Efficiently compute WorkdaysDiff() and AddWorkdays() with unit tests.

DateTimeExtensions.cs

  public static class DateTimeExtensions
  {
      #region Add/Diff Weekdays
  
      // Transformed for SQL which numbers Mon=0 and Sun=6
      // 0123444401233334012222340111123400012345001234550
  
      private static readonly int[,] _addOffset = 
      {
        // 0  1  2  3  4
          {0, 1, 2, 3, 4}, // Su  0
          {0, 1, 2, 3, 4}, // M   1
          {0, 1, 2, 3, 6}, // Tu  2
          {0, 1, 4, 5, 6}, // W   3
          {0, 1, 4, 5, 6}, // Th  4
          {0, 3, 4, 5, 6}, // F   5
          {0, 2, 3, 4, 5}, // Sa  6
      };
  
      public static DateTime AddWeekdays(this DateTime date, int weekdays)
      {
          int extraDays = weekdays % 5;
          int addDays = weekdays >= 0
              ? (weekdays / 5) * 7 + _addOffset[(int)date.DayOfWeek, extraDays]
              : (weekdays / 5) * 7 - _addOffset[6 - (int)date.DayOfWeek, -extraDays];
          return date.AddDays(addDays);
      }
  
      static readonly int[,] _diffOffset = 
      {
        // Su M  Tu W  Th F  Sa
          {0, 1, 2, 3, 4, 5, 5}, // Su
          {4, 0, 1, 2, 3, 4, 4}, // M 
          {3, 4, 0, 1, 2, 3, 3}, // Tu
          {2, 3, 4, 0, 1, 2, 2}, // W 
          {1, 2, 3, 4, 0, 1, 1}, // Th
          {0, 1, 2, 3, 4, 0, 0}, // F 
          {0, 1, 2, 3, 4, 5, 0}, // Sa
      };
  
      public static int GetWeekdaysDiff(this DateTime dtStart, DateTime dtEnd)
      {
          int daysDiff = (int)(dtEnd - dtStart).TotalDays;
          return daysDiff >= 0
              ? 5 * (daysDiff / 7) + _diffOffset[(int) dtStart.DayOfWeek, (int) dtEnd.DayOfWeek]
              : 5 * (daysDiff / 7) - _diffOffset[6 - (int) dtStart.DayOfWeek, 6 - (int) dtEnd.DayOfWeek];
      }
  
      #endregion  // Weekdays
  }

DateTimeExtensionsTests.cs

  using System;
  using System.Collections.Generic;
  using System.Linq;
  using NUnit.Framework;
  using ShouldBe;
  
  [TestFixture]
  public class DateTimeExtensionsTests
  {
      /// <summary>
      /// Exclude start date, Include end date
      /// </summary>
      /// <param name="dtStart"></param>
      /// <param name="dtEnd"></param>
      /// <returns></returns>
      private IEnumerable<DateTime> GetDateRange(DateTime dtStart, DateTime dtEnd)
      {
          Console.WriteLine(@"dtStart={0:yy-MMM-dd ddd}, dtEnd={1:yy-MMM-dd ddd}", dtStart, dtEnd);
  
          TimeSpan diff = dtEnd - dtStart;
          Console.WriteLine(diff);
  
          if (dtStart <= dtEnd)
          {
              for (DateTime dt = dtStart.AddDays(1); dt <= dtEnd; dt = dt.AddDays(1))
              {
                  Console.WriteLine(@"dt={0:yy-MMM-dd ddd}", dt);
                  yield return dt;
              }
          }
          else
          {
              for (DateTime dt = dtStart.AddDays(-1); dt >= dtEnd; dt = dt.AddDays(-1))
              {
                  Console.WriteLine(@"dt={0:yy-MMM-dd ddd}", dt);
                  yield return dt;
              }
          }
      }
  
      [Test, Combinatorial]
      public void TestGetWeekdaysDiff(
          [Values(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 20, 30)]
          int startDay,
          [Values(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 20, 30)]
          int endDay,
          [Values(7)]
          int startMonth,
          [Values(7)]
          int endMonth)
      {
          // Arrange
          DateTime dtStart = new DateTime(2016, startMonth, startDay);
          DateTime dtEnd = new DateTime(2016, endMonth, endDay);
  
          int nDays = GetDateRange(dtStart, dtEnd)
              .Count(dt => dt.DayOfWeek != DayOfWeek.Saturday && dt.DayOfWeek != DayOfWeek.Sunday);
  
          if (dtEnd < dtStart) nDays = -nDays;
  
          Console.WriteLine(@"countBusDays={0}", nDays);
  
          // Act / Assert
          dtStart.GetWeekdaysDiff(dtEnd).ShouldBe(nDays);
      }
  
      [Test, Combinatorial]
      public void TestAddWeekdays(
          [Values(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 20, 30)]
          int startDay,
          [Values(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 20, 30)]
          int weekdays)
      {
          DateTime dtStart = new DateTime(2016, 7, startDay);
          DateTime dtEnd1 = dtStart.AddWeekdays(weekdays);
          dtStart.GetWeekdaysDiff(dtEnd1).ShouldBe(weekdays);  // ADD
  
          DateTime dtEnd2 = dtStart.AddWeekdays(-weekdays);
          dtStart.GetWeekdaysDiff(dtEnd2).ShouldBe(-weekdays); // SUBTRACT
      }
  }