using System;

public int ClosestIndexOf(string input, char value, int startIndex)

{

    if (startIndex > input.Length)

        throw new IndexOutOfRangeException("Start index was outside the length of input string.");

​

    // get closest index below and above specified "startIndex" position

    int below = input.LastIndexOf(value, startIndex - 1);

    int above = input.IndexOf(value, startIndex);

    var mid = (below + above) / 2;

    int result;

​

    if (mid > startIndex)

        result = below;

    else

        result = above;

    Console.WriteLine(string.Format("StartIndex: {0}, Below: {1}, Above: {2}, Mid: {3}, Result: {4}", startIndex, below, above, mid, result));

​

    return result;

}

​

string test = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";

​

Console.WriteLine("0         1         2         3         4         5     ");

Console.WriteLine("01234567890123456789012345678901234567890123456789012345");

Console.WriteLine("————————————————————————————————————————————————————————");

Console.WriteLine(test);

Console.WriteLine("————————————————————————————————————————————————————————");

​

ClosestIndexOf(test, ' ', 30); // 27

ClosestIndexOf(test, ' ', 35); // 39

ClosestIndexOf(test, ' ', 50); // 50

ClosestIndexOf(test, ' ', 19); // 17 & 21 have same offset, return 21

ClosestIndexOf(test, 'x', 30); // -1

ClosestIndexOf(test, 'a', 45); // 40

ClosestIndexOf(test, 'a', 20); // 22

ClosestIndexOf(test, ' ', 60); // IndexOutOfRangeException

​