IEnumerableExtensions.cs

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

namespace Extensions
{
    public static class IEnumerableExtensions
    {
        /// <summary>
        /// Flats to one level an IEnumrable
        /// </summary>
        /// <typeparam name="T">any type</typeparam>
        /// <param name="rootLevel">The main IEnumerable to flat</param>
        /// <returns>Returns the flattend IEnumerable</returns>
        public static IEnumerable<object> Flatten<T>(this IEnumerable<T> rootLevel)
        {
            List<object> flatted = new List<object>();
            FlatLevel(flatted, rootLevel);
            return flatted;
        }

       
        private static void FlatLevel(List<object> mainIEnumerable, IEnumerable currentLevel)
        {
            foreach (var item in currentLevel ?? Enumerable.Empty<object>())
            {
                IEnumerable enumerable = item as IEnumerable;
                if (enumerable != null)
                {
                    FlatLevel(mainIEnumerable, enumerable);
                }
                else
                {
                    mainIEnumerable.Add(item);
                }
            }
        }
    }
}

TestIEnumerableExtensions.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Extensions;
using System.Collections.Generic;
using System.Linq;

namespace UnitTestProject1
{
    [TestClass]
    public class TestIEnumerableExtensions
    {
        [TestMethod]
        public void TestFlatten()
        {
            var list1 = new List<object> { new List<object> { 1, 2, new List<object> { 3 }, 4 } };

            CollectionAssert.AreEqual(list1.Flatten().ToList(), new List<object> { 1, 2, 3, 4 });

            List<object> list2 = null;

            CollectionAssert.AreEqual(list2.Flatten().ToList(), new List<object>());

            var list3 = new List<object> { new List<object> { 1, 2, new List<int> { 3, 4 }, 5 } };

            CollectionAssert.AreEqual(list3.Flatten().ToList(), new List<object> { 1, 2, 3, 4, 5 });

            var list4 = new List<object> { new List<object> { 1, 2, new int[] { 3, 4 }, 5 } };

            CollectionAssert.AreEqual(list4.Flatten().ToList(), new List<object> { 1, 2, 3, 4, 5 });
        }
    }
}