C# Performance Standards · CS-08

When a method or other member returns an IEnumerable or other collection class that does not expose a Count property, use the Any() extension method rather than Count() to determine whether the collection contains items. If you do use Count(), you risk that iterating over the entire collection might have a significant impact (such as when it really is an IQueryable to a persistent store).

NOTE: If you return an IEnumerable to prevent editing from outside the owner as explained in here and you’re developing in .NET 4.5+, consider the new read-only classes.

Roslyn Analyzer Rule CA1827

The usage of async won’t automatically run something on a worker thread like Task.Run does. It just adds the necessary logic to allow releasing the current thread and marshal the result back on that same thread if a long-running asynchronous operation has completed. In other words, use async only for I/O bound operations.

If you do need to execute a CPU bound operation, use Task.Run to offload the work to a thread from the Thread Pool. Just don’t forget that you have to marshal the result back to your main thread manually.

await will not block the current thread but simply instruct to compiler to generate a state-machine. However, Task.Wait will block the thread and may even cause deadlocks.