InterfaceStudy/InterfaceStudy.Core/ChildA.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace InterfaceStudy.Core
{
    public class ChildA : ExplicitBase, IBase
    {
        int Number = -1;

        public override int GetNumber()
        {
            return --Number;
        }
    }
}