Delete duplicate nodes from sorted linked list in c#

Csharp program for Delete duplicate nodes from sorted linked list. Here problem description and explanation.

// Include namespace system
using System;
/*
    Csharp program for
    Delete duplicate nodes in sorted linked list
*/
// Linked list node
public class LinkNode
{
	public int data;
	public LinkNode next;
	public LinkNode(int data)
	{
		this.data = data;
		this.next = null;
	}
}
public class SingleLL
{
	public LinkNode head;
	public LinkNode tail;
	public SingleLL()
	{
		this.head = null;
		this.tail = null;
	}
	// Add new node at the end of linked list
	public void addNode(int value)
	{
		// Create a new node
		var node = new LinkNode(value);
		if (this.head == null)
		{
			this.head = node;
		}
		else
		{
			this.tail.next = node;
		}
		this.tail = node;
	}
	// Display linked list element
	public void display()
	{
		if (this.head == null)
		{
			return;
		}
		var temp = this.head;
		// iterating linked list elements
		while (temp != null)
		{
			Console.Write(temp.data.ToString() + " → ");
			// Visit to next node
			temp = temp.next;
		}
		Console.WriteLine(" NULL");
	}
	// Remove the duplicate nodes from 
  	// sorted singly linked list
	public void deleteDuplicate()
	{
		if (this.head == null)
		{
			return;
		}
		else
		{
			// Auxiliary variables
			var temp = this.head.next;
			var current = this.head;
			LinkNode hold = null;
			// Find and remove duplicate
			while (temp != null)
			{
				// Check duplicate node 
				if (current.data == temp.data)
				{
					//  When node key are same
					hold = temp;
				}
				else
				{
					// When node key are not same
					current = temp;
				}
				// Visit to next node
				temp = temp.next;
				if (hold != null)
				{
					// Modified link value
					current.next = temp;
					hold = null;
				}
				else
				{
					// Change last node
					this.tail = current;
				}
			}
		}
	}
	public static void Main(String[] args)
	{
		var sll = new SingleLL();
		// Sorted Linked list node
		//  1 → 1 → 2 → 3 → 4 → 4 → 4 → 5 → 6 → 7 → NULL
		sll.addNode(1);
		sll.addNode(1);
		sll.addNode(2);
		sll.addNode(3);
		sll.addNode(4);
		sll.addNode(4);
		sll.addNode(4);
		sll.addNode(5);
		sll.addNode(6);
		sll.addNode(7);
		Console.WriteLine(" Before Delete");
		sll.display();
		sll.deleteDuplicate();
		Console.WriteLine(" After Delete");
		// 1 → 2 → 3 → 4 → 5 → 6 → 7 → NULL
		sll.display();
	}
}

Output

 Before Delete
1 → 1 → 2 → 3 → 4 → 4 → 4 → 5 → 6 → 7 →  NULL
 After Delete
1 → 2 → 3 → 4 → 5 → 6 → 7 →  NULL