Thursday, June 14, 2018

C#–Task vs ValueTask

The combination of async/await with the power of the TPL(Task Parallel Library) makes async programming in .NET a breeze. The only disadvantage of using the Task based programming model, is that it’s like a virus that starts to spread all around your code. Since Task (Task) is a reference type, returning Task object from async method means allocating it on heap every time. And this is needed in many cases.

In these cases where the async method returns a result immediately or completes synchronously, this allocation is unnecessary and can become costly. To avoid this extra allocation a ValueTask structure was added to .NET 4.7.

The compiler doesn’t care if your async method returns a Task or ValueTask, so there are completely interchangeable;

So why shouldn’t I switch to ValueTask everywhere?

There are some tradeoffs you must take into consideration before using ValueTask. ValueTask is a value type with 2 fields. The Task is a reference type with just one field. When you use ValueTask we have more data to handle and take care of. If such method is awaited within an async method, the state machine for that async method will be larger as well, because storing the whole structure, in general, requires more space than storing a single reference.

Conclusion

The recommendation is to use Task as default return type, only after performance analysis, you should consider using ValueTask instead.

More information: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/async-return-types

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