C#中的谓词委托

Question

提问by Canavar

Can you explain to me:

你能给我解释一下吗：

What is a Predicate Delegate?
Where should we use predicates?
Any best practices when using predicates?

什么是谓词委托？
我们应该在哪里使用谓词？
使用谓词时有什么最佳实践吗？

Descriptive source code will be appreciated.

描述性源代码将不胜感激。

Answer 1

采纳答案by Andrew Hare

A predicate is a function that returns trueor false. A predicate delegate is a reference to a predicate.

谓词是一个返回trueor的函数false。谓词委托是对谓词的引用。

So basically a predicate delegate is a reference to a function that returns trueor false. Predicates are very useful for filtering a list of values - here is an example.

所以基本上谓词委托是对返回trueor的函数的引用false。谓词对于过滤值列表非常有用 - 这是一个示例。

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List<int> list = new List<int> { 1, 2, 3 };

        Predicate<int> predicate = new Predicate<int>(greaterThanTwo);

        List<int> newList = list.FindAll(predicate);
    }

    static bool greaterThanTwo(int arg)
    {
        return arg > 2;
    }
}

Now if you are using C# 3 you can use a lambda to represent the predicate in a cleaner fashion:

现在，如果您使用 C# 3，您可以使用 lambda 以更简洁的方式表示谓词：

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List<int> list = new List<int> { 1, 2, 3 };

        List<int> newList = list.FindAll(i => i > 2);
    }
}

Answer 2

回答by Adam Carr

Just a delegate that returns a boolean. It is used a lot in filtering lists but can be used wherever you'd like.

只是一个返回布尔值的委托。它在过滤列表中被大量使用，但可以在您想要的任何地方使用。

List<DateRangeClass>  myList = new List<DateRangeClass<GetSomeDateRangeArrayToPopulate);
myList.FindAll(x => (x.StartTime <= minDateToReturn && x.EndTime >= maxDateToReturn):

Answer 3

回答by LukeH

There's a good article on predicates here, although it's from the .NET2 era, so there's no mention of lambda expressions in there.

有一个关于谓词的好文章在这里，虽然它从.NET2时代的，所以有lambda表达式没有提及在那里。

Answer 4

回答by WestDiscGolf

Leading on from Andrew's answer with regards to c#2 and c#3 ... you can also do them inline for a one off search function (see below).

从 Andrew 对 c#2 和 c#3 的回答开始……您也可以将它们内联用于一次性搜索功能（见下文）。

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List<int> list = new List<int> { 1, 2, 3 };

        List<int> newList = list.FindAll(delegate(int arg)
                           {
                               return arg> 2;
                           });
    }
}

Hope this helps.

希望这可以帮助。

Answer 5

回答by danlash

If you're in VB 9 (VS2008), a predicate can be a complex function:

如果您使用的是 VB 9 (VS2008)，谓词可以是一个复杂的函数：

Dim list As New List(Of Integer)(New Integer() {1, 2, 3})
Dim newList = list.FindAll(AddressOf GreaterThanTwo)
...
Function GreaterThanTwo(ByVal item As Integer) As Boolean
    'do some work'
    Return item > 2
End Function

Or you can write your predicate as a lambda, as long as it's only one expression:

或者，您可以将谓词编写为 lambda，只要它只有一个表达式：

Dim list As New List(Of Integer)(New Integer() {1, 2, 3})
Dim newList = list.FindAll(Function(item) item > 2)

Answer 6

回答by lukaszk

The predicate-based searching methods allow a method delegate or lambda expression to decide whether a given element is a “match.” A predicate is simply a delegate accepting an object and returning true or false: public delegate bool Predicate (T object);

基于谓词的搜索方法允许方法委托或 lambda 表达式决定给定元素是否是“匹配”。谓词只是一个接受对象并返回 true 或 false 的委托： public delegate bool Predicate (T object);

   static void Main()
        {
            string[] names = { "Lukasz", "Darek", "Milosz" };
            string match1 = Array.Find(names, delegate(string name) { return name.Contains("L"); });
            //or
            string match2 = Array.Find(names, delegate(string name) { return name.Contains("L"); });
            //or
            string match3 = Array.Find(names, x => x.Contains("L"));


            Console.WriteLine(match1 + " " + match2 + " " + match3);     // Lukasz Lukasz Lukasz
        }
        static bool ContainsL(string name) { return name.Contains("L"); }

Answer 7

回答by Gul Md Ershad

What is Predicate Delegate?

什么是谓词委托？

1) Predicate is a feature that returns true or false.This concept has come in .net 2.0 framework. 2) It is being used with lambda expression (=>). It takes generic type as an argument. 3) It allows a predicate function to be defined and passed as a parameter to another function. 4) It is a special case of a Func, in that it takes only a single parameter and always returns a bool.

1) Predicate 是一个返回true 或false 的特性。这个概念在.net 2.0 框架中已经出现。2) 它与 lambda 表达式 (=>) 一起使用。它接受泛型类型作为参数。3) 它允许定义一个谓词函数并将其作为参数传递给另一个函数。4) 这是 a 的一个特例Func，因为它只接受一个参数并且总是返回一个布尔值。

In C# namespace:

在 C# 命名空间中：

namespace System
{   
    public delegate bool Predicate<in T>(T obj);
}

It is defined in the System namespace.

它在 System 命名空间中定义。

Where should we use Predicate Delegate?

我们应该在哪里使用谓词委托？

We should use Predicate Delegate in the following cases:

我们应该在以下情况下使用 Predicate Delegate：

1) For searching items in a generic collection. e.g.

1) 用于搜索通用集合中的项目。例如

var employeeDetails = employees.Where(o=>o.employeeId == 1237).FirstOrDefault();

2) Basic example that shortens the code and returns true or false:

2) 缩短代码并返回 true 或 false 的基本示例：

Predicate<int> isValueOne = x => x == 1;

now, Call above predicate:

现在，调用上面的谓词：

Console.WriteLine(isValueOne.Invoke(1)); // -- returns true.

3) An anonymous method can also be assigned to a Predicate delegate type as below:

3) 匿名方法也可以分配给 Predicate 委托类型，如下所示：

Predicate<string> isUpper = delegate(string s) { return s.Equals(s.ToUpper());};
    bool result = isUpper("Hello Chap!!");

Any best practices about predicates?

关于谓词的任何最佳实践？

Use Func, Lambda Expressions and Delegates instead of Predicates.

使用 Func、Lambda 表达式和委托而不是谓词。

Answer 8

回答by parijat mishra

A delegate defines a reference type that can be used to encapsulate a method with a specific signature. C# delegate Life cycle:The life cycle of C# delegate is

委托定义了一种引用类型，可用于封装具有特定签名的方法。 C# 委托生命周期：C# 委托的生命周期是

Declaration
Instantiation
INVACATION

宣言
实例化
度假中

learn more form http://asp-net-by-parijat.blogspot.in/2015/08/what-is-delegates-in-c-how-to-declare.html

了解更多信息 http://asp-net-by-parijat.blogspot.in/2015/08/what-is-delegates-in-c-how-to-declare.html

Answer 9

回答by dexterous

Predicate falls under the category of generic delegates in C#. This is called with one argument and always return boolean type. Basically, the predicate is used to test the condition - true/false. Many classes support predicate as an argument. For e.g. list.findall expects the parameter predicate. Here is an example of the predicate.

谓词属于 C# 中的泛型委托类别。这是用一个参数调用的，并且总是返回布尔类型。基本上，谓词用于测试条件 - 真/假。许多类支持谓词作为参数。例如 list.findall 需要参数谓词。下面是谓词的一个例子。

Imagine a function pointer with the signature -

想象一个带有签名的函数指针 -

bool delegate myDelegate(T match);

bool 委托 myDelegate(T 匹配);

Here is the example

这是例子

Node.cs

节点.cs

namespace PredicateExample
{
    class Node
    {
        public string Ip_Address { get; set; }
        public string Node_Name { get; set; }
        public uint Node_Area { get; set; }
    }
}

Main class -

主课——

using System;
using System.Threading;
using System.Collections.Generic;

namespace PredicateExample
{
    class Program
    {
        static void Main(string[] args)
        {
            Predicate<Node> backboneArea = Node =>  Node.Node_Area == 0 ;
            List<Node> Nodes = new List<Node>();
            Nodes.Add(new Node { Ip_Address = "1.1.1.1", Node_Area = 0, Node_Name = "Node1" });
            Nodes.Add(new Node { Ip_Address = "2.2.2.2", Node_Area = 1, Node_Name = "Node2" });
            Nodes.Add(new Node { Ip_Address = "3.3.3.3", Node_Area = 2, Node_Name = "Node3" });
            Nodes.Add(new Node { Ip_Address = "4.4.4.4", Node_Area = 0, Node_Name = "Node4" });
            Nodes.Add(new Node { Ip_Address = "5.5.5.5", Node_Area = 1, Node_Name = "Node5" });
            Nodes.Add(new Node { Ip_Address = "6.6.6.6", Node_Area = 0, Node_Name = "Node6" });
            Nodes.Add(new Node { Ip_Address = "7.7.7.7", Node_Area = 2, Node_Name = "Node7" });

            foreach( var item in Nodes.FindAll(backboneArea))
            {
                Console.WriteLine("Node Name " + item.Node_Name + " Node IP Address " + item.Ip_Address);
            }

            Console.ReadLine();
        }
    }
}

Answer 10

回答by Shubham Khare

Simply -> they provide True/False values based on condition mostly used for querying. mostly used with delegates

简单 - >它们根据主要用于查询的条件提供真/假值。主要与代表一起使用

consider example of list

考虑列表示例

List<Program> blabla= new List<Program>();
        blabla.Add(new Program("shubham", 1));
        blabla.Add(new Program("google", 3));
        blabla.Add(new Program("world",5));
        blabla.Add(new Program("hello", 5));
        blabla.Add(new Program("bye", 2));

contains names and ages. Now say we want to find names on condition So I Will use,

包含姓名和年龄。现在说我们想在条件下找到名字所以我会使用，

    Predicate<Program> test = delegate (Program p) { return p.age > 3; };
        List<Program> matches = blabla.FindAll(test);
        Action<Program> print = Console.WriteLine;
        matches.ForEach(print);

tried to Keep it Simple!

试图保持简单！

C#中的谓词委托

提问by Canavar

采纳答案by Andrew Hare

回答by Adam Carr

回答by LukeH

回答by WestDiscGolf

回答by danlash

回答by lukaszk

回答by Gul Md Ershad

回答by parijat mishra

回答by dexterous

回答by Shubham Khare

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C#中的谓词委托

提问by Canavar

采纳答案by Andrew Hare

回答by Adam Carr

回答by LukeH

回答by WestDiscGolf

回答by danlash

回答by lukaszk

回答by Gul Md Ershad

回答by parijat mishra

回答by dexterous

回答by Shubham Khare

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