FindIndexOfAll · Software Engineering Wiki

Returns all indices in an IList that match 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 and return only matching indices.

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

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

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

_30s.FindIndexOfAll(nums, x => x % 2 != 0); // {0, 3}