Question

I have the following code:

var actionsToExecute = _messagesToExecute.Where(m => m.CanExecute).ToList();

It runs fine 99% of the time, but every once in a while it will crash with the exception:

Collection was modified; enumeration operation may not execute

I am a bit lost as it seems a bit random. This is the first line in the method. What might cause a lambda expression to throw this exception?

Answer 1

You either need to use lock() around all your references to _messagesToExecute, or you could use something from System.Collections.Concurrent which handle locking the collection internally.

e.g.

_messagesToExecute = new ConcurrentBag<TMessage>();

or if you’d rather use locking:

static readonly object m_lock = new object();

then whenever you update the list:

lock(m_lock){
  _messagesToExecute.Add(item);
}

and then when you’re pulling out the list:

lock(m_lock){
    var actionsToExecute = _messagesToExecute.Where(m => m.CanExecute).ToList();
}