CommandBase and ModelBase

namespace SpaceTakeover
{
    public abstract class CommandBase : ICommand
    {

        public abstract bool CanExecute(object parameter);

        public event EventHandler CanExecuteChanged;

        public abstract void Execute(object parameter);

        public string Text { get; internal set; }
    }
    public abstract class ModelBase : INotifyPropertyChanged, IDataErrorInfo
    {

        public event PropertyChangedEventHandler PropertyChanged;

        public virtual string Error
        {
            get { return null; }
        }

        public abstract string this[string columnName] { get; }

        internal void NotifyOnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler temp = PropertyChanged;
            if (temp != null)
            {
                temp(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}