Sample Calculator MVVM in VB.NET

ViewModelBase

Public Property Display As String
Get
Return _Display
End Get
Set(ByVal value As String)
_Display = value
OnPropertyChanged("Display")
End Set
End Property
Public Property LastOperator As String
Get
Return _LastOperator
End Get
Set(ByVal value As String)
_LastOperator = value
OnPropertyChanged("LastOperator")
End Set
End Property
Public Property FirstNumber As Double
Get
Return _FirstNumber
End Get
Set(ByVal value As Double)
_FirstNumber = value
OnPropertyChanged("FirstNumber")
End Set
End Property

Public ReadOnly Property NumberPress As ICommand
Get
If _NumberPress Is Nothing Then
_NumberPress = New MVVM.RelayCommand(AddressOf ExecuteNumberPress)
End If
Return _NumberPress
End Get
End Property
Public ReadOnly Property OperatorPress As ICommand
Get
If _OperatorPress Is Nothing Then
_OperatorPress = New MVVM.RelayCommand(AddressOf ExecuteOperatorPress)
End If
Return _OperatorPress
End Get
End Property

Sub ExecuteNumberPress(ByVal Param As Object)
Select Case Param
Case Is = ","
If Display.Contains(CStr(Param)) = False Then
LastEspecial = ","
Display = Display & LastEspecial
End If
Case Is = "00"
LastEspecial = "00"
Display = Display & LastEspecial
Case Is = "CE"
If String.IsNullOrEmpty(LastOperator) Then
FirstNumber = 0
Display = 0
Else
LastNumber = 0
End If
Case Is = "C"
LastNumber = 0
FirstNumber = 0
Display = 0
LastOperator = String.Empty
Case Is = "←"
If Display.Length <= 1 OrElse CDbl(Display) = 0 Then
Display = "0,00"
Else
Display = Left(Display, Display.Length - 1)
End If
Case Is = "+/-"
If CDbl(Display) <> 0 Then
Display = CDbl(Display) * -1
End If
Case Else
If String.IsNullOrEmpty(LastOperator) Then
If CDbl(Display) = 0 OrElse (FirstNumber = 0 And LastNumber = 0) Then
Display = CStr(Param)
Else
Display = Display & CStr(Param)
End If
_FirstNumber = CDbl(Display)
Else
If LastNumber <> 0 Then
Display = Display & CStr(Param)
Else
Display = CStr(Param)
End If
LastNumber = CDbl(Display)
End If
End Select

End Sub
Private Sub ExecuteOperatorPress(ByVal Param As Object)
If String.IsNullOrEmpty(LastOperator) Then
FirstNumber = CDbl(Display)
If CStr(Param) <> "=" Then
LastOperator = CType(Param, String)
End If

If LastOperator = "√" Then
Calculate()
End If
Else
LastNumber = CDbl(Display)
If CStr(Param) <> "=" Then
LastOperator = CType(Param, String)
End If
Calculate()
End If

End Sub
Private Sub Calculate()
Select Case LastOperator
Case Is = "%"
Result = FirstNumber * LastNumber / 100

Case Is = "X"
Result = FirstNumber * LastNumber

Case Is = "/"
Result = FirstNumber / LastNumber
Case Is = "+"
Result = FirstNumber + LastNumber

Case Is = "-"
Result = FirstNumber - LastNumber

Case Is = "√"
Result = CDbl(Display)
Result = Math.Sqrt(Result)
End Select
Display = Result
LastOperator = String.Empty
FirstNumber = 0
LastNumber = 0

End Sub
End Class

And it links commands to the buttons on the view like this

<Button Content="-" Height="30" Width="30" Command="{Binding Path=OperatorPress}" CommandParameter="{Binding Path=Content, RelativeSource={RelativeSource Self}}"/>

And

<Button Width="45" Height="45" Content="CE" Command="{Binding Path=NumberPress}" CommandParameter="{Binding Path=Content, RelativeSource={RelativeSource Self}}"/>

After, we binds the keypress (this is new in wpf 4.0)

<KeyBinding Key="NumPad0" Command="{Binding Path=NumberPress}" CommandParameter="0"/>
<KeyBinding Key="Decimal" Command="{Binding Path=NumberPress}" CommandParameter=","/>

<KeyBinding Key="Add" Command="{Binding Path=OperatorPress}" CommandParameter="+"/>

Can download source code from here.