C# Linq Find all indexes of item in List<int> within another List<int>

public static IEnumerable<int> GetIndices<T>(this IEnumerable<T> items, Func<T, bool> predicate)
{
    return items.Select( (item, index) => new { Item = item, Index = index })
                     .Where(p => predicate(p.Item))
                     .Select(p => p.Index);
}

List<Integer> results = new ArrayList<>();
for (int i = 0; i < list.size(); i++) {
    if (search == list.get(i).intValue()) {
        // found value at index i
        results.add(i);
    }
}

List<Integer> list = Arrays.asList(10, 6, 16, 46, 5, 16, 7);
int search = 16;
int[] indices = IntStream.range(0, list.size())
                .filter(i -> list.get(i) == search)
                .toArray();
System.out.printf("Found %d at indices %s%n", search, Arrays.toString(indices));

Found 16 at indices [2, 5]

    def find(target, myList):
        for i in range(len(myList)):
            if myList[i] == target:
                yield i

    def find_with_list(myList, target):
         inds = []
         for i in range(len(myList)):
             if myList[i] == target:
                 inds += i,
         return inds


In [8]: x = range(50)*200
In [9]: %timeit [i for i,j in enumerate(x) if j == 3]
1000 loops, best of 3: 598 us per loop

In [10]: %timeit list(find(3,x))
1000 loops, best of 3: 607 us per loop
In [11]: %timeit find(3,x)
1000000 loops, best of 3: 375 ns per loop

In [55]: %timeit find_with_list(x,3)
1000 loops, best of 3: 618 us per loop

var testFirstList = myListCollectionObj.firstList.Where(x => x == 3).FirstOrDefault(); //then i want to get "33", and 333 from secondList and thirdList respectively
var index = myListCollectionObj.firstList.IndexOf(testFirstList);