Exactly - the problem I'm usually facing with concurrent collection access in C# and Java is that the concurrent ccess wasn't even known or even intended. Having a mode or a special set of collections that guarantee that they are never accessed from two threads would be very useful to detect unintended sharing. It wouldn't help in scenarios where things are passed between threads sequentially but for that case at least the sharing is intended and can be reasoned about. The bugs are invariably in code no one even considered involved two threads at all. Having e.g a debug-only optional mode that verifies all calls to collection classes are from the same thread would provide a way of trapping many of these.
C# makes it very easy to avoid the normal threading issues by using Task<T> and other high level helpers - which is excellent. The problem I'm usually facing is isolating what the code within the task can "reach". Normally the main program code is a huge amount of code that must be single threaded. Tons of caches using vanilla dictionaries and so on. Now there is a small perf critical thing I need to do with Tasks. The failure mode is that the code from in the task somehow indirectly (e.g through some property on an object ending up in a cache backed by a dictionary) ends up accessing shared mutable state. Soparating the code used concurrently from the rest becomes a mental overhead. So anything that helps me do that would help.
C# makes it very easy to avoid the normal threading issues by using Task<T> and other high level helpers - which is excellent. The problem I'm usually facing is isolating what the code within the task can "reach". Normally the main program code is a huge amount of code that must be single threaded. Tons of caches using vanilla dictionaries and so on. Now there is a small perf critical thing I need to do with Tasks. The failure mode is that the code from in the task somehow indirectly (e.g through some property on an object ending up in a cache backed by a dictionary) ends up accessing shared mutable state. Soparating the code used concurrently from the rest becomes a mental overhead. So anything that helps me do that would help.