Exception: Index outside the bounds of the array when materializing objects

Using LightSpeed 4.0.1195.19954

I'm seeing the following exception occasionally, especially under initial load (i.e. seeing multiple similar requests after an IIS reset):

System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at Mindscape.LightSpeed.Linq.Utils.PartialEvaluator.SubtreeEvaluator.Evaluate(Expression e)
   at Mindscape.LightSpeed.Linq.Utils.PartialEvaluator.SubtreeEvaluator.Visit(Expression exp)
   at Mindscape.LightSpeed.Linq.Plan.ExpressionVisitor.VisitBinary(BinaryExpression binary)
   at Mindscape.LightSpeed.Linq.Plan.ExpressionVisitor.VisitLambda(LambdaExpression lambda)
   at Mindscape.LightSpeed.Linq.Plan.ExpressionVisitor.VisitUnary(UnaryExpression unary)
   at Mindscape.LightSpeed.Linq.Plan.ExpressionVisitor.VisitExpressionList(ReadOnlyCollection`1 original)
   at Mindscape.LightSpeed.Linq.Plan.ExpressionVisitor.VisitMethodCall(MethodCallExpression methodCall)
   at Mindscape.LightSpeed.Linq.LinqQueryProvider.GetExecutionPlan(Expression expression)
   at Mindscape.LightSpeed.Linq.LinqQueryProvider.System.Linq.IQueryProvider.Execute[S](Expression expression)
   at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)

Looking through Reflector, this code isn't protecting its Dictionary (where I've seen this error in code before).

Is there anything I should be doing in a thread-safe App_Start or similar to prevent this occurring?

Hmm, looks like there's a race condition deep within Dictionary that means the indexer isn't last-one-wins but can fail when called concurrently. Fix going in tonight. The only workaround is to ensure that the first execution of any given LINQ query is done in a thread-safe way, which is probably not practical unless you have a very limited number of queries.

Yes, the .NET 3.5 dictionary is a real pain - "Not thread safe" doesn't mean "results are unpredictable", it means "we will crash and burn".

I've been hit by it a few times in the past - hence why they brought out the ConcurrentDictionary in 4.

Cheers!