Displaying a currency value

Question

When a user clicks on "Case", it pops up a box where they have to input a number, which is later used to calculate the value to Euro and around to other values. After this, it checks which value is larger than 0 and writes it to the display box with a white line between each value.

I want to optimize this code. What's the best way to do it (meaning fewer double lines)? I am reading through the book but as far as I know, there's nothing on optimizing the code (it seems ridiculous to write those lines every time).

Private Sub munteenheidlistbox_SelectedIndexChanged(sender As Object, e As EventArgs) Handles munteenheidlistbox.SelectedIndexChanged
 ' On case selection pops up the Inputbox to set a number
 ' When a number is set it checks if its inside the paramets if not pops a msgbox and repops the Inputbox for new user input
 ' On case else pops up a msgbox saying the user should click on a valuta instead.
 Select Case munteenheidlistbox.SelectedIndex
 Case 0
 usdvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Amerikaanse dollars (1 Eur = .... Amerikaanse Dollars")
 Do Until usdvalue >= 0 And usdvalue <= 500
 MsgBox("De parameters vallen buiten de wisselkoers. Lees de text nog maals in het volgende scherm")
 usdvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Amerikaanse dollars (1 Eur = .... Amerikaanse Dollars")
 Loop
 Case 2
 gbpvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Britse Ponden (1 Eur = .... Britse Ponden")
 Do Until gbpvalue >= 0 And gbpvalue <= 500
 MsgBox("De parameters vallen buiten de wisselkoers. Lees de text nog maals in het volgende scherm")
 gbpvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Amerikaanse dollars (1 Eur = .... Amerikaanse Dollars")
 Loop
 Case 4
 eurovalue = MsgBox("Wissel koers van Euro naar Euro is altijd 1.")
 eurovalue = 1
 Case 6
 rusvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Russische Roebel (1 Eur = .... Russische Roebel")
 Do Until rusvalue >= 0 And rusvalue <= 500
 MsgBox("De parameters vallen buiten de wisselkoers. Lees de text nog maals in het volgende scherm")
 rusvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Amerikaanse dollars (1 Eur = .... Amerikaanse Dollars")
 Loop
 Case 8
 japyenvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Japanese Yen (1 Eur = .... Japanse Yen")
 Do Until japyenvalue >= 0 And japyenvalue <= 500
 MsgBox("De parameters vallen buiten de wisselkoers. Lees de text nog maals in het volgende scherm")
 japyenvalue = InputBox("Geef de wisselkoers (> 0 en < 500) voor de Euro tov Amerikaanse dollars (1 Eur = .... Amerikaanse Dollars")
 Loop
 Case Else
 MsgBox("Klik alstublieft op een valuta")
 End Select
 'Clear input and check which values are greater -> and rebuild textbox.
 Wisselkoersenlistbox.Clear()
 If usdvalue > 0 Then
 Wisselkoersenlistbox.Text = Wisselkoersenlistbox.Text & "1 euro = " & usdvalue & usdtext & vbCrLf
 Wisselkoersenlistbox.AppendText(Environment.NewLine & "")
 End If
 If gbpvalue > 0 Then
 Wisselkoersenlistbox.Text = Wisselkoersenlistbox.Text & "1 euro = " & gbpvalue & gbptext & vbCrLf
 Wisselkoersenlistbox.AppendText(Environment.NewLine & "")
 End If
 If eurovalue > 0 Then
 Wisselkoersenlistbox.Text = Wisselkoersenlistbox.Text & "1 euro = " & eurovalue & eurtext & vbCrLf
 Wisselkoersenlistbox.AppendText(Environment.NewLine & "")
 End If
 If rusvalue > 0 Then
 Wisselkoersenlistbox.Text = Wisselkoersenlistbox.Text & "1 euro = " & rusvalue & rustext & vbCrLf
 Wisselkoersenlistbox.AppendText(Environment.NewLine & "")
 End If
 If japyenvalue > 0 Then
 Wisselkoersenlistbox.Text = Wisselkoersenlistbox.Text & "1 euro = " & japyenvalue & japtext & vbCrLf
 Wisselkoersenlistbox.AppendText(Environment.NewLine & "")
 End If
End Sub

Answer 1

Here is a possible solution:

Private currencies As New Dictionary(Of String, CurrencyItem)
Private Class CurrencyItem
 Public CurrencyName As String
 Public CurrencyPrompt As String
 Public Sub New(currencyName As String, currencyPrompt As String)
 Me.CurrencyName = currencyName
 Me.CurrencyPrompt = currencyPrompt
 End Sub
End Class
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
 currencies.Add("USD", New CurrencyItem("USD", "Amerikaanse dollars"))
 currencies.Add("GBP", New CurrencyItem("GBP", "Britse Ponden"))
 currencies.Add("RBL", New CurrencyItem("RBL", "Russische Roebel"))
End Sub
Private Sub munteenheidlistbox_SelectedIndexChanged(sender As Object, e As EventArgs) Handles munteenheidlistbox.SelectedIndexChanged
 Try
 If isValidSelection(munteenheidlistbox.SelectedIndex) Then
 calculateCurrency(translateCurrency(munteenheidlistbox.SelectedIndex))
 End If
 Catch ex As Exception
 MessageBox.Show(String.Concat("An error occurred: ", ex.Message))
 End Try
End Sub
''' <summary>
''' check if the selection is value
''' </summary>
''' <param name="selectedIndex"></param>
''' <returns></returns>
Private Function isValidSelection(selectedIndex As Integer) As Boolean
 Select Case selectedIndex
 Case 0, 2, 4, 6, 8
 Return True
 Case Else
 MsgBox("Klik alstublieft op een valuta")
 Return False
 End Select
End Function
''' <summary>
''' get a currency item
''' </summary>
''' <param name="listIndex"></param>
''' <returns></returns>
Private Function translateCurrency(listIndex As Integer) As CurrencyItem
 Select Case listIndex
 Case 0
 Return currencies("USD")
 Case 2
 Return currencies("GBP")
 Case Else 'handle not found
 '''
 End Select
End Function
''' <summary>
''' prompt and display
''' </summary>
''' <param name="currency"></param>
Private Sub calculateCurrency(currency As CurrencyItem)
 Dim curValue As Integer = CInt(InputBox(String.Format("Geef de wisselkoers (> 0 en < 500) voor de Euro tov {0} (1 Eur = .... {1}", currency.CurrencyPrompt, currency.CurrencyPrompt)))
 Do Until curValue >= 0 And curValue <= 500
 MsgBox("De parameters vallen buiten de wisselkoers. Lees de text nog maals in het volgende scherm")
 curValue = CInt(InputBox(String.Format("Geef de wisselkoers (> 0 en < 500) voor de Euro tov {0} (1 Eur = .... {1}", currency.CurrencyPrompt, currency.CurrencyPrompt)))
 Loop
 Wisselkoersenlistbox.Clear()
 If curValue > 0 Then
 Wisselkoersenlistbox.Text = String.Concat(Wisselkoersenlistbox.Text, "1 euro = ", curValue, Environment.NewLine)
 Wisselkoersenlistbox.AppendText(Environment.NewLine & "")
 End If
End Sub

My insightful observation is that the code can be reduced to a number of simpler methods.

The purpose of the CurrencyItem class is to support future changes without having to hard code a number of if statements. Separating the selected index validation, and the "calculation" on the currency allows the program to have distinct purposes for distinct methods.