FindIndexOfFirstBy · Software Engineering Wiki

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