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.

Such a behaviour should be implemented in the view model. When e.g. the `Start` property changes, the view model also updates its `Duration` property internally. The properties should of course also fire PropertyChanged events. — Clemens, Oct 22 '15 at 08:49
@Clemens That sounds reasonable. This means, I add a third property End and put all logic in the setters of those properties, right? — Tim Pohlmann, Oct 22 '15 at 08:50
Exactly. And the view model class should implement the INotifyPropertyChanged interface. — Clemens, Oct 22 '15 at 08:53

score 1 · Accepted Answer · answered Oct 22 '15 at 08:54

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));
        }
    }
}

While I agree that this is probably the nicest solution following MVVM, this is not really an answer to the question if it is possible with MultiBinding. Thanks anyway, looks pretty good, I might end up using it. — Tim Pohlmann, Oct 22 '15 at 08:56
@TimPohlmann This answer is: not at all. MultiBinding is meant for binding multiple source properties (e.g. from a view model) to a single target property in the view, i.e. the other way round. — Clemens, Oct 22 '15 at 09:42

How to bind Start, End and Duration with Multibinding?

1 Answers1