IndexOfAll · Software Engineering Wiki

Returns all indices of n in an IList.

Use Enumerable.Range() to iterate over all indices in data.
Use Enumerable.Where() in combination with object.Equals() to compare each value in data to n and return only matching indices.

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

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

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

_30s.IndexOfAll(_30s.IndexOfAll(nums, 2)); // {1, 4, 5}