c# – 从一个实现INotifyPropertyChanged的基类继承？

public class BaseViewModel : INotifyPropertyChanged
{
    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,new PropertyChangedEventArgs(propertyName));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

和另一个类：

public class SchemaDifferenceViewModel : BaseViewModel
{
    private string firstSchemaToCompare;

    public string FirstSchemaToCompare
    {
        get { return firstSchemaToCompare; }
        set
        {
            firstSchemaToCompare = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this,new PropertyChangedEventArgs("FirstSchemaToCompare"));
                //StartCommand.RaiseCanExecuteChanged();
            }
        }
    }

PropertyChanged在这里(2次),红色下划线,它说：

Error   1   The event BaseViewModel.PropertyChanged' can only appear on the left hand side of += or -= (except when used from within the type 'SchemaDifferenceFinder.ViewModel.BaseViewModel')

我做错了什么？我只将PropertyChangedEvent扫描到一个新类：BaseViewModel ..