I recently needed to do a running total on a report. Where for each group, I order the rows and then calculate the running total based on the previous rows within the group. Aha! I thought, a perfect use case for PLINQ!

However, when I wrote up the code, I got some strange behavior. The values I was modifying showed as being modified when stepping through the debugger, but when they were accessed they were always zero.

Sample Code:

 class Item
 {
  public int PortfolioID;
  public int TAAccountID;
  public DateTime TradeDate;
  public decimal Shares;
  public decimal RunningTotal;
 }

    List<Item> itemList = new List<Item>
   {
    new Item
    {
     PortfolioID = 1,
     TAAccountID = 1,
     TradeDate = new DateTime(2010, 5, 1),
     Shares = 5.335m,
    },
    new Item
    {
     PortfolioID = 1,
     TAAccountID = 1,
     TradeDate = new DateTime(2010, 5, 2),
     Shares = -2.335m,
    },
    new Item
    {
     PortfolioID = 2,
     TAAccountID = 1,
     TradeDate = new DateTime(2010, 5, 1),
     Shares = 7.335m,
    },
    new Item
    {
     PortfolioID = 2,
     TAAccountID = 1,
     TradeDate = new DateTime(2010, 5, 2),
     Shares = -3.335m,
    },

   };

   var found = (from i in itemList
      where i.TAAccountID == 1
      select new Item
      {
       TAAccountID = i.TAAccountID,
       PortfolioID = i.PortfolioID,
       Shares = i.Shares,
       TradeDate = i.TradeDate,
       RunningTotal = 0
      });




   found.AsParallel().ForAll(x =>
   {
    var prevItems =  found.Where(i => i.PortfolioID == x.PortfolioID
     && i.TAAccountID == x.TAAccountID 
     && i.TradeDate <= x.TradeDate);
    x.RunningTotal = prevItems.Sum(s => s.Shares);
   });

   foreach (Item i in found)
   {
    Console.WriteLine("Running total: {0}", i.RunningTotal);
   }

   Console.ReadLine();

If I change the select for found to be .ToArray(), then it works fine and I get calculated reuslts.

Any ideas what I am doing wrong?