await await vs Unwrap()

问题

Given a method such as

public async Task<Task> ActionAsync()
{
    ...
}

What is the difference between

await await ActionAsync();

and

await ActionAsync().Unwrap();

if any.

回答1:

Unwrap() creates a new task instance that represent whole operation on each call. In contrast to await task created in such a way is differ from original inner task. See the Unwrap() docs, and consider the following code:

private async static Task Foo()
{
    Task<Task<int>> barMarker = Bar();

    Task<int> awaitedMarker = await barMarker;
    Task<int> unwrappedMarker = barMarker.Unwrap();

    Console.WriteLine(Object.ReferenceEquals(originalMarker, awaitedMarker));
    Console.WriteLine(Object.ReferenceEquals(originalMarker, unwrappedMarker));
}

private static Task<int> originalMarker;
private static Task<Task<int>> Bar()
{
    originalMarker = Task.Run(() => 1);;
    return originalMarker.ContinueWith((m) => m);
}

Output is:

True
False

Update with benchmark for .NET 4.5.1: I tested both versions, and it turns out that version with double await is better in terms of memory usage. I used Visual Studio 2013 memory profiler. Test includes 100000 calls of each version.

x64:

╔══════════════════╦═══════════════════════╦═════════════════╗
║ Version          ║ Inclusive Allocations ║ Inclusive Bytes ║
╠══════════════════╬═══════════════════════╬═════════════════╣
║ await await      ║ 761                   ║ 30568           ║
║ await + Unwrap() ║ 100633                ║ 8025408         ║
╚══════════════════╩═══════════════════════╩═════════════════╝

x86:

╔══════════════════╦═══════════════════════╦═════════════════╗
║ Version          ║ Inclusive Allocations ║ Inclusive Bytes ║
╠══════════════════╬═══════════════════════╬═════════════════╣
║ await await      ║ 683                   ║ 16943           ║
║ await + Unwrap() ║ 100481                ║ 4809732         ║
╚══════════════════╩═══════════════════════╩═════════════════╝

回答2:

There won't be any functional difference.

来源：https://stackoverflow.com/questions/34816628/await-await-vs-unwrap

标签

async-await