C＃中谓词和Func委托之间的区别 [英] differences between Predicate and Func delegate in C#

问题描述

我是C＃的初学者，只是一个关于C＃中的谓词和Func委托之间的区别的问题

I'm a beginner in C#, just a question on differences between Predicate and Func delegate in C#

我们知道：

public delegate bool Predicate<in T>(T obj);

public delegate TResult Func<in T, out TResult>(T arg);

所以，如果我喜欢

Func<Product, bool> firstdelegate = ...

和

Predicate<Product> secondpredicate = ...

它们不一样吗？我是说它们都做同样的事情，所以为什么要在LINQ的Where（）方法中使用Func类型委托而不是谓词呢？

aren't they the same? I mean they all do the same thing, so why in LINQ' Where() method takes Func type delegate instead of Predicate?

推荐答案

谓词T< code> 是在.NET 2.0中引入泛型的。这是一个接受一个参数并返回 bool 的委托。

Predicate<T> was introduced in .NET 2.0 with the introduction of generics. It's a delegate taking one parameter and returning a bool.

不过，.NET 3.5中引入了LINQ ，则确定需要两个通用类型的家族- Func 和 Action （区别在于它们是否返回任何东西）最多占用 16 4 ¹ 个通用输入参数，并且在返回类型上是通用的。如果先存在 Func ，则首先不会创建 Predicate< T> 。

However, with the introduction of LINQ in .NET 3.5, a need was identified for two families of generic types - Func and Action (the difference being on whether they return anything) taking up to 16 4¹ generic input parameters and being generic in their return types. If Func had existed first, Predicate<T> would never have been created in the first place. It's an unnecessarily specialized delegate type.

尽管出于向后兼容的原因，它们现在不能删除 Predicate< T> 。 有些可能会争辩说它的名称确实传达了特定的语义含义，但我很难确定许多情况，其中任何 Func< T，bool> （或 Func< T1，T2，bool> 等）不被视为谓词。

For backwards compatibility reasons though, they cannot now remove Predicate<T> from the framework. Some may argue that it's name does convey specific semantic meaning but I'd struggle to identify many situations where any Func<T,bool> (or Func<T1,T2,bool>, etc) wouldn't be considered a predicate.

¹ 4，.NET 4及更高版本中的16。

¹4 in .NET 3.5, 16 in .NET 4 and later.

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