Best way to notify property change when field is depending on another

Question

What is the best way in c# to notify property changed on an item\'s field without set but get depends on other fields ?

For example :

Answer 1

Even though in this solution the event is still propagated from the setter (so not exactly what the question is about), it provides a nice, more manageable way for representing dependencies. Someone might find it useful.

The solution is to create a custom wrapper for triggering INotifyPropertyChanged events. Instead of calling OnPropertyChanged manually we can define following mathods (preferably inside a base class that we will reuse later):

public abstract class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    internal void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    protected ViewModelPropertyChange SetPropertyValue(ref T property, T value, [CallerMemberName] string propertyName = null)
    {
        property = value;
        OnPropertyChanged(propertyName);

        return new ViewModelPropertyChange(this);
    }
}

This class provides us with a way of setting a value of a given field without a need for providing the name of a proparty the call comes from.

We also have to define a class that will enable use to define dependent properties (instance of this class is returned from SetPropertyValue mathod).

public class ViewModelPropertyChange
{
    private readonly ViewModelBase _viewModel;

    public ViewModelPropertyChange(ViewModelBase viewModel)
    {
        _viewModel = viewModel;
    }

    public ViewModelPropertyChange WithDependent(string name)
    {
        _viewModel.OnPropertyChanged(name);

        return this;
    }
}

It simply stores a reference to an object that is being changed and makes it possible to propagate an event to next properties.

With this we can create a class derived from ViewModelBase like this:

class OurViewModel : ViewModelBase
{
    private int _partOne;
    public int PartOne
    {
        get => _partOne;
        set => SetPropertyValue(ref _partOne, value)
            .WithDependent(nameof(Total));
    }

    private int _partTwo;
    public int PartTwo
    {
        get => _partTwo;
        set => SetPropertyValue(ref _partTwo, value)
            .WithDependent(nameof(Total))
            .WithDependent(nameof(PartTwoPlus2));
    }

    public int Total {
        get => PartOne + PartTwo;
    }

    public int PartTwoPlus2 {
        get => PartTwo + 2;
    }
}