I noticed an invalid implementation of a reader / writer lock, that I wanted to bring to your attention, due to potential scalability issues.

In the included method below, an exclusive lock on the instance variable, _readerLockerDefs is obtained, preventing any other threads from reading the cache at the same time. The _writerLockerDefs is actually redundant here, since it is also an instance variable.

Either a ReaderWriterLock or a custom implementation of the Reader/Writer pattern is needed.

Cheers,

Stuart

private WorkflowFoundationPageFlowDefinition GetOrCreateDefinition(Type pageFlowType, WorkflowInstance instance)
{

WorkflowFoundationPageFlowDefinition cachedDefinition;
lock (_readerLockerDefs)
{
    if (!_workflowDefinitionsCache.TryGetValue(pageFlowType, out cachedDefinition))
    {
        lock (_writerLockerDefs)
        {
            if (!_workflowDefinitionsCache.TryGetValue(pageFlowType, out cachedDefinition))
            {
                cachedDefinition = new WorkflowFoundationPageFlowDefinition((Activities.PageFlow)instance.GetWorkflowDefinition());
                _workflowDefinitionsCache.Add(pageFlowType, cachedDefinition);
            }
        }
    }
}
return cachedDefinition;

}