Performance Guidelines

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Consider using `Any()` to determine whether an `IEnumerable` is empty (AV1800)

When a member or local function returns an IEnumerable<T> 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<T> to a persistent store).

Note: If you return an IEnumerable<T> to prevent changes from calling code as explained in AV1130, and you’re developing in .NET 4.5 or higher, consider the new read-only classes.

Only use `async` for low-intensive long-running activities (AV1820)

The usage of async won’t automagically 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.

Prefer `Task.Run` or `Task.Factory.StartNew` for CPU-intensive activities (AV1825)

If you do need to execute a CPU bound operation, use Task.Run to offload the work to a thread from the Thread Pool. For long-running operations use Task.Factory.StartNew with TaskCreationOptions.LongRunning parameter to create a new thread. Remember that you have to marshal the result back to your main thread manually.

Beware of mixing up `async`/`await` with `Task.Wait` (AV1830)

await does not block the current thread but simply instructs the compiler to generate a state-machine. However, Task.Wait blocks the thread and may even cause deadlocks (see AV1835).

Beware of `async`/`await` deadlocks in single-threaded environments (AV1835)

Consider the following asynchronous method:

private async Task GetDataAsync()
	var result = await MyWebService.GetDataAsync();
	return result.ToString();

Now when an ASP.NET MVC controller action does this:

public ActionResult ActionAsync()
	var data = GetDataAsync().Result;
	return View(data);  

You end up with a deadlock. Why? Because the Result property getter will block until the async operation has completed, but since an async method could automatically marshal the result back to the original thread (depending on the current SynchronizationContext or TaskScheduler) and ASP.NET uses a single-threaded synchronization context, they’ll be waiting on each other. A similar problem can also happen on UWP, WPF or a Windows Store C#/XAML app. Read more about this here.