using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var l = new List<double>() {.0, .1, .2, .3, .4, .5, .6, .7 };

        var v = .45;

        var index = l.FindIndexOfFirstValueLessThan(v);

        Console.Write($"l[{index}] = ");

        Console.WriteLine($"{l[index]}");

    }

}

​

public static class ListExtensions

{

    public static int FindIndexOfFirstValueLessThan<T>(this List<T> sortedList, T value, IComparer<T> comparer = null)

    {

        var index = sortedList.BinarySearch(value, comparer);

        // The value was found in the list. Just return its index.

        if (index >= 0)

            return index;

        // The value was not found and "~index" is the index of the next value greater than the search value.

        index = ~index;

        // There are values in the list less than the search value. Return the index of the closest one.

        if (index > 0)

            return index - 1;

        // All values in the list are greater than the search value.

        return -1;

    }

}