等待await vs Unwrap()

本文介绍了等待await vs Unwrap()的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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 ║ ╚══════════════════╩═══════════════════════╩═════════════════╝