from vb 5/6 I guess. I will look into these "type" class patterns and see if that makes for better coding.

  do you know of any specific information in MSDN I can look up

   

  Duncan

  
  

• demercurio

  before you say 'but you make it so complicated' I say guess again. . .

  what happens when you decide to add a new currency type

  total code rewrite using the select case. . . just add a couple of lines of code using 'TypeCode' class pattern.

  Plus the code reads better. . . and is faster!

  
  
  

• Daniel Holt

  here is an extremely good investigation of the pattern:

   Right Click and Save this PDF

  
  
  

• Ken D.

  Hi Blair

  I am a little confused by this abstract classing, I can see how it works, just not in this case, perhaps I should have provided more information.

  Select Case Me.optCurrency.Value
  Case 2
  Me.optCurrency.Value = CurrencyType.Euros
  'load combo box
  Call LoadFundId(CurrencyType.Euros)
  Case 3
  Me.optCurrency.Value = CurrencyType.Sterling
  'load combo box
  Call LoadFundId(CurrencyType.Sterling)
  End Select

  the me.optCurrency was a group of radio buttons, each having a value, depending on which one has been selected, this calls loadFundId(currencyType.XXXX) and populates a dropdown. Me is the current form (I guess it would "this" in c#)

  So not sure whether this needs to be an abstract class, so to get over the issue being thrown by VS2K5 I've changed to an if statement rather than a select case.

  I will certianly look into abstract class in more detail, as I can see how they would benefit an application, thanks for your help

  Duncan

  
  

• waterhunter

  Hi Blair

  Well I didn't expect that, wow thanks for that, my first reaction is that is a lot of code just to replace a select case statement, but the more I look at it the more cleaner and readable it becomes, and makes for a better application in the long run, so I will look at implmenting this into the app (got probably 4 or 5 areas to clean up).

  I have also decided to step my learning of c#, been reading some interesting articles about where Microsoft is taking VB and C#, so might be better to start now.

  Thanks once again

  Duncan

  
  

• Ed Reid

  also. . . you could override the Derived CurrencyType.ToString such as:

  Public Class USDollarCurrency
    Inherits CurrencyType
    Public Overrides Function GetFundList() As IList(Of CurrencyFund)
      Dim result As New List(Of CurrencyFund)
      result.Add(New CurrencyFund(Guid.NewGuid, "Goldman Sachs Global Currency Fund-Dollar Plus", "14.56", "Goldman Sachs"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds US Dollar Cash Fund", "11.929", "Fidelity International"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds II US Dollar Currency Fund", "30.207", "Fidelity International"))
      Return result
    End Function

    Public Overrides ReadOnly Property Code() As CurrencyCode
      Get
        Return CurrencyCode.UsDollar
      End Get
    End Property

    Public Overrides Function ToString() As String
      Return "US Dollar ($)"
    End Function
  End Class

  and change the assignbutton function:

  Private Sub AssignButton(ByVal abutton As RadioButton, ByVal aCurrency As CurrencyType)
      abutton.Text = aCurrency.ToString()
      abutton.Tag = aCurrency
    End Sub

  and the viewer:
  

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
      If CurrentCurrencyFund Is Nothing Then Return
      With CurrentCurrencyFund
        MessageBox.Show(String.Format("{1}{0}{2}{0}{3}", _
              System.Environment.NewLine, .FundName, .FundManager, .Value), _currentCurrencyType.ToString())
      End With
    End Sub

  
  
  

• Andres Yepes

  what is optCurrency an integer or a currency type

  on another note. . .  I can't stand select case statements. . . typically bad style.

  I will assume that Me in your example is a class called CurrencyUser

  the right way is to make Currency an abstract class with an abstract method LoadFundId(ByVal aUser as CurrencyUser)

  inherit from Currency for all your different types.

  Each CurrencyUser has a Currency field.

  CurrencyUser user just calls Me.Currency.LoadFundID(Me)

  done!

  
  
  

• nhaas

  google "Replace Conditional with Polymorphism" or "Replace Type Code with State/Strategy"

  here's an example: Replace Conditional with Polymorphism

  Get Fowler's Refactoring: Improving the Design of Existing Code and the orginal Gang Of Four:

  Design Patterns: Elements of Reusable Object-Oriented Software

   

  
  
  

• Wes Williamson

  create a new windows app.

  add a class module call CurrencyType and add this code:

  Public MustInherit Class CurrencyType
    Public Enum CurrencyCode As Integer
      Euro
      Sterling
      UsDollar
    End Enum
    Public MustOverride Function GetFundList() As IList(Of CurrencyFund)
    Public MustOverride ReadOnly Property Code() As CurrencyCode
  End Class

  Public Class EuroCurrency
    Inherits CurrencyType
    Public Overrides Function GetFundList() As IList(Of CurrencyFund)
      Dim result As New List(Of CurrencyFund)
      result.Add(New CurrencyFund(Guid.NewGuid, "Goldman Sachs Global Currency Fund - Euro Plus ", "11.73", "Goldman Sachs"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds Euro Cash Fund  ", "11.064", "Fidelity International"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds II Euro Currency Fund  ", "19.428", "Fidelity International"))
      result.Add(New CurrencyFund(Guid.NewGuid, "STANLIB Euro Currency Fund of Funds", "73.68", "Standard Bank"))
      Return result
    End Function

    Public Overrides ReadOnly Property Code() As CurrencyCode
      Get
        Return CurrencyCode.Euro
      End Get
    End Property
  End Class

  Public Class SterlingCurrency
    Inherits CurrencyType
    Public Overrides Function GetFundList() As IList(Of CurrencyFund)
      Dim result As New List(Of CurrencyFund)
      result.Add(New CurrencyFund(Guid.NewGuid, "Goldman Sachs Sterling Liquid Reserves Fund", "4.16", "Goldman Sachs"))
      result.Add(New CurrencyFund(Guid.NewGuid, "STANLIB Offshore Sterling Fund", "13.797", "Standard Bank"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds II - Sterling Currency Fund", "19.0290", "Fidelity International"))
      Return result
    End Function

    Public Overrides ReadOnly Property Code() As CurrencyCode
      Get
        Return CurrencyCode.Sterling
      End Get
    End Property
  End Class

  Public Class USDollarCurrency
    Inherits CurrencyType
    Public Overrides Function GetFundList() As IList(Of CurrencyFund)
      Dim result As New List(Of CurrencyFund)
      result.Add(New CurrencyFund(Guid.NewGuid, "Goldman Sachs Global Currency Fund-Dollar Plus", "14.56", "Goldman Sachs"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds US Dollar Cash Fund", "11.929", "Fidelity International"))
      result.Add(New CurrencyFund(Guid.NewGuid, "Fidelity Funds II US Dollar Currency Fund", "30.207", "Fidelity International"))
      Return result
    End Function

    Public Overrides ReadOnly Property Code() As CurrencyCode
      Get
        Return CurrencyCode.UsDollar
      End Get
    End Property
  End Class

  
  Public Class CurrencyFund
    Private _id As Guid
    Private _fundName As String
    Private _value As Double
    Private _fundManager As String

    Public ReadOnly Property ID() As System.Guid
      Get
        Return _id
      End Get
    End Property

    Public ReadOnly Property FundManager() As String
      Get
        Return _fundManager
      End Get
    End Property

    Public ReadOnly Property FundName() As String
      Get
        Return _fundName
      End Get
    End Property

    Public ReadOnly Property Item() As CurrencyFund
      Get
        Return Me
      End Get
    End Property

    Public Property Value() As Double
      Get
        Return _value
      End Get
      Set(ByVal value As Double)
        _value = value
      End Set
    End Property

    Public Sub New(ByVal aId As Guid, ByVal aName As String, ByVal initialValue As Double, ByVal fundManager As String)
      _id = aId
      _fundName = aName
      _value = initialValue
      _fundManager = fundManager
    End Sub
  End Class

  in your main form. . . drop three radio buttons a combobox and a button. . .

  put this code in it:

  Public Class Form1

    Private _currentCurrencyType As CurrencyType

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
      Button1.Text = "View"
      ComboBox1.DropDownStyle = ComboBoxStyle.DropDownList
      ComboBox1.DisplayMember = "FundName"
      ComboBox1.ValueMember = "Item"
      AssignButton(RadioButton1, New EuroCurrency)
      AssignButton(RadioButton2, New SterlingCurrency)
      AssignButton(RadioButton3, New USDollarCurrency)
      RadioButton1.Checked = True
    End Sub

    Private Sub AssignButton(ByVal abutton As RadioButton, ByVal aCurrency As CurrencyType)
      abutton.Text = aCurrency.Code.ToString()
      abutton.Tag = aCurrency
    End Sub

    Private Sub RadioButton1_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
        Handles RadioButton1.CheckedChanged, RadioButton2.CheckedChanged, RadioButton3.CheckedChanged
      If Not (TypeOf sender Is RadioButton) Then Return
      Dim button As RadioButton = CType(sender, RadioButton)
      If Not button.Checked Then
        _currentCurrencyType = Nothing
        Return
      End If
      _currentCurrencyType = CType(button.Tag, CurrencyType)
      ComboBox1.DataSource = _currentCurrencyType.GetFundList()
    End Sub

    Private ReadOnly Property CurrentCurrencyFund() As CurrencyFund
      Get
        Return CType(ComboBox1.SelectedItem, CurrencyFund)
      End Get
    End Property

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
      If CurrentCurrencyFund Is Nothing Then Return
      With CurrentCurrencyFund
        MessageBox.Show(String.Format("{1}{0}{2}{0}{3}", _
              System.Environment.NewLine, .FundName, .FundManager, .Value), _currentCurrencyType.Code.ToString)
      End With
    End Sub
  End Class

  compile and run. . .

  code can be found here: http://www.obj-tec.com/MSDNForums/TestOptsForDuncan/testopts.zip
  

  
  
  

• 10wattmindtrip

  Hi

  point taken about the language, yes vb should have been changed long before vb.net, too much of a hand holding thing I feel. and to a lesser extent they are still but thats another topic.

  anyway thanks for the advice I will look at the information you have provided and hopefully that will put me straight.

  Once again thanks

  Duncan

  
  

• Archana Kamath

  now you ready. . .lets say you want to add the foo currency type. . . In CurrencyTypes.vb, change this:

  Public MustInherit Class CurrencyType
    Public Enum CurrencyCode As Integer
      Euro
      Sterling
      UsDollar
    End Enum
    Public MustOverride Function GetFundList() As IList(Of CurrencyFund)
    Public MustOverride ReadOnly Property Code() As CurrencyCode
  End Class

  to

  Public MustInherit Class CurrencyType
    Public Enum CurrencyCode As Integer
      Euro
      Sterling
      UsDollar
      Foo
    End Enum
    Public MustOverride Function GetFundList() As IList(Of CurrencyFund)
    Public MustOverride ReadOnly Property Code() As CurrencyCode
  End Class

  and add this class:

  Public Class FooCurrency
    Inherits CurrencyType
    Public Overrides Function GetFundList() As IList(Of CurrencyFund)
      Dim result As New List(Of CurrencyFund)
      result.Add(New CurrencyFund(Guid.NewGuid, "Foobar Foo Currency Fund", "100", "FooBar Holdings"))
      Return result
    End Function

    Public Overrides ReadOnly Property Code() As CurrencyCode
      Get
        Return CurrencyCode.Foo
      End Get
    End Property
  End Class

  and simply add a new radiobutton and an assignment in the FormLoad:

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
      Button1.Text = "View"
      ComboBox1.DropDownStyle = ComboBoxStyle.DropDownList
      AssignButton(RadioButton1, New EuroCurrency)
      AssignButton(RadioButton2, New SterlingCurrency)
      AssignButton(RadioButton3, New USDollarCurrency)
      AssignButton(RadioButton4, New FooCurrency)
      RadioButton1.Checked = True
    End Sub

  
  
  

• AndyMc

  Hi

  I guess you're using c#, I am learning c# but have a better knowledge of VB so logical choice for me. I didn't think the type of language used mattered such much these days as everything is compiled into MSIL.

  
  
• Error 27 Option Strict On disallows operands of type Object for operator '='. Use the 'Is' operator to test for object identity.