List Model --> Orderbydescending System. Datetime Does Not Work?

Sorry if this is a duplicate but I can't find a StackOverflow post that works for me.

I'm getting miffed over learning how to use list model w/ linq. My problem here is sorting by DateTime had no effect. I'm using .NET framework v4.5. I'm using SQL DataReader to read data into list model, but instead of writing/posting sql object, I'm gonna manually specify the adding of data to list model manually for this posting.

public MyInventory : IDisposable
{
    public MyInventory {
        PurchaseId = -1;
        StockDate = null;
    }  
    public void Dispose() {
        //PurchaseId...
        StockDate = null;
    } 
    public long PurchaseId { get; set; }
    public DateTime? StockDate { get; set; }
}

List<MyInventory> modelMyInventory = new List<MyInventory>();

modelMyInventory.Add(new MyInventory { PurchaseId = 2, StockDate = DateTime.Parse("01-02-2010") });
modelMyInventory.Add(new MyInventory { PurchaseId = 5, StockDate = DateTime.Parse("01-03-2011") });
modelMyInventory.Add(new MyInventory { PurchaseId = 7, StockDate = DateTime.Parse("01-01-2010") });

modelMyInventory.OrderByDescending(m => m.StockDate);

Thanks...