Parallel.ForEach local storage

Question

I recently changed a For Each loop to a Parallel.ForEach loop. I'm concerned about an object being declared outside the loop but assigned while iterating in the loop. Here is the simplified code.

Dim results As ModelResults
Dim noResultsModel As New List(Of ModelResults)
Dim lock As New Object
Parallel.ForEach(_modelEngines,
    Sub(model)

    results = model.Execute

    If results IsNot Nothing Then

        SyncLock lock
            noResultsModel.Add(results)
        End SyncLock

    End If

    results = Nothing

End Sub)

Is there a potential race condition with the results object? Would anything be different if I moved the declaration of results into the for loop?

Answer 1

Yes, there definitely is a race condition with the variable declared outside the loop:

Thread 1: results = model.Execute ' results are from Thread1's current modelEngine
Thread 2: results = model.Execute ' results are from Thread2's current modelEngine
Thread 2: If results IsNot Nothing Then ' results are from Thread2's current modelEngine
Thread 1: If results IsNot Nothing Then ' results are from Thread2's current modelEngine(!)

Just move it inside, I don't see why you'd want to declare it outside the loop anyways.