November 2020
Intermediate to advanced
226 pages
5h 34m
Chinese
Content preview from C#并发编程经典实例（第2版）
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44
｜
第
3
章
3.7
对异步流使用
LINQ
问题
假设要使用经过妥善定义以及完善测试的运算符，来处理异步流。
解决方案
IEnumerable<T>
具有
LINQ to Objects
，而
IObservable<T>
具有
LINQ to Events
。两者都有
扩展方法库，这些方法所定义的运算符可供建立查询。
IAsyncEnumerable<T>
也具备
LINQ
支持，该支持由
.NET
社区提供，包含于
System.Linq.Async
NuGet
包中。
举个例子，关于
LINQ
的一个常见疑问是，如果
Where
的谓词是异步的，该如何使用
Where
运算符。假设要根据一些异步条件过滤序列，比如需要在数据库或
API
中查询每个元素，
判断是否应将其包含在结果序列里。
Where
并不能在异步条件下工作，这是因为
Where
运
算符的委托必须立即返回同步的答案。
异步流的支持库定义了许多有用的运算符。在下面的示例中，
WhereAwait
是个恰当的选择：
IAsyncEnumerable<
int
> values = SlowRange().WhereAwait(
async value
=>
{
// 执行某些异步操作，判断是否应当包含该元素
await
Task.Delay(10);
return value
% 2 == 0;
});
await foreach
(
int
result
in
values)
{
Console.WriteLine(result);
}
// 生成序列，该序列逐渐减缓 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.