MergeList2.cs

void MergeListImproved(ref List<int> list1, List<int> list2)
{
    int list1Size = list1.Count;
    int list2Size = list2.Count;
    int totalSize = list1Size + list2Size;

    var newList = new List<int>(totalSize);

    int index1 = 0;
    int index2 = 0;

    while (index1 < list1Size && index2 < list2Size)
        if (list1[index1] < list2[index2])
        {
            newList.Add(list1[index1]);
            index1++;
        }
        else
        {
            newList.Add(list2[index2]);
            index2++;
        }


    while (index1 < list1Size)
    {
        newList.Add(list1[index1]);
        index1++;
    }

    while (index2 < list2Size)
    {
        newList.Add(list2[index2]);
        index2++;
    }

    list1.Clear();
    list1.Capacity = totalSize;
    list1.AddRange(newList);
}