What is ValueTask? Why ValueTask Could Be Your Key to Faster Async Code ?

#csharp #threading #parallel #microsoft

ValueTask is a structure introduced in .NET for scenarios requiring micro-optimizations in asynchronous programming. It provides an alternative to Task with better performance for specific use cases, such as when the result of an asynchronous operation is already available or can be synchronously produced.

Key Characteristics:

ValueTask can be used for both asynchronous and synchronous results, while Task is inherently asynchronous.
Unlike Task, which always allocates a heap object, ***ValueTask* **avoids allocation when the result is immediately available.
It’s particularly useful in high-performance scenarios to reduce unnecessary allocations.

Common Use Cases:

Operations that are frequently completed synchronously, where allocation overhead from Task would be significant.
APIs implementing IAsyncEnumerable, which often return ValueTask for efficiency.

Precautions When Using ValueTask
Avoid Awaiting the Same Instance Multiple Times

Unlike Task, a ValueTask is not reusable. Awaiting it multiple times can lead to undefined behavior.

ValueTask<int> task = SomeMethodAsync();
await task; // Valid
await task; // Undefined behavior

Avoid .GetAwaiter().GetResult() If the Operation Isn’t Completed
Avoid Using the Instance After It’s Awaited
Once awaited, the state of ValueTask can no longer be relied upon, unlike Task, which can be awaited multiple times.
Avoiding excessive bouncing
For methods that are called many times in a loop, you can avoid the cost of
repeatedly bouncing to a UI message loop by calling ConfigureAwait. This forces
a task not to bounce continuations to the synchronization context, cutting the
overhead closer to the cost of a context switch