有没有办法为 VS2015 实现类似的 Raise 属性更改？

Question

Seems I have syntax errors in VS2015 for the code below. The error sounds like:

A value of type '?' cannot be used as a default parameter because there are no standard conversions to type 'T'

Is there a way to implement similar "Raise property changed" in VS2015?

 public class ViewModel : ViewModelBase
    {

        // Property        
        public const string SelectedItemPropertyName = "SelectedItem";

        private int? _selectedItem;       
        public int? SelectedItem
        {
            get
            {
                return _selectedItem;
            }
            set
            {
                Set(SelectedItemPropertyName, ref _selectedItem, value);
            }
        }

        // Method
        private void Load()
        {
            int number = SelectedItem ?? -1;

            // do your work
        }


       // Raise property changed
        public override void RaisePropertyChanged<T>([CallerMemberName] string propertyName = null, T oldValue = default, T newValue = default, bool broadcast = false)
        {
            base.RaisePropertyChanged(propertyName, oldValue, newValue, broadcast);
            if (propertyName == nameof(SelectedItem))
                Load();
        }
    }

Answer 1

It looks like you are using a C# version under 7.1. In that case, you will need to use default(T) , rather than simply default . Here is a link to the docs on default value expressions.