Update Entity Framework property when different property in model is changed

Question

I have an Entity Framework model

public partial class Product
{
    public Product()
    {
        this.Designs = new HashSet<Design>();
    }

    public int BookingProductId { get; set; }
    public System.Guid BookingId { get; set; }
    public decimal Price { get; set; }

    public virtual Booking Booking { get; set; }
    public virtual ICollection<Design> Designs { get; set; }
}

...in which I would like to update the Price property in response to a new Design being added to the Product. I've tried to follow this example:

How can I change an Entity Framework ICollection to be an ObservableCollection?

so my class reads as follows:

public partial class Product : INotifyPropertyChanged
{
    public Product()
    {
        this.Designs = new ObservableCollection<Design>();
    }

    public int BookingProductId { get; set; }
    public System.Guid BookingId { get; set; }
    public decimal Price { get; set; }

    public virtual Booking Booking { get; set; }
    public virtual ObservableCollection<Design> Designs { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

but if I add a new Design to the Product as so

product.Designs.Add(new Design());

then the NotifyPropertyChanged method is never fired. Does anyone know why???

Thanks in advance

Answer 1

NotifyPropertyChanged never fires because adding another element to your Designs collection doesn't actually change any Product's properties. If you want to track changes within the collection itself, it should implement INotifyCollectionChanged interface. ObservableCollection already implements that, so you can simply handle CollectionChanged event.

Answer 2

NotfiyPropertyChanged will only called when you set the whole collection and not the included items.

This is a workaround for the problme:

public partial class Product : INotifyPropertyChanged
{
    public Product()
    {
        this.Designs = new ObservableCollection<Design>();
        this.Designs.CollectionChanged += ContentCollectionChanged;
    }

    public void ContentCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        // This will get called when the collection is changed
        // HERE YOU CAN ALSO FIRE THE PROPERTY CHANGED 
    }

    public int BookingProductId { get; set; }
    public System.Guid BookingId { get; set; }
    public decimal Price { get; set; }

    public virtual Booking Booking { get; set; }
    public virtual ObservableCollection<Design> Designs { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}