Calculates the sum of all the integers in the range between two integers, with the option to exclude them. License: MIT

SumBetween.cs

/// <summary>
/// Calculates the sum of all the integers in the range between two integers.
/// </summary>
/// <param name="start">The starting integer.</param>
/// <param name="end">The ending integer.</param>
/// <param name="exclusive">Whether or not the <paramref name="start"/> and <paramref name="end"/> integers are excluded from the sum.</param>
/// <returns>Returns the sum of the integers in the range.</returns>
private static long SumBetween(int start, int end, bool exclusive = false)
{
  if (start > end) { throw new ArgumentException("End value must be more than or equal to start value."); }

  // Create long (Int64) representations of both 'start' and 'end' int parameters
  long min = start;
  long max = end;

  // Adjust 'min' and 'max' values if the series is exclusive of the start and end values
  if (exclusive)
  {
    min++;
    max--;
    if (min > max) { return 0; }
  }

  // Calculate the number of values in the series between the two given integers
  long seriesLength = (max - min) + 1;
  // Return the sum of every value in the series length
  return (seriesLength * (min + max)) / 2;
}