merge.cs

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

namespace Merge
{
    class Program
    {
        public static IEnumerable<int> Merge(IEnumerable<int> a, IEnumerable<int> b)
        {
            var enumerable1 = a as IList<int> ?? a.ToList(); // Preventing multiple enumeration
            var enumerable2 = b as IList<int> ?? b.ToList();
            var en1 = enumerable1.GetEnumerator();
            var en2 = enumerable2.GetEnumerator();
            var current1 = (en1.MoveNext()) ? (int?)en1.Current : null; // Using C# advantages for more appropriate interface for current values
            var current2 = (en2.MoveNext()) ? (int?)en2.Current : null; // e.g checking termination without moving the enumerator's cursor
                                                                        // and "third-party" variables
            while (current1.HasValue && current2.HasValue)
            {
                while (current1.HasValue && (current1.Value < current2.Value))
                {
                    yield return current1.Value;
                    current1 = (en1.MoveNext()) ? (int?)en1.Current : null;
                }

                while (current1.HasValue && current2.HasValue && (current2.Value < current1.Value))
                {
                    yield return current2.Value;
                    current2 = (en2.MoveNext()) ? (int?)en2.Current : null;
                }

                if (current1.HasValue && current2.HasValue && (current2.Value == current1.Value))
                {
                    yield return current1.Value;
                    current1 = (en1.MoveNext()) ? (int?)en1.Current : null;
                    current2 = (en2.MoveNext()) ? (int?)en2.Current : null;
                }
            }
            var notEmpty = (current1.HasValue) ? en1 : en2;
            do yield return notEmpty.Current; while (notEmpty.MoveNext());
        }
        static void Main()
        {
            var list1 = new List<int> {1,    3,    5, 6, 7,    9,    11};
            var list2 = new List<int> {1, 2,    4,    6,    8, 9, 10   };
            // the result should be something like 1,2,3,4,5,6,7,8,9,10,11
            foreach (var i in Merge(list1, list2))
            {
                Console.WriteLine(i);
            }
        }
    }
}