C# Action<T>、Func<T>、Predicate<T>委托的区别、用法

参考：https://social.technet.microsoft.com/wiki/contents/articles/22418.c-action-func-tresult-and-predicate-t-delegate.aspx下面通过例子进行介绍：class Program{static void Main(string[] args){List<Employee> employees = Employee.GetEmployees();//Action委托//employees.ForEachAction<Employee> empAction = new Action<Employee>(Employee.CalculateAge);employees.ForEach(empAction);foreach (var e in employees){Console.WriteLine(e.Age);}//使用 Lambda表达式：employees.ForEach(e => e.Age = DateTime.Now.Year - e.Birthday.Year);//Func 委托: Enumerable.First<TSource> 方法Func<Employee, bool> myFunc = new Func<Employee, bool>(Employee.NameIsEqual);Console.WriteLine(employees.First(myFunc).FirstName);//使用 Lambda表达式：Console.WriteLine(employees.First(e => e.FirstName == "Jaliya").FirstName);Console.ReadKey();}}/// <summary>/// 自定义 员工 类/// </summary>public class Employee{public string FirstName { get; set; }public string LastName { get; set; }public DateTime Birthday { get; set; }public int Age { get; set; }public static List<Employee> GetEmployees(){return new List<Employee>(){new Employee(){FirstName = "Jaliya",LastName = "Udagedara",Birthday = Convert.ToDateTime("1986-09-11")},new Employee(){FirstName = "Gary",LastName = "Smith",Birthday = Convert.ToDateTime("1988-03-11")}};}public static void CalculateAge(Employee emp){emp.Age = DateTime.Now.Year - emp.Birthday.Year;}public static bool NameIsEqual(Employee emp){return emp.FirstName == "Jaliya";}}在主函数中，可以通过下面语句，获取一个ListList<Employee> employees = Employee.GetEmployees();Action：            接收一个或多个输入参数，但不返回任何参数，可传递至多16种不同类型的参数类型。      例如List<T>.ForEach方法，接收一个类型为T的Action，例子如下，其中Employee是自定义类型

static void CalculateAge(Employee emp)

{

emp.Age = DateTime.Now.Year - emp.Birthday.Year;

}

可以创建一个Action，指向上面的方法：

Action<Employee> empAction = new Action<Employee>(CalculateAge);

employees.ForEach(empAction); 

foreach (Employee e in employees)

{

Console.WriteLine(e.Age);

}

使用Lambda表达式，可以简化代码：

employees.ForEach(e => e.Age = DateTime.Now.Year - e.Birthday.Year);

Func<TResult>：这种委托指向具有0、 1或多个输入参数，并返回TResult类型参数的方法。以Enumeralbe.First<TSource>方法为例，该方法接收一个Func，并返回第一个满足条件的元素。在这个例子中，this particular method accepts Func which accepts an Employee and returns a bool value.，因此我们需要创建一个方法，并用Func指向它。

static bool NameIsEqual(Employee emp)

{

return emp.FirstName == "Jaliya";

}

现在我们可以创建一个Func，并获取满足Func条件的Employee：

Func<Employee, bool> myFunc = new Func<Employee, bool>(NameIsEqual);

Console.WriteLine(employees.First(myFunc).FirstName);

同样，可以使用 Lambda 表达式简化代码：

Console.WriteLine(employees.First(e => e.FirstName == "Jaliya").FirstName);

Predicate<T>：参考文章最开始的链接