等待await vs Unwrap() [英] await await vs Unwrap()

问题描述

给出诸如

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

两者之间有什么区别

await await ActionAsync();

和

await ActionAsync().Unwrap();

(如果有).

推荐答案

Unwrap()创建一个新的任务实例，该实例代表每个调用的整个操作.与以这种方式创建的await任务相反，它与原始内部任务不同.请参见 Unwrap()文档，以及考虑以下代码:

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);
}

输出为:

True
False

使用.NET 4.5.1的基准更新:我测试了两个版本，事实证明，具有双await的版本在内存使用方面更好. 我使用了Visual Studio 2013内存探查器.测试包括每个版本的100000次调用.

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         ║
╚══════════════════╩═══════════════════════╩═════════════════╝

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