.NET 純原生實現 Cron 定時任務執行，未依賴第三方組件

常用的定時任務組件有 Quartz.Net 和 Hangfire 兩種，這兩種是使用人數比較多的定時任務組件，個人以前也是使用的 Hangfire ，慢慢的發現自己想要的其實只是一個能夠根據 Cron 表達式來定時執行函數的功能，Quartz.Net 和 Hangfire 雖然都能實現這個目的，但是他們都只用來實現 Cron表達式解析定時執行函數就顯得太笨重了，所以想著以 解析 Cron表達式定期執行函數為目的，編寫了下麵的一套邏輯。

首先為瞭解析 Cron表達式，我們需要一個CronHelper ,代碼如下

using System.Globalization;
using System.Text;
using System.Text.RegularExpressions;

namespace Common
{

    public class CronHelper
    {


        /// <summary>
        /// 獲取當前時間之後下一次觸發時間
        /// </summary>
        /// <param name="cronExpression"></param>
        /// <returns></returns>
        public static DateTimeOffset GetNextOccurrence(string cronExpression)
        {
            return GetNextOccurrence(cronExpression, DateTimeOffset.UtcNow);
        }



        /// <summary>
        /// 獲取給定時間之後下一次觸發時間
        /// </summary>
        /// <param name="cronExpression"></param>
        /// <param name="afterTimeUtc"></param>
        /// <returns></returns>
        public static DateTimeOffset GetNextOccurrence(string cronExpression, DateTimeOffset afterTimeUtc)
        {
            return new CronExpression(cronExpression).GetTimeAfter(afterTimeUtc)!.Value;
        }



        /// <summary>
        /// 獲取當前時間之後N次觸發時間
        /// </summary>
        /// <param name="cronExpression"></param>
        /// <param name="count"></param>
        /// <returns></returns>
        public static List<DateTimeOffset> GetNextOccurrences(string cronExpression, int count)
        {
            return GetNextOccurrences(cronExpression, DateTimeOffset.UtcNow, count);
        }



        /// <summary>
        /// 獲取給定時間之後N次觸發時間
        /// </summary>
        /// <param name="cronExpression"></param>
        /// <param name="afterTimeUtc"></param>
        /// <returns></returns>
        public static List<DateTimeOffset> GetNextOccurrences(string cronExpression, DateTimeOffset afterTimeUtc, int count)
        {
            CronExpression cron = new(cronExpression);

            List<DateTimeOffset> dateTimeOffsets = new();

            for (int i = 0; i < count; i++)
            {
                afterTimeUtc = cron.GetTimeAfter(afterTimeUtc)!.Value;

                dateTimeOffsets.Add(afterTimeUtc);
            }

            return dateTimeOffsets;
        }



        private class CronExpression
        {

            private const int Second = 0;

            private const int Minute = 1;

            private const int Hour = 2;

            private const int DayOfMonth = 3;

            private const int Month = 4;

            private const int DayOfWeek = 5;

            private const int Year = 6;

            private const int AllSpecInt = 99;

            private const int NoSpecInt = 98;

            private const int AllSpec = AllSpecInt;

            private const int NoSpec = NoSpecInt;

            private SortedSet<int> seconds = null!;

            private SortedSet<int> minutes = null!;

            private SortedSet<int> hours = null!;

            private SortedSet<int> daysOfMonth = null!;

            private SortedSet<int> months = null!;

            private SortedSet<int> daysOfWeek = null!;

            private SortedSet<int> years = null!;

            private bool lastdayOfWeek;

            private int everyNthWeek;

            private int nthdayOfWeek;

            private bool lastdayOfMonth;

            private bool nearestWeekday;

            private int lastdayOffset;

            private static readonly Dictionary<string, int> monthMap = new Dictionary<string, int>(20);

            private static readonly Dictionary<string, int> dayMap = new Dictionary<string, int>(60);

            private static readonly int MaxYear = DateTime.Now.Year + 100;

            private static readonly char[] splitSeparators = { ' ', '\t', '\r', '\n' };

            private static readonly char[] commaSeparator = { ',' };

            private static readonly Regex regex = new Regex("^L-[0-9]*[W]?", RegexOptions.Compiled);

            private static readonly TimeZoneInfo timeZoneInfo = TimeZoneInfo.Local;


            public CronExpression(string cronExpression)
            {
                if (monthMap.Count == 0)
                {
                    monthMap.Add("JAN", 0);
                    monthMap.Add("FEB", 1);
                    monthMap.Add("MAR", 2);
                    monthMap.Add("APR", 3);
                    monthMap.Add("MAY", 4);
                    monthMap.Add("JUN", 5);
                    monthMap.Add("JUL", 6);
                    monthMap.Add("AUG", 7);
                    monthMap.Add("SEP", 8);
                    monthMap.Add("OCT", 9);
                    monthMap.Add("NOV", 10);
                    monthMap.Add("DEC", 11);

                    dayMap.Add("SUN", 1);
                    dayMap.Add("MON", 2);
                    dayMap.Add("TUE", 3);
                    dayMap.Add("WED", 4);
                    dayMap.Add("THU", 5);
                    dayMap.Add("FRI", 6);
                    dayMap.Add("SAT", 7);
                }

                if (cronExpression == null)
                {
                    throw new ArgumentException("cronExpression 不能為空");
                }

                CronExpressionString = CultureInfo.InvariantCulture.TextInfo.ToUpper(cronExpression);
                BuildExpression(CronExpressionString);
            }





            /// <summary>
            /// 構建表達式
            /// </summary>
            /// <param name="expression"></param>
            /// <exception cref="FormatException"></exception>
            private void BuildExpression(string expression)
            {
                try
                {
                    seconds ??= new SortedSet<int>();
                    minutes ??= new SortedSet<int>();
                    hours ??= new SortedSet<int>();
                    daysOfMonth ??= new SortedSet<int>();
                    months ??= new SortedSet<int>();
                    daysOfWeek ??= new SortedSet<int>();
                    years ??= new SortedSet<int>();

                    int exprOn = Second;

                    string[] exprsTok = expression.Split(splitSeparators, StringSplitOptions.RemoveEmptyEntries);
                    foreach (string exprTok in exprsTok)
                    {
                        string expr = exprTok.Trim();

                        if (expr.Length == 0)
                        {
                            continue;
                        }
                        if (exprOn > Year)
                        {
                            break;
                        }

                        if (exprOn == DayOfMonth && expr.IndexOf('L') != -1 && expr.Length > 1 && expr.IndexOf(",", StringComparison.Ordinal) >= 0)
                        {
                            throw new FormatException("不支持在月份的其他日期指定“L”和“LW”");
                        }
                        if (exprOn == DayOfWeek && expr.IndexOf('L') != -1 && expr.Length > 1 && expr.IndexOf(",", StringComparison.Ordinal) >= 0)
                        {
                            throw new FormatException("不支持在一周的其他日期指定“L”");
                        }
                        if (exprOn == DayOfWeek && expr.IndexOf('#') != -1 && expr.IndexOf('#', expr.IndexOf('#') + 1) != -1)
                        {
                            throw new FormatException("不支持指定多個“第N”天。");
                        }

                        string[] vTok = expr.Split(commaSeparator);
                        foreach (string v in vTok)
                        {
                            StoreExpressionVals(0, v, exprOn);
                        }

                        exprOn++;
                    }

                    if (exprOn <= DayOfWeek)
                    {
                        throw new FormatException("表達式意料之外的結束。");
                    }

                    if (exprOn <= Year)
                    {
                        StoreExpressionVals(0, "*", Year);
                    }

                    var dow = GetSet(DayOfWeek);
                    var dom = GetSet(DayOfMonth);

                    bool dayOfMSpec = !dom.Contains(NoSpec);
                    bool dayOfWSpec = !dow.Contains(NoSpec);

                    if (dayOfMSpec && !dayOfWSpec)
                    {
                        // skip
                    }
                    else if (dayOfWSpec && !dayOfMSpec)
                    {
                        // skip
                    }
                    else
                    {
                        throw new FormatException("不支持同時指定星期和日參數。");
                    }
                }
                catch (FormatException)
                {
                    throw;
                }
                catch (Exception e)
                {
                    throw new FormatException($"非法的 cron 表達式格式 ({e.Message})", e);
                }
            }



            /// <summary>
            /// Stores the expression values.
            /// </summary>
            /// <param name="pos">The position.</param>
            /// <param name="s">The string to traverse.</param>
            /// <param name="type">The type of value.</param>
            /// <returns></returns>
            private int StoreExpressionVals(int pos, string s, int type)
            {
                int incr = 0;
                int i = SkipWhiteSpace(pos, s);
                if (i >= s.Length)
                {
                    return i;
                }
                char c = s[i];
                if (c >= 'A' && c <= 'Z' && !s.Equals("L") && !s.Equals("LW") && !regex.IsMatch(s))
                {
                    string sub = s.Substring(i, 3);
                    int sval;
                    int eval = -1;
                    if (type == Month)
                    {
                        sval = GetMonthNumber(sub) + 1;
                        if (sval <= 0)
                        {
                            throw new FormatException($"無效的月份值：'{sub}'");
                        }
                        if (s.Length > i + 3)
                        {
                            c = s[i + 3];
                            if (c == '-')
                            {
                                i += 4;
                                sub = s.Substring(i, 3);
                                eval = GetMonthNumber(sub) + 1;
                                if (eval <= 0)
                                {
                                    throw new FormatException(
                                        $"無效的月份值： '{sub}'");
                                }
                            }
                        }
                    }
                    else if (type == DayOfWeek)
                    {
                        sval = GetDayOfWeekNumber(sub);
                        if (sval < 0)
                        {
                            throw new FormatException($"無效的星期幾值： '{sub}'");
                        }
                        if (s.Length > i + 3)
                        {
                            c = s[i + 3];
                            if (c == '-')
                            {
                                i += 4;
                                sub = s.Substring(i, 3);
                                eval = GetDayOfWeekNumber(sub);
                                if (eval < 0)
                                {
                                    throw new FormatException(
                                        $"無效的星期幾值： '{sub}'");
                                }
                            }
                            else if (c == '#')
                            {
                                try
                                {
                                    i += 4;
                                    nthdayOfWeek = Convert.ToInt32(s.Substring(i), CultureInfo.InvariantCulture);
                                    if (nthdayOfWeek is < 1 or > 5)
                                    {
                                        throw new FormatException("周的第n天小於1或大於5");
                                    }
                                }
                                catch (Exception)
                                {
                                    throw new FormatException("1 到 5 之間的數值必須跟在“#”選項後面");
                                }
                            }
                            else if (c == '/')
                            {
                                try
                                {
                                    i += 4;
                                    everyNthWeek = Convert.ToInt32(s.Substring(i), CultureInfo.InvariantCulture);
                                    if (everyNthWeek is < 1 or > 5)
                                    {
                                        throw new FormatException("每個星期<1或>5");
                                    }
                                }
                                catch (Exception)
                                {
                                    throw new FormatException("1 到 5 之間的數值必須跟在 '/' 選項後面");
                                }
                            }
                            else if (c == 'L')
                            {
                                lastdayOfWeek = true;
                                i++;
                            }
                            else
                            {
                                throw new FormatException($"此位置的非法字元：'{sub}'");
                            }
                        }
                    }
                    else
                    {
                        throw new FormatException($"此位置的非法字元：'{sub}'");
                    }
                    if (eval != -1)
                    {
                        incr = 1;
                    }
                    AddToSet(sval, eval, incr, type);
                    return i + 3;
                }

                if (c == '?')
                {
                    i++;
                    if (i + 1 < s.Length && s[i] != ' ' && s[i + 1] != '\t')
                    {
                        throw new FormatException("'?' 後的非法字元: " + s[i]);
                    }
                    if (type != DayOfWeek && type != DayOfMonth)
                    {
                        throw new FormatException(
                            "'?' 只能為月日或周日指定。");
                    }
                    if (type == DayOfWeek && !lastdayOfMonth)
                    {
                        int val = daysOfMonth.LastOrDefault();
                        if (val == NoSpecInt)
                        {
                            throw new FormatException(
                                "'?' 只能為月日或周日指定。");
                        }
                    }

                    AddToSet(NoSpecInt, -1, 0, type);
                    return i;
                }

                var startsWithAsterisk = c == '*';
                if (startsWithAsterisk || c == '/')
                {
                    if (startsWithAsterisk && i + 1 >= s.Length)
                    {
                        AddToSet(AllSpecInt, -1, incr, type);
                        return i + 1;
                    }
                    if (c == '/' && (i + 1 >= s.Length || s[i + 1] == ' ' || s[i + 1] == '\t'))
                    {
                        throw new FormatException("'/' 後面必須跟一個整數。");
                    }
                    if (startsWithAsterisk)
                    {
                        i++;
                    }
                    c = s[i];
                    if (c == '/')
                    {
                        // is an increment specified?
                        i++;
                        if (i >= s.Length)
                        {
                            throw new FormatException("字元串意外結束。");
                        }

                        incr = GetNumericValue(s, i);

                        i++;
                        if (incr > 10)
                        {
                            i++;
                        }
                        CheckIncrementRange(incr, type);
                    }
                    else
                    {
                        if (startsWithAsterisk)
                        {
                            throw new FormatException("星號後的非法字元：" + s);
                        }
                        incr = 1;
                    }

                    AddToSet(AllSpecInt, -1, incr, type);
                    return i;
                }
                if (c == 'L')
                {
                    i++;
                    if (type == DayOfMonth)
                    {
                        lastdayOfMonth = true;
                    }
                    if (type == DayOfWeek)
                    {
                        AddToSet(7, 7, 0, type);
                    }
                    if (type == DayOfMonth && s.Length > i)
                    {
                        c = s[i];
                        if (c == '-')
                        {
                            ValueSet vs = GetValue(0, s, i + 1);
                            lastdayOffset = vs.theValue;
                            if (lastdayOffset > 30)
                            {
                                throw new FormatException("與最後一天的偏移量必須 <= 30");
                            }
                            i = vs.pos;
                        }
                        if (s.Length > i)
                        {
                            c = s[i];
                            if (c == 'W')
                            {
                                nearestWeekday = true;
                                i++;
                            }
                        }
                    }
                    return i;
                }
                if (c >= '0' && c <= '9')
                {
                    int val = Convert.ToInt32(c.ToString(), CultureInfo.InvariantCulture);
                    i++;
                    if (i >= s.Length)
                    {
                        AddToSet(val, -1, -1, type);
                    }
                    else
                    {
                        c = s[i];
                        if (c >= '0