設計原則：消除Switch...Case的過程，可能有點過度設計了。

備注

不要重復自己，也不要重復別人，一旦養成了“拷貝和粘貼”的習慣，寫程序的時候非常容易導致重復，好在一直暗示自己要稍后進行重構，本文給出一個重構的示例。

需求

需求：按照年、月和日顯示銷售數據，根據不同的周期類型，有三個問題需要注意：

1. 默認的日期范圍不同
2. 圖表中顯示的格式不同
3. 默認的模擬數據不同（發布環境會使用真實的數據）

如下圖：

 

第一遍代碼（重復的代碼）

最愛的拷貝和粘貼。

默認的日期范圍不同

        private void ResetStartDateAndEndDate()
        {
            this.EndDate = DateTime.Now;

            switch (_currentCircle)
            {
                case "日":
                    this.StartDate = this.EndDate.AddMonths(-1);
                    break;
                case "月":
                    this.StartDate = this.EndDate.AddMonths(-12);
                    break;
                case "年":
                    this.StartDate = this.EndDate.AddMonths(-12 * 3);
                    break;
            }

            this.StartDate = this.StartDate.AddDays(1);
        }

圖表中顯示的格式不同

        public string DisplayDate
        {
            get
            {
                switch (this.Cycle)
                {
                    case "日":
                        return this.Date.ToString("yyyy-MM-dd");
                    case "月":
                        return this.Date.ToString("yyyy-MM");
                    case "年":
                        return this.Date.ToString("yyyy");
                    default:
                        throw new InvalidOperationException("周期類型不匹配");
                }
            }
        }

默認的模擬數據不同

        public IEnumerable<SalesViewModel> Find(string cycle, DateTime startDate, DateTime endDate)
        {
            switch (cycle)
            {
                case "日":
                    return new List<SalesViewModel>
                    {
                        new SalesViewModel{ Date = DateTime.Now.AddDays(-2).AddDays(1), Total = 100, Cycle = cycle },
                        new SalesViewModel{ Date = DateTime.Now, Total = 200, Cycle = cycle }
                    };
                case "月":
                    return new List<SalesViewModel>
                    {
                        new SalesViewModel{ Date = DateTime.Now.AddMonths(-2).AddDays(1), Total = 100, Cycle = cycle },
                        new SalesViewModel{ Date = DateTime.Now, Total = 200, Cycle = cycle }
                    };
                case "年":
                    return new List<SalesViewModel>
                    {
                        new SalesViewModel{ Date = DateTime.Now.AddYears(-2).AddDays(1), Total = 100, Cycle = cycle },
                        new SalesViewModel{ Date = DateTime.Now, Total = 200, Cycle = cycle }
                    };
                default:
                    return new List<SalesViewModel>();
            }
        }

第二遍代碼（消除重復）

“門面類型+多態+私有內部類”消除重復

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Collections.Generic;

namespace Marking.Dashboard.Infrastructures
{
    public static class CycleTypeHelper
    {
        private static Dictionary<string, CycleType> _CycleTypeMaps = new Dictionary<string, CycleType>
        {
            { "日", new DayCycleType() },
            { "月", new MonthCycleType() },
            { "年", new YearCycleType() }
        };

        public static IEnumerable<string> CircleTypes
        {
            get
            {
                return _CycleTypeMaps.Keys;
            }
        }

        public static DateTime GetDefaultStartDate(string cycleType, DateTime endDate)
        {
            return _CycleTypeMaps[cycleType].GetDefaultStartDate(endDate);
        }

        public static string GetDisplayDateString(string cycleType, DateTime date)
        {
            return _CycleTypeMaps[cycleType].GetDisplayDateString(date);
        }

        public static IEnumerable<DateTime> SimulateDates(string cycleType, DateTime startDate, DateTime endDate)
        {
            return _CycleTypeMaps[cycleType].SimulateDates(startDate, endDate);
        }

        private abstract class CycleType
        {
            public abstract DateTime GetDefaultStartDate(DateTime endDate);

            public abstract string GetDisplayDateString(DateTime date);

            public abstract IEnumerable<DateTime> SimulateDates(DateTime startDate, DateTime endDate);
        }

        private class YearCycleType : CycleType
        {
            public override DateTime GetDefaultStartDate(DateTime endDate)
            {
                return endDate.AddMonths(-12 * 3);
            }

            public override string GetDisplayDateString(DateTime date)
            {
                return date.ToString("yyyy");
            }

            public override IEnumerable<DateTime> SimulateDates(DateTime startDate, DateTime endDate)
            {
                for (var i = startDate; i <= endDate; i = i.AddYears(1))
                {
                    yield return i;
                }
            }
        }

        private class MonthCycleType : CycleType
        {
            public override DateTime GetDefaultStartDate(DateTime endDate)
            {
                return endDate.AddMonths(-12);
            }

            public override string GetDisplayDateString(DateTime date)
            {
                return date.ToString("yyyy-MM");
            }

            public override IEnumerable<DateTime> SimulateDates(DateTime startDate, DateTime endDate)
            {
                for (var i = startDate; i <= endDate; i = i.AddMonths(1))
                {
                    yield return i;
                }
            }
        }

        private class DayCycleType : CycleType
        {
            public override DateTime GetDefaultStartDate(DateTime endDate)
            {
                return endDate.AddMonths(-1);
            }

            public override string GetDisplayDateString(DateTime date)
            {
                return date.ToString("MM-dd");
            }

            public override IEnumerable<DateTime> SimulateDates(DateTime startDate, DateTime endDate)
            {
                for (var i = startDate; i <= endDate; i = i.AddDays(1))
                {
                    yield return i;
                }
            }
        }
    }
}

備注

完成第一遍后，差點不想進行重構了，戰勝自己非常不容易，繼續努力。