本例使用的規則引擎包是RulesEngine，關於RulesEngine的介紹，請自行補充，這裡只是一個極簡單使用場景。

　　例子大體就是使用者有一些優惠券，系統會根據使用者訂單情況，篩選可以使用的優惠券供使用者選擇，使用者選擇後會計算出優惠後金額。

　　程式碼相對簡單，就不一一介紹了，奉上service中的程式碼：

using Microsoft.Extensions.Logging;
using RulesEngine.Extensions;
using RulesEngine.Models;
using System;
using System.Collections.Generic;
 
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;

namespace RulesEngineSample.Services
{
    public class CouponService : ICouponService
    {
        private readonly ILogger<CouponService> _logger;
        private readonly List<Coupon> _userCoupons;
        
 
private readonly Order _order;
        private readonly User _user;
        public CouponService(ILogger<CouponService> logger)
        {
            _logger = logger;
            //假裝使用者用的優惠券
            _userCoupons = new List<Coupon>() {
                new Coupon
                {
                    ID 
 
= 1,
                    Code="YH01",
                    Name = "滿500減20",
                    BeginTime = DateTime.Parse("2021-01-01"),
                    EndTime = DateTime.Parse("2021-06-01"),
                    Expression = "input1.Details.Sum(s => s.Price * Convert.ToDecimal(s.Quantity))>= 500",
                    Symbol="-",
                    Number=20

                },
                new Coupon
                {
                    ID = 2,
                    Code="YH02",
                    Name = "新人5元券",
                    BeginTime = DateTime.Parse("2021-01-01"),
                    EndTime = DateTime.Parse("2021-06-01"),
                    Expression = "input1.Details.Count>0",
                    Symbol="-",
                    Number=5
                },
                new Coupon
                {
                    ID = 3,
                    Code="YH03",
                    Name = "商品C五折",
                    BeginTime = DateTime.Parse("2021-01-01"),
                    EndTime = DateTime.Parse("2021-06-01"),
                    Expression = "input1.Details.Where(s=>s.GoodsID==\"SP0000003\").Count()>0",
                    Symbol="*",
                    Number=0.5f,
                    GoodsID="SP0000003"
                },
            };
            //假裝當前使用者的訂單
            _order = new Order
            {
                OrderNo = "NO00000001",
                OrderTime = DateTime.Now,
                Details = new List<Detail>
                {
                    new Detail{ GoodsID="SP0000001", Name="商品A", Price=12.5m, Quantity=3},
                    new Detail{ GoodsID="SP0000002", Name="商品B", Price=100m, Quantity=4},
                    new Detail{ GoodsID="SP0000003", Name="商品C", Price=22.3m, Quantity=5},
                }
            };
            _user = new User { ID = 1, UserName = "zhangsan" };
        }
        /// <summary>
        /// 適配優惠券
        /// </summary>
        /// <returns></returns>
        public async Task<string> SelectCouponAsync()
        {
            var workRules = new RulesEngine.Models.WorkflowRules();
            workRules.WorkflowName = "優惠券";
            var rules = new List<Rule>();
            foreach (var coupon in _userCoupons.Where(s => s.BeginTime < DateTime.Now && s.EndTime > DateTime.Now))
            {
                var rule = new Rule
                {
                    RuleName = coupon.Name,
                    SuccessEvent = coupon.Code,
                    ErrorMessage = "規則應用失敗",
                    ErrorType = ErrorType.Error,
                    RuleExpressionType = RuleExpressionType.LambdaExpression,
                    Expression = coupon.Expression
                };
                rules.Add(rule);
            }
            workRules.Rules = rules;
            var rulesEngine = new RulesEngine.RulesEngine(new WorkflowRules[] { workRules }, _logger, new ReSettings());
            var ruleResults = await rulesEngine.ExecuteAllRulesAsync("優惠券", _order, _user);
           // var valueCoupons = new List<string>();
            //處理結果
            var discountCoupons = new StringBuilder();
            foreach (var ruleResult in ruleResults)
            {
                if (ruleResult.IsSuccess)
                {
                    discountCoupons.AppendLine($"可以使用的優惠券 “{_userCoupons.SingleOrDefault(s => s.Code == ruleResult.Rule.SuccessEvent)?.Name}”, Code是：{ruleResult.Rule.SuccessEvent}");
                    //valueCoupons.Add(ruleResult.Rule.SuccessEvent);
                }
            }
            //resultList.OnSuccess((eventName) =>
            //{
            //    discountOffered += $"可以使用的優惠券“{userCoupons.SingleOrDefault(s => s.Code == eventName)?.Name}”,Code是：{eventName} ";
            //});
            ruleResults.OnFail(() =>
            {
                discountCoupons.AppendLine("您沒有適合的優惠券！");
            });
            return discountCoupons.ToString();
        }
        /// <summary>
        /// 計算訂單支付總額
        /// </summary>
        /// <param name="code"></param>
        /// <returns></returns>
        public string GetOrderAmount(string code)
        {            
            var selectCoupon = _userCoupons.SingleOrDefault(s => s.Code == code);
            var orderAmount = 0m;
            switch (selectCoupon.Symbol)
            {
                case "-":
                    orderAmount = _order.Details.Sum(s => s.Price * Convert.ToDecimal(s.Quantity)) - Convert.ToDecimal(selectCoupon.Number);
                    break;
                case "*":
                    if (!string.IsNullOrWhiteSpace(selectCoupon.GoodsID))
                    {
                        orderAmount = _order.Details.Sum(s => s.Price * Convert.ToDecimal(s.Quantity));
                        var detail = _order.Details.SingleOrDefault(s => s.GoodsID == selectCoupon.GoodsID);
                        if (detail != null)
                        {
                            orderAmount -= detail.Price * Convert.ToDecimal(detail.Quantity) * Convert.ToDecimal(1 - selectCoupon.Number);
                        }
                    }
                    else
                    {
                        orderAmount = _order.Details.Sum(s => s.Price * Convert.ToDecimal(s.Quantity)) * Convert.ToDecimal(selectCoupon.Number);
                    }
                    break;
            }
            return $"訂單總金額：{_order.Details.Sum(s => s.Price * Convert.ToDecimal(s.Quantity)) }，優惠後請支付：{orderAmount}";
        }

    }
    public class Coupon
    {
        public int ID { get; set; }
        public string Code { get; set; }
        public string Name { get; set; }
        public DateTime BeginTime { get; set; }
        public DateTime EndTime { get; set; }
        public string Expression { get; set; }
        public string Symbol { get; set; }
        public float Number { get; set; }
        public string GoodsID { get; set; }
    }

    public class User
    {
        public int ID { get; set; }
        public string UserName { get; set; }
    }
    public class Order
    {
        public string OrderNo { get; set; }
        public DateTime OrderTime { get; set; }
        public List<Detail> Details { get; set; }
    }
    public class Detail
    {
        public string GoodsID { get; set; }
        public float Quantity { get; set; }
        public decimal Price { get; set; }
        public string Name { get; set; }

    }
}

api /selectcoupon是查詢可使用的優惠券

/getamount按優惠券code，計算本次訂單的應付金額：

優惠券YH01

優惠券YH02

優惠券YH03