MergeList1.cs

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

    int index1 = 0;
    int index2 = 0;

    while (index2 < list2Size)
    {
        while (list2[index2] > list1[index1])
            index1++;
        list1.Insert(index1, list2[index2]);
        index2++;
        index1++;
    }
}