Implementing INotifyPropertyChanged using LINQ-to-SQL

Question

I'm builing a Windows Phone 8 app using LINQ-to-SQL for my entities. My current implementation uses the simple INotifyPropertyChanging/INotififyPropertyChanged methods:

[Table]
public class Item : INotifyPropertyChanged, INotifyPropertyChanging
{
    private int _itemId;

    [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "INT NOT NULL Identity", AutoSync = AutoSync.OnInsert)]
    public int ItemId
    {
        get { return _itemId; }
        set
        {
            if (_itemId != value)
            {
                NotifyPropertyChanging("ItemId");
                _itemId = value;
                NotifyPropertyChanged("ItemId");
            }
        }
    }

    [Column]
    internal int? _groupId;

    private EntityRef<Board> _group;

    [Association(Storage = "_group", ThisKey = "_groupId", OtherKey = "GroupId", IsForeignKey = true)]
    public Group Group
    {
        get { return _group.Entity; }
        set
        {
            NotifyPropertyChanging("Group");
            _group.Entity = value;

            if (value != null)
            {
                _groupId = value.BoardId;
            }

            NotifyPropertyChanging("Group");
        }
    }
}

I want to change the property setter to my new method, I've founded here: http://danrigby.com/2012/04/01/inotifypropertychanged-the-net-4-5-way-revisited/

protected bool SetProperty<T>(ref T storage, T value, [CallerMemberName] String propertyName = null)
{
    if (object.Equals(storage, value)) return false;

    this.OnPropertyChanging(propertyName);
    storage = value;
    this.OnPropertyChanged(propertyName);
    return true;
}

It's easy to implement for a property like ItemId, but I don't know how to implement it for GroupId because the value should be set on _group.Entity and this can not be passed as reference.

This is my workaround (not tested yet), but I think the PropertyChanging/PropertyChanged events will be raised to early:

    public Group Group
    {
        get { return _group.Entity; }
        set
        {
            var group = _group.Entity;

            if (SetProperty(ref group, value))
            {
                _group.Entity = group;

                if (value != null)
                {
                    _groupId = value.GroupId;
                }
            }
        }
    }

Is there clear solution for this issue?

Answer 1

I have found a solution. Just overload the method with another implementation:

    protected T SetProperty<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
    {
        if (EqualityComparer<T>.Default.Equals(field, value))
        {
            return value;
        }

        NotifyPropertyChanging(propertyName);
        field = value;
        NotifyPropertyChanged(propertyName);

        return value;
    }

    protected T SetProperty<T>(ref EntityRef<T> field, T value, [CallerMemberName] string propertyName = null) where T : class
    {
        NotifyPropertyChanging(propertyName);
        field.Entity = value;
        NotifyPropertyChanged(propertyName);

        return value;
    }

Call it like this:

    public Group Group
    {
        get { return _board.Entity; }
        set
        {
            if (SetProperty(ref _group, value) != null)
            {
                _groupId = value.GroupId;
            }
        }
    }