Queue in C#

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

public class Queue<T> : IEnumerable<T>
{
   public class Node
   {
       public T value;
       public Node next;

       public Node(T value, Node next)
       {
           this.value = value;
           this.next = next;
       }
   }

   private Node head;
   private Node tail;
   public int Size { get; private set; }

   public bool IsEmpty()
   {
       return Size <= 0;
   }

   public T Peek()
   {
       if (IsEmpty())
       {
           throw new InvalidOperationException();
       }
       return head.value;
   }

   public void Enqueue(T value)
   {
       if (IsEmpty())
       {
           tail = new Node(value, null);
           head = tail;
           Size++;
           return;
       }
       tail.next = new Node(value, null);
       tail = tail.next;
       Size++;
       return;
   }

   public T Dequeue()
   {
       if (IsEmpty())
       {
           throw new InvalidOperationException();
       }
       if(Size == 1)
       {
           Node tmp = head;
           head = null;
           tail = null;
           Size = 0;
           return tmp.value;
       }
       else
       {
           Node tmp = head;
           head = head.next;
           Size--;
           return tmp.value;
       }
   }

   public T Poll()
   {
       if (IsEmpty())
       {
           return default(T);
       }
       if(Size == 1)
       {
           Node tmp = head;
           head = null;
           tail = null;
           Size = 0;
           return tmp.value;
       }
       else
       {
           Node tmp = head;
           head = head.next;
           Size--;
           return tmp.value;
       }
   }

   override
   public string ToString()
   {
       string res = "{";
       Node tmp = head;
       while(tmp != null)
       {
           res += tmp.value.ToString();
           if (tmp.next != null) res += ", ";
           tmp = tmp.next;
       }
       res += "}";
       return res;
   }

   public IEnumerator<T> GetEnumerator()
   {
       Node node = this.head;
       while (node != null)
       {
           yield return node.value;
           node = node.next;
       }
   }

   IEnumerator IEnumerable.GetEnumerator()
   {
       return this.GetEnumerator();
   }
}