Menu
This blog post mentions the new Task APIs, including a new Task.CompletedTask property introduced in .NET 4.6.
Why was this added? How is this better than, say, Task.FromResult(whatever)?
659
Task. CompletedTask property is important when you need to give a caller a dummy Task (that doesn't return a value/result) that's already completed. This might be necessary to fulfill an "interface" contract or testing purposes. Task. FromResult(data) also returns a dummy Task, but this time with data or a result.
We can return null from a method that returns a Task because Task is a reference type. In our previous example, we return null from NonAsyncFoo() . But, awaiting null isn't legal, so await NonAsyncFoo() throws a NullReferenceException .
The recommended return type of an asynchronous method in C# is Task. You should return Task<T> if you would like to write an asynchronous method that returns a value. If you would like to write an event handler, you can return void instead. Until C# 7.0 an asynchronous method could return Task, Task<T>, or void.
Task.FromResult(whatever) works for Task<TResult>, but until 4.6 there was nothing for the nongeneric task. You could use FromResult with a dummy value and implicitly cast it to Task, but that somewhat obfuscates the intent (you're not really returning any asynchronous value) and allocates objects underneath (whereas CompletedTask can be cached and shared between all callers).
It's not uncommon to see custom static completed tasks in current (4.5.2 and older) codebases, so in my opinion it makes sense to incorporate it to the framework itself.
151
Task.CompletedTask property is important when you need to give a caller a dummy Task (that doesn't return a value/result) that's already completed. This might be necessary to fulfill an "interface" contract or testing purposes.
Task.FromResult(data) also returns a dummy Task, but this time with data or a result. You probably would be doing this because you already have the data and have no need to perform any operation to get it.
Example - Task.CompletedTask
public Task DoSomethingAsync() { return Task.CompletedTask; // null would throw exception on await } Example - Task.FromResult(data)
public Task<User> GetUserAsync() { if(cachedUser != null) { return Task.FromResult(cachedUser); } else { return GetUserFromDb(); } } If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
