How to implement a merge sort on a Doubly Linked List using C#

I have been working on a custom linked list recently and required a sorting algorithm to sort the doubly linked list.

Mergesort works better on linked lists than it does on arrays. It avoids the need for the auxiliary space, and becomes a simple, reliably O(N log N) sorting algorithm. And as an added bonus, it's stable too.

Here is a quick merge sort algorithm in C#.Net 3.5 (Note - T is my custom node interface) :

private T Merge_Sort(T first)
{
    if (first == null || first.Next == null)
        return first;

    T second = Split(first);
 
    first = Merge_Sort(first);
    second = Merge_Sort(second);
 
    return Merge(first, second);
}
 
private T Split(T item)
{
    if (item == null || item.Next == null)
        return null;
 
    T second = item.Next;
 
    item.Next = second.Next;

    second.Next = Split(second.Next);
             
    return second;
}
 
private T Merge(T first, 
      T second)
{
    if (first == null)
        return second;

    if (second == null)
        return first;
 
    // If both are not null, we will run the comparer
    // If first is less than second
    if (first.CompareTo(second) < 0)
    {
        first.Next = Merge(first.Next, second);
        first.Next.Previous = first;
        first.Previous = default(T);

        return first;
    } 
    else
    {
        second.Next = Merge(first, second.Next);
        second.Next.Previous = second;
        second.Previous = default(T);
        
        return second;
    } 
}
For the full implementation of the Custom Doubly Linked List, look at this post.