问题描述
有真实的例子和他们的使用，有人可以帮助我理解:
With real examples and their use, can someone please help me understand:
- 我们什么时候需要
Func - 我们什么时候需要
Action<T>委托? - 我们什么时候需要
谓词<T>委托?
推荐答案
Func 和 Action 的区别只是你是否希望委托返回一个值(使用Func) 与否(使用 Action).
The difference between Func and Action is simply whether you want the delegate to return a value (use Func) or not (use Action).
Func 可能是 LINQ 中最常用的——例如在投影中:
Func is probably most commonly used in LINQ - for example in projections:
list.Select(x => x.SomeProperty)
或过滤:
list.Where(x => x.SomeValue == someOtherValue)
或按键选择:
list.Join(otherList, x => x.FirstKey, y => y.SecondKey, ...)
Action 更常用于 List<T>.ForEach 之类的事情:对列表中的每个项目执行给定的操作.我使用它的频率低于 Func，尽管我 确实 有时将无参数版本用于 Control.BeginInvoke 和 Dispatcher.BeginInvoke.
Action is more commonly used for things like List<T>.ForEach: execute the given action for each item in the list. I use this less often than Func, although I do sometimes use the parameterless version for things like Control.BeginInvoke and Dispatcher.BeginInvoke.
Predicate 实际上只是一个特殊的 Func，在所有 Func 和大部分 之前引入>Action 代表出现了.我怀疑如果我们已经有各种形式的 Func 和 Action ，那么 Predicate 就不会被引入......尽管它确实赋予委托的使用某种意义，而 Func 和 Action 用于广泛不同的目的.
Predicate is just a special cased Func<T, bool> really, introduced before all of the Func and most of the Action delegates came along. I suspect that if we'd already had Func and Action in their various guises, Predicate wouldn't have been introduced... although it does impart a certain meaning to the use of the delegate, whereas Func and Action are used for widely disparate purposes.
Predicate 主要用于 List<T> 中，用于 FindAll 和 RemoveAll 等方法.
Predicate is mostly used in List<T> for methods like FindAll and RemoveAll.
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