How to do arithmetic with textboxes

Question

I have two textboxes on a wpf window that contain numeric values; call them txt1 and txt2. I want a third textbox (txt3) to show the difference between those two values.

I created a Property in code-behind called Diff defined as:

Diff = Cdec(txt1.Text) - Cdec(txt2.Text)

and bound that value as follows:

txt3.Text={Binding Path=Diff

However, at run-time the Property Diff is not found. How do I make that value available to be bound?

Answer 1

Hi Roger, don't forget:

• set DataContext or Source
• Notify property changed
• check input for invalid decimal numbers

Try following simple demo.

XAML:

<Window x:Class="Window014"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="Window014" Height="450" Width="800" Loaded="Window_Loaded">
    <StackPanel>
    <TextBox x:Name="txt1" Margin="5" />
    <TextBox x:Name="txt2" Margin="5"/>
    <TextBox Text="{Binding Diff}" Margin="5"/>
  </StackPanel>
</Window>

CodeBehind:

Imports System.ComponentModel
Imports System.Runtime.CompilerServices

Public Class Window014
  Implements INotifyPropertyChanged

  Private Sub Window_Loaded(sender As Object, e As RoutedEventArgs)
    Me.DataContext = Me
  End Sub

  Private Sub txt_TextChanged(sender As Object, e As TextChangedEventArgs) Handles txt1.TextChanged, txt2.TextChanged
    Dim dec1 As Decimal = 0
    Decimal.TryParse(txt1.Text, dec1)
    Dim dec2 As Decimal = 0
    Decimal.TryParse(txt2.Text, dec2)
    Diff = dec1 - dec2
    OnPropChanged(NameOf(Diff))
  End Sub

  Public Property Diff As Decimal

  Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
  Private Sub OnPropChanged(<CallerMemberName> Optional propName As String = "")
    RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propName))
  End Sub

End Class

Answer 2

Welcome to our Microsoft Q&A platform!

I think you can use MultiBinding:

Imports System.ComponentModel  
Imports System.Globalization  
Imports System.Runtime.CompilerServices  
  
Class MainWindow  
    Implements INotifyPropertyChanged  
    Public Sub New()  
  
        ' This call is required by the designer.  
        InitializeComponent()  
  
        ' Add any initialization after the InitializeComponent() call.  
        Me.DataContext = Me  
  
  
    End Sub  
  
    Private _tb1 As String  
    Public Property tb1() As String  
        Get  
            Return _tb1  
        End Get  
        Set(value As String)  
            _tb1 = value  
            NotifyPropertyChanged("tb1")  
        End Set  
    End Property  
  
  
    Private _tb2 As String  
    Public Property tb2() As String  
        Get  
            Return _tb2  
        End Get  
        Set(value As String)  
            _tb2 = value  
            NotifyPropertyChanged("tb2")  
        End Set  
    End Property  
  
    Private _Diff As String  
  
    Public Property Diff() As String  
        Get  
            Return _Diff  
  
        End Get  
        Set(value As String)  
            _Diff = value  
            NotifyPropertyChanged("Diff")  
        End Set  
    End Property  
    Public Event PropertyChanged As PropertyChangedEventHandler _  
          Implements INotifyPropertyChanged.PropertyChanged  
  
    Private Sub NotifyPropertyChanged(ByVal info As String)  
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(info))  
    End Sub  
  
  
  
  
End Class  
Public Class MultiStringStateConverter  
    Implements IMultiValueConverter  
  
  
    Private Function IMultiValueConverter_Convert(value() As Object, targetType As Type, parameter As Object, culture As CultureInfo) As Object Implements IMultiValueConverter.Convert  
  
        Return (System.Convert.ToInt32(value(0)) - System.Convert.ToInt32(value(1))).ToString()  
  
    End Function  
  
    Private Function IMultiValueConverter_ConvertBack(value As Object, targetTypes() As Type, parameter As Object, culture As CultureInfo) As Object() Implements IMultiValueConverter.ConvertBack  
        Throw New NotImplementedException()  
    End Function  
End Class  
  

xaml:

  <Window.Resources>  
        <local:MultiStringStateConverter x:Key="MultiStringStateConverter"/>  
    </Window.Resources>  
    <StackPanel>  
        <TextBox Name="txt1" Text="{Binding tb1,UpdateSourceTrigger=PropertyChanged}"/>  
        <TextBox Name="txt2" Text="{Binding tb2,UpdateSourceTrigger=PropertyChanged}"/>  
        <TextBox Name="txt3">  
            <TextBox.Text>  
                <MultiBinding UpdateSourceTrigger="PropertyChanged" Converter="{StaticResource MultiStringStateConverter}">  
                    <Binding Path="tb1"/>  
                    <Binding Path="tb2"/>  
                </MultiBinding>  
            </TextBox.Text>  
        </TextBox>  
    </StackPanel>  

Answer 3

If you don't want set the DataContent to itself,you can use the following code:

Imports System.ComponentModel
Imports System.Globalization
Imports System.Runtime.CompilerServices

Class MainWindow
    Public Sub New()
        ' This call is required by the designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.
        Me.DataContext = New MyViewModel()
    End Sub
End Class
Public Class MyViewModel
    Implements INotifyPropertyChanged

    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
    Private Sub NotifyPropertyChanged(ByVal info As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(info))
    End Sub
    Private _tb1 As String = 0
    Public Property tb1() As String
        Get
            Return _tb1
        End Get
        Set(value As String)
            _tb1 = value
            NotifyPropertyChanged("tb1")
        End Set
    End Property


    Private _tb2 As String = 0
    Public Property tb2() As String
        Get
            Return _tb2
        End Get
        Set(value As String)
            _tb2 = value
            NotifyPropertyChanged("tb2")
        End Set
    End Property

    Private _Diff As String

    Public Property Diff() As String
        Get
            Return _Diff

        End Get
        Set(value As String)
            _Diff = value
            NotifyPropertyChanged("Diff")
        End Set
    End Property
End Class
Public Class MultiStringStateConverter
    Implements IMultiValueConverter


    Private Function IMultiValueConverter_Convert(value() As Object, targetType As Type, parameter As Object, culture As CultureInfo) As Object Implements IMultiValueConverter.Convert

        Return (System.Convert.ToInt32(value(0)) - System.Convert.ToInt32(value(1))).ToString()

    End Function

    Private Function IMultiValueConverter_ConvertBack(value As Object, targetTypes() As Type, parameter As Object, culture As CultureInfo) As Object() Implements IMultiValueConverter.ConvertBack
        Throw New NotImplementedException()
    End Function
End Class