Notify and update a source through Multibinding with using StringFormat in WPF

Question

Please analyze following code snippet:

<TextBox x:Name="TxtBox_CommandInfo"  Style="{DynamicResource MetroTextBox}"
    IsReadOnly="True" Controls:TextBoxHelper.Watermark="This is a textbox" 
    HorizontalAlignment="Left" Margin="0,236,0,0" TextWrapping="Wrap"
    VerticalAlignment="Top" Height="154" Width="780" 
    ScrollViewer.HorizontalScrollBarVisibility="Auto" 
    ScrollViewer.VerticalScrollBarVisibility="Auto">
  <TextBox.Text>
    <MultiBinding StringFormat="{}{0} {1} {2}" NotifyOnTargetUpdated="True">
      <Binding  Path="A" />
      <Binding  Path="B" />
      <Binding  Path="C"/>
      <Binding Path="D" Mode="TwoWay" NotifyOnTargetUpdated="True"
          UpdateSourceTrigger="PropertyChanged"/>
    </MultiBinding>
  </TextBox.Text>
</TextBox>

Simply as you can seeTxtBox_CommandInfo textbox text(Target) value will be modified/formatted with only by A,B,C , but the updated text should be notified to D via binding.Model as below:

A,B,C => [TexBox.Text] => D

Problem is respective source D in C# side won't get notified and updated when TxtBox_CommandInfo.Text is getting changed.

1. Without any Converters how to achieve a solution to this issue?
2. Did I mis-configured the Multibinding setting ?

It really requires StringFormat feature,therefore without injecting conversion do we have any alternatives with StringFormat?

Answer 1

I am still confused. Don't know exactly what you mean:

What I tried to explain in the comments, here is a solution for that.

XAML:

 <Grid>
    <StackPanel>
        <ComboBox ItemsSource="{Binding Path=MyTexts}" SelectedItem="{Binding Path=A}" Margin="10"/>
        <ComboBox ItemsSource="{Binding Path=MyTexts}" SelectedItem="{Binding Path=B}" Margin="10"/>
        <ComboBox ItemsSource="{Binding Path=MyTexts}" SelectedItem="{Binding Path=C}" Margin="10"/>

        <TextBox Height="80" Margin="10" Text="{Binding Path=D, Mode=OneWay}" IsReadOnly="True" AcceptsReturn="True" TextWrapping="Wrap">
        </TextBox>
    </StackPanel>
</Grid>

C#: (ViewModel)

 public class MyViewModel : INotifyPropertyChanged
{
    private string _a;
    private string _b;
    private string _c;

    public string A
    {
        get => _a;
        set
        {
            _a = value;
            OnPropertyChanged(nameof(A));
            OnPropertyChanged(nameof(D));
        }
    }
    public string B
    {
        get => _b;
        set
        {
            _b = value;
            OnPropertyChanged(nameof(B));
            OnPropertyChanged(nameof(D));
        }
    }
    public string C
    {
        get => _c;
        set
        {
            _c = value;
            OnPropertyChanged(nameof(C));
            OnPropertyChanged(nameof(D));
        }
    }
    public string D => $"{A} {B} {C}";

    public List<string> MyTexts { get; }

    public MyViewModel()
    {
        MyTexts = new List<string>() { "FHAKWEFJ AWKEEF AWEKF LAEWKF LAWEF", "AAAAAAAAAAAAAAAAAAAAAA", "BBBBBBBBBBBBBBBBBBBBBBBBB", "EEEEEEEEEEEEEEEEEEEEEEEEE", "GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG", "GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG" };
    }

    #region INotifyPropertyChanged
    public event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    #endregion
}

No hacks, no magic and no events. Plain MVVM

EDIT: I guess this is what you want (only showing modifications to above code):

<TextBox Height="80" Margin="10" TextChanged="TextBoxBase_OnTextChanged" IsReadOnly="True" AcceptsReturn="True" TextWrapping="Wrap">
   <TextBox.Text>
                <MultiBinding StringFormat="{}{0} {1} {2}" NotifyOnTargetUpdated="True" Mode="OneWay">
                    <Binding  Path="A" Mode="OneWay"/>
                    <Binding  Path="B"  Mode="OneWay"/>
                    <Binding  Path="C" Mode="OneWay"/>
                </MultiBinding>
            </TextBox.Text>
 </TextBox>

C#:

private void TextBoxBase_OnTextChanged(object sender, TextChangedEventArgs e)
{
    if (this.DataContext is MyViewModel dataContext)
        dataContext.D = ((TextBox) sender).Text;
}

C# (VM):

    private string _d;

    public string D
    {
        get => _d;
        set
        {
            _d = value;
            OnPropertyChanged(nameof(D));
        }
    }