当前上下文不存在GetByIndex！

我在弄虚拟线性表！代码很多！那就复制出错误的一个吧！using System;
using System.Drawing;

namespace WindowsApplication1
{
    public class LinkedList
    {
        private int count;
        private MemoryUnit _head;
        private Heap _heap;
        private Color _background;

        public LinkedList(Heap heap, Color color)
        {
            _heap = heap;
            _background = color;
        }
        public void Add(string value)
        {
            MemoryUnit newNode = _heap.NewUnit(_background);
            newNode.Value = value;
            if (_head == null)
            {
                _head = newNode;
            }
            else
            {
                GetByIndex(count - 1).Next = newNode;                                                   
            }
            count++;
        }
        public void Insert(int index, string value)
        {
            MemoryUnit newNode = _heap.NewUnit(_background);
            newNode.Value = value;
            if (index == 0)
            {
                if (_head == null)
                {
                    _head = newNode;
                }
                else
                {
                    newNode.Next = _head;
                    _head = newNode;
                }
            }
            else
            {
                MemoryUnit PrevNode = GetByIndex(index - 1);
                MemoryUnit nextNode = PrevNode.Next;
                PrevNode.Next = newNode;
                newNode.Next = nextNode;
            }
            count++;
        }
        public void RemoveAt(int index)
        {
            if (index == 0)
            {
                _head.SetUnuse();
                _head = _head.Next;
            }
            else
            {
                MemoryUnit prevNode = GetByIndex(index - 1);
                if (prevNode.Next == null)
                {
                    throw new ArgumentOutOfRangeException("索引超出范围");
                }
                prevNode.Next.SetUnuse();
                prevNode.Next = prevNode.Next.Next;
            }
            count--;
        }
        private MemoryUnit GetByUndex(int index)
         {
             if((index<0)||(index>=this.count))
             {
                 throw new ArgumentOutOfRangeException("索引超出范围");
             }
             MemoryUnit tempNode=this._head;
             for(int i=0;i<index;i++)
             {
                 tempNode=tempNode.Next;
             }
             return tempNode;
         }
    }
}