Returning the innerlist from a custom collection inheriting from List<T>

I have made a custom collection that inherits from List, I have created a methods that does some work to my data. All my manipulation is done from a private field inside the Collection.

I have set my Private field to reference the this object, I then need to sort my data so I can do my calculation. I do it with Linq, thinking that *this and my Private Field are the same. But changes made with Linq does not change the this object. While every other change I made does change it.

class OrdersDetailCollection : List<OrdersDetail>
{
    List<OrdersDetail> _List;
    public OrdersDetailCollection(IEnumerable<OrdersDetail> input)
    {
        this.AddRange(input);
        _List = this;
        _List = (from l in _List
                 orderby l.Item, l.DateExp
                 select l).ToList();
    }

    public void CalculateQty()
    {
        RemoveDuplicate();

        _List = (from l in _List
                 where double.开发者_运维百科Parse(l.QteBO) > 0
                 orderby l.DateExp, l.Item
                 select l).ToList();
    }

    private void RemoveDuplicate()
    {
       //Do some stuff to _List
    }

Shouldn't this and _List be the same since they reference the same space in memory? How to assign _List to this?

What is missing from with my Custom Collection so that I can use it the same way as a List and do the following (simplified)?

OrdersDetailCollection ShipList = new OrdersDetailCollection(/*Value from SQL*/)
ShipList.CalculateQty();
return ShipList;

I want to know how to do this without a extension method, I am thinking yield may have something to do with it, but don't understand it really well.

Changing _List is simply changing the field (not the actual list) to a completely unrelated list. In short inheritance isn't going to work well for what you want here; you can't reassign "this" (at least, not for a class).

I suspect encapsulating the list would work better for you.

Your call to ToList creates a new reference which is no longer equal to this.

It looks like you probably would be better off with this class being a "helper" by holding onto the list:

public class OrderDetailCollection
{
    private List<OrdersDetail> _List;
    public OrdersDetailCollection(IEnumerable<OrdersDetail> input)
    {
        this._List = input.OrderBy(xx => xx.Item)
                          .ThenBy(xx => xx.DateExp)
                          .ToList();
       RemoveDuplicates();
    }

    public IEnumerable<OrdersDetail> CalculateQuantity()
    {
         return from xx in this._List
               where double.Parse(xx.QteBO) > 0.0
               orderby xx.DateExp, xx.Item
               select xx;
    }
}

From the suggestion I got the problems seems to be that the .ToList in the Linq query reference a new list. So I just have added a Clear and a AddRange after the Linq query. But the best way would have been to not try inheritance. So here is the answer.

class OrdersDetailCollection : List<OrdersDetail>
{
    List<OrdersDetail> _List;
    public OrdersDetailCollection(IEnumerable<OrdersDetail> input)
{
    this.AddRange(input);
    _List = this;
    _List = (from l in _List
             orderby l.Item, l.DateExp
             select l).ToList();

    this.Clear();
    this.AddRange(_List);
}

public void CalculateQty()
{
    RemoveDuplicate();

    _List = (from l in _List
             where double.Parse(l.QteBO) > 0
             orderby l.DateExp, l.Item
             select l).ToList();

    this.Clear();
    this.AddRange(_List);
}