C#中的异步/等待和并行 [英] Async/await and parallel in C#
问题描述
何时应在C#中使用async/await，何时应使用parallel.foreach?并行和异步/等待是否达到相同的目的? 它们有什么区别?
When should I use async/await and when should I use parallel.foreach in C#? Are parallel and async/await serve the same purpose? What are the differences in them?
推荐答案
异步/等待与异步有关，而Parallel.ForEach与并行性有关.它们是相关的概念，但并不相同.
async/await is about asynchrony, whereas Parallel.ForEach is about parallelism. They're related concepts, but not the same.
Parallel.ForEach用于当您要对集合中的所有项目并行执行相同的操作时，阻塞当前线程，直到所有操作完成.
Parallel.ForEach is used when you want to execute the same operation on all the items in a collection, in parallel, blocking the current thread until all operations have completed.
async/await用于在特定异步操作完成之前当前操作无法进行任何操作，但又不想阻塞当前线程的情况.这在两种情况下特别有用:
async/await is used when the current operation can't make any more progress until a particular asynchronous operation has completed, but you don't want to block the current thread. This is particularly useful in two situations:
- 基本上在UI线程上一个接一个地启动各种异步操作的编写代码，在两次操作之间短暂地访问UI. (比这更笼统，但这是一个简单的示例.)您不想阻塞UI线程，但是管理所有这些异步操作则很痛苦. 一次
- 一次处理很多长时间运行的操作-例如在Web服务器中.由于调用其他Web服务，数据库等，每个单独的请求可能会花费很长时间-但您不希望每个请求都有一个线程，因为线程是一种相对昂贵的资源.
- Writing code which basically kicks off various asynchronous operations, one after the other, on the UI thread, accessing the UI briefly between operations. (It's more general than that, but that's a simple example.) You don't want to block the UI thread, but managing all of those asynchronous operations is a pain otherwise.
- Handling lots of long-running operations at a time - e.g. in a web server. Each individual request may take a long time due to calling into other web service, databases etc - but you don't want to have one thread per request, as threads are a relatively expensive resource.
您可以通过启动一个将调用Parallel.ForEach的新任务，然后等待该任务来结合并行性和异步性. (仅举一个例子.)
You can combine parallelism and asynchrony by kicking off a new task which will call Parallel.ForEach, and then awaiting that task. (Just as one example.)
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