Adding to a generic dictionary causes IndexOutOfRangeException

Question

I'm using a dictionary inside of some Task.

Logically I have set it up so that my Keys will never clash, though sometimes when I am adding to the dictionary I get this Exception.

Index was outside the bounds of the array.
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Rpc.<MapIntoRpc>b__4[T](Object x) in Rpc.cs:line 113
   at System.Threading.Tasks.Task`1.InvokeFuture(Object futureAsObj)
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()

I understand there can be concurrency issues from trying to remove or add the same Key multiple times, but I have accounted for that algorithmically.

What causes the add to sometimes fail? What is the best way to work around that?

Answer 1

You should have looked to the documentation. That what it says:

A Dictionary can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization. For a thread-safe alternative, see ConcurrentDictionary.