Question

I’ve been using the MVVM pattern for a little while now and frequently run into a scenario where the value of one property depends on the value of another property. For instance, I have a control with a height and width, and I want to display the height and width on the control as a formatted string, “{height} x {width}”. So I include the following properties in my view model:

public class FooViewModel : INotifyPropertyChanged
{
    // . . .

    private double _width;
    public double Width
    {
        get { return _width; }
        set
        {
            if(_width != value)
            {
                _width = value;
                NotifyPropertyChanged("Width");
                NotifyPropertyChanged("DisplayString"); // I had to remember to
                                                        // do this.
            }
        }
    }

    public string DisplayString
    {
        get
        {
            return string.Format("{0} x {1}", _width, _height);
        }
    }

    // . . .
}

Then I bind to content of my Label to the DisplayString property, which seems a lot more convenient than using a IMultiValueConverter to convert from the Width and Height properties. The inconvenient part is that anywhere I need to NotifyPropertyChanged for “Width” or “Height”, I also have to remember to notify for “DisplayString”. I can think of myriad ways to automate this, to varying degrees, but my question is whether there is a standard practice that people generally use to do this under MVVM in WPF?

Answer 1

No there is no standard way of doing this.

You can write a base viewmodel class which has PropertyChanged helper method. It will look at the properties of class with DependsOn attribute (which also you’ll create) and fire event for all the properties that depend on the updated property.