Differences
This shows you the differences between two versions of the page.
| — |
csharp:async_await [2019/10/08 18:45] (current) admin created |
||
|---|---|---|---|
| Line 1: | Line 1: | ||
| + | ====== async/await ====== | ||
| + | |||
| + | ''await'' is an //asynchronous wait//: | ||
| + | <code csharp> | ||
| + | await operation(); | ||
| + | </code> | ||
| + | The operation called after the await keyword is an "awaitable". When execution reaches ''await'': | ||
| + | * if the awaitable operation has already completed, execution proceeds to the next statement in the containing method (like any normal synchronous operation); | ||
| + | * if the awaitable operation has //not// already completed, the containing method returns. The rest of the method is executed as soon as the awaitable has completed. | ||
| + | |||
| + | Methods containing ''await'' must: | ||
| + | * return an [[#Awaitable expression types|awaitable expression type]] or ''void'' (''void'' only really makes sense in fire-and-forget notification/event handling scenarios); | ||
| + | * be flagged ''async'' in method signature -- this indicates to the compiler (and to human readers) that the method contains an ''await''. (This also removes ambiguity by preventing the ''await'' keyword from being used as an identifier in the body of an ''async'' method -- elsewhere it acts as any other identifier). | ||
| + | |||
| + | ===== Awaitable expression types ===== | ||
| + | |||
| + | An expression is awaitable if it is of a type that provides a [[https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/expressions#awaitable-expressions|specific set of properties]]. | ||
| + | |||
| + | The types ''[[https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task|Task]]'' and ''Task<T>'' are awaitable; you could also roll your own. | ||
| + | |||
| + | If you don't need a result immediately (so can call an operation asynchronously) but will need it by some future time, ''Task.Wait()'' is a blocking call that returns only when the asynchronous operation has completed. | ||
| + | |||
| + | ==== Task<T> ==== | ||
| + | |||
| + | ''Task<T>'' enables you to return a value (of type ''T'') from an ''async'' method. The Task wrapper provides a reference to the awaited operation, allowing you to access the returned value when the asynchronous operation has completed. | ||
| + | |||
| + | Access to the returned value is provided by the ''Task.Result'' property; calling the property's get accessor also blocks the calling thread (equivalent to calling ''Task.Wait()''). | ||
| + | |||
| + | ===== See also ===== | ||
| + | |||
| + | [[https://blog.stephencleary.com/2012/02/async-and-await.html]] | ||
| + | |||
| + | {{tag> csharp }} | ||
| + | ~~DISCUSSION~~ | ||
| + | |||