Dev Blog

Summary ​

Conclusions ​

Ordinary .NET collections are not designed for concurrent writes: they lose data, throw exceptions, or hang, and compound check-then-act operations are unsafe even with thread-safe collections. The concurrent collections in System.Collections.Concurrent offer atomic Try…, GetOrAdd, and AddOrUpdate methods; some use lock-free algorithms based on CAS, such as the Treiber stack. Dictionary factories execute outside locks, so expensive computations are wrapped in Lazy<T>. Immutable and frozen collections allow lock-free reads, with updates performed by atomically replacing a reference. The producer–consumer pattern is implemented using BlockingCollection<T> for threads or channels for asynchronous code; bounded capacity provides backpressure, and a completion signal allows the pipeline to stop correctly. False sharing destroys scalability when threads write to neighboring variables on the same cache line; eliminate it with local variables and structure padding, and detect it using measurements, the dotTrace profiler, and perf c2c.

Self-check questions ​

  1. Why must List<T> and Dictionary<TKey, TValue> not be modified from multiple threads?
  2. What is a compound check-then-act operation? How can it be fixed?
  3. What does Interlocked.CompareExchange do? What is a lock-free algorithm?
  4. How does the Treiber stack work?
  5. What is the ABA problem, and why is it less severe in .NET?
  6. How do ConcurrentQueue<T>, ConcurrentStack<T>, and ConcurrentBag<T> differ?
  7. When is ConcurrentBag<T> efficient, and when is it not?
  8. Why can a GetOrAdd factory execute several times? How can this be prevented?
  9. What requirements must an AddOrUpdate update function meet?
  10. How do immutable collections allow lock-free reads? What is ImmutableInterlocked for?
  11. Which scenarios is FrozenDictionary<TKey, TValue> designed for?
  12. What capabilities does BlockingCollection<T> add? How do you stop consumers?
  13. What is backpressure? Compare a completion signal with a “poison pill.”
  14. What full modes does a bounded channel provide? How does a channel differ from BlockingCollection<T>?
  15. What are a cache line and false sharing? How can false sharing be detected and eliminated?