.net中 接口可以有默认实现了

从 .NET Core 3.0 上的 C# 8.0 开始，可以在声明接口成员时定义实现。 最常见的方案是安全地将成员添加到已经由无数客户端发布并使用的接口。

示例：

 internal interface ICustomer
    {
    
        IEnumerable<IOrder> PreviousOrders {
     get; }

        DateTime DateJoined {
     get; }
        DateTime? LastOrder {
     get; }
        string Name {
     get; }
        IDictionary<DateTime, string> Reminders {
     get; }

    }
    
  internal interface IOrder
    {
    
        DateTime Purchased {
     get; }
        decimal Cost {
     get; }
    }

C# 中比较常见的两个接口。 然后新特性来了。

 internal interface ICustomer
    {
    
        IEnumerable<IOrder> PreviousOrders {
     get; }

        DateTime DateJoined {
     get; }
        DateTime? LastOrder {
     get; }
        string Name {
     get; }
        IDictionary<DateTime, string> Reminders {
     get; }

        // Version 1:
        public decimal ComputeLoyaltyDiscountVersionOne()
        {
    
            DateTime TwoYearsAgo = DateTime.Now.AddYears(-2);
            if ((DateJoined < TwoYearsAgo) && (PreviousOrders.Count() > 10))
            {
    
                return 0.10m;
            }
            return 0;
        }

        // Version 2:
        public static void SetLoyaltyThresholds(
            TimeSpan ago,
            int minimumOrders = 10,
            decimal percentageDiscount = 0.10m)
        {
    
            length = ago;
            orderCount = minimumOrders;
            discountPercent = percentageDiscount;
        }
        private static TimeSpan length = new TimeSpan(365 * 2, 0, 0, 0); // two years
        private static int orderCount = 10;
        private static decimal discountPercent = 0.10m;

        public decimal ComputeLoyaltyDiscountVersionTwo()
        {
    
            DateTime start = DateTime.Now - length;

            if ((DateJoined < start) && (PreviousOrders.Count() > orderCount))
            {
    
                return discountPercent;
            }
            return 0;
        }

        //另外一种方式
        public decimal ComputeLoyaltyDiscount() => DefaultLoyaltyDiscount(this);
        protected static decimal DefaultLoyaltyDiscount(ICustomer c)
        {
    
            DateTime start = DateTime.Now - length;

            if ((c.DateJoined < start) && (c.PreviousOrders.Count() > orderCount))
            {
    
                return discountPercent;
            }
            return 0;
        }

    }```