FindIndexOfFirstBy

Returns the first index in an IList that matches the given predicate function, match.

• Use Enumerable.Range() to iterate over all indices in data.
• Use IEnumerable.Where() to filter out all values in data for which match returns false.
• Use IEnumerable.First() to return only the first matching index.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static int FindIndexOfFirstBy<T>(IList<T> data, Predicate<T> match)
  {
    return Enumerable
      .Range(0, data.Count())
      .Where(i => match(data[i]))
      .First();
  }
}

int[] nums = {1, 2, 4, 5, 2, 2, 4};

_30s.FindIndexOfFirstBy(nums, x => x % 2 == 0); // 1