How to flatten an array of arbitrarily nested arrays

flatten_array.cs

private void Flatten(object item, List<int> flattened)
{
    if (item is int i)
    {
        flattened.Add(i);
        return;
    }

    if (!(item is List<object> list)) return;

    foreach (var itm in list)
    {
        Flatten(itm, flattened);
    }
}

[TestMethod]
public void Flatten_Array()
{
    // Arrange
    var head = new List<int>();
    var first = new List<object> { 1, 2, new List<object> { 3 } };
    var second = new List<object> { first, new List<object> { 4 } };
    var third = new List<object> { second };

    // Act
    Flatten(third, head);

    // Assert
    Assert.IsNotNull(head);
    Assert.IsTrue(head.Any());
    Assert.AreEqual(4, head.Count);
}