How to bind Start, End and Duration with Multibinding?

Question

I have two properties in my ViewModel:

public double Start { get; set; }
public double Duration { get; set; }

I have three textboxes in my View:

<TextBox Name="StartTextBox" Text="{Binding Start}" />
<TextBox Name="EndTextBox" />
<TextBox Name="DurationTextBox" Text="{Binding Duration} />

I want to achieve the following behavior:

• When the user changes the content of the Start- or the EndTextBox, the DurationTextBox should be updated accordingly (Duration = End - Start).
• When the user changes the DurationTextBox, the EndTextBox should be updated accordingly (End = Start + Duration).

I can achieve this by listening to the TextChanged events in code behind. I would prefer to achieve this via MultiBinding, though. Is it possible?

The problems I encounter, when I try to use MultiBinding:

• If I put the MultiBinding on the DurationTextBox I cannot bind it to the Duration property.
• If I put the MultiBinding on the EndTextBox and the user changes the StartTextBox, the EndTextBox gets updated instead of the DurationTextBox.
• In any case I'm not able to implement the ConvertBack method.

Answer 1

I think blow codes are much easier to achieve the behavior which you want:

XAML:

<TextBox Name="StartTextBox" Text="{Binding Start}" />
<TextBox Name="EndTextBox" Text="{Binding End}" />
<TextBox Name="DurationTextBox" Text="{Binding Duration}" />

ViewModel:

    private double _start;
    private double _duration;
    private double _end;
    public double Start
    {
        get
        {
            return _start;
        }
        set
        {
            if (_start != value)
            {
                _start = value;
                Duration = _end - _start;
                OnPropertyChanged("Start");
            }
        }
    }
    public double Duration
    {
        get
        {
            return _duration;
        }
        set
        {
            if (_duration != value)
            {
                _duration = value;
                End = _duration + _start;
                OnPropertyChanged("Duration");
            }
        }
    }
    public double End
    {
        get
        {
            return _end;
        }
        set
        {
            if (_end != value)
            {
                _end = value;
                Duration = _end - _start;
                OnPropertyChanged("End");
            }
        }
    }

The OnPropertyChanged in ViewModel implements the INotifyPropertyChanged interface like this:

public class ViewModelBase:INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}