Old vs New ViewModelBase

// old usage
 public class PersonViewModel : ViewModelBase<Person>
    {
        private int _id;

        public int ID
        {
            get { return _id; }
            set { Set(ref _id, value); }
        }
// ....
public abstract class ViewModelBase<T> : INotifyPropertyChanged
    {
        public T Model { get; private set; }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual bool Set<K>(ref K storage, K value, [CallerMemberName] string propertyName = null)
        {
            if (!object.Equals(storage, value))
            {
                storage = value;
                OnPropertyChanged(propertyName);
                return true;
            }

            return false;
        }

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

        protected ViewModelBase(T model)
        {
            this.Model = model;
        }

// NEW USAGE - workaround for passing Properties "as" reference
 public class Foo : VMBase<Person>
    {

        public int Age
        {
            get { return Model.Age; }
            set { Set(value, Model, x => x.Age, "Age"); }
        }


        public Foo(Person model)
            : base(model)
        {
        }
    }

//and viewmodel base and Set method
    public T Model { get; set; }
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void Set<T, I>(I input, T output, Expression<Func<T, I>> expr, [CallerMemberName] string propertyName = null)
        {

            var member = (MemberExpression)expr.Body;
            var prop = (PropertyInfo)member.Member;
            prop.SetValue(output, input, null);
            OnPropertyChanged(propertyName);
        }

// WHERE T is type of model, I is type of input (property of model)