Under what circumstance System.Collections.ArrayList.Add throws IndexOutOfRangeException?

Question

We are experiencing weird bug at production environment we cannot debug nor inject logging code. I am trying to figure this up but following stack trace confuse me.

Answer 1

It boils down to List not being thread safe. I have had IndexOutOfRangeException occuring when iterating over a list after adding items using multiple threads without synchronization. The code below could corrupt the items count and result in IndexOutOfRangeException while iterating the list subsequently,

List updatedFills = new List();
Parallel.ForEach (trades, (trade) =>
{
    TradeFillInfo fill = new TradeFillInfo();

    //do something

    updatedFills.Add(fill); //NOTE:Adding items without synchronization
});

foreach (var fill in updatedFills) //IndexOutOfRangeException here sometimes
{
    //do something
}

Synchronizing the Add() with a lock statement will fix the issue.

lock (updatedFills)
{
    updatedFills.Add(fill);
}