Delegate Func Action Predicate default() 知识点
看仓储模式,有代码写到这几个关键字,陌生,记录下来。
定义一个类型,此类型抽象化了相似结构的某一类方法,因此我们能将此类型代表的方法作为参数进行传递。
Delegate至少0个参数,至多32个参数,可以无返回值,也可以指定返回值类型
Func可以接受0个至16个传入参数,必须具有返回值 用于泛型
Action可以接受0个至16个传入参数,无返回值 用于泛型
Predicate只能接受一个传入参数,返回值为bool类型 用于泛型
default(T) 针对泛型默认值的初始化处理。 当T不确认是值类型或引用类型时,他的默认值会给我们造成困扰。既可以是NULL又可能是0。当使用default(T)处理后T temp = default(T);
temp的默认值会根据T的类型自动赋值。
//下面代码来自msdn.microsoft.com
namespace ConsoleApplication1 { class Program { static void Main(string[] args) { // Test with a non-empty list of integers. GenericList<int> gll = new GenericList<int>(); gll.AddNode(5); gll.AddNode(4); gll.AddNode(3); int intVal = gll.GetLast(); // The following line displays 5. 结果将返回5 System.Console.WriteLine(intVal); // Test with an empty list of integers.用空值进行测试 GenericList<int> gll2 = new GenericList<int>(); intVal = gll2.GetLast(); // The following line displays 0. 结果返回0 //此处根据T的类型返回了int型的默认值0 System.Console.WriteLine(intVal); // Test with a non-empty list of strings. GenericList<string> gll3 = new GenericList<string>(); gll3.AddNode("five"); gll3.AddNode("four"); string sVal = gll3.GetLast(); // The following line displays five. System.Console.WriteLine(sVal); // Test with an empty list of strings. GenericList<string> gll4 = new GenericList<string>(); sVal = gll4.GetLast(); // The following line displays a blank line. //此处根据T的类型返回string的默认值“空白” System.Console.WriteLine(sVal); } } // T is the type of data stored in a particular instance of GenericList. public class GenericList<T> { private class Node { // Each node has a reference to the next node in the list. public Node Next; // Each node holds a value of type T. public T Data; } // The list is initially empty. private Node head = null; // Add a node at the beginning of the list with t as its data value. public void AddNode(T t) { Node newNode = new Node(); newNode.Next = head; newNode.Data = t; head = newNode; } // The following method returns the data value stored in the last node in // the list. If the list is empty, the default value for type T is // returned. public T GetLast() { // The value of temp is returned as the value of the method. // The following declaration initializes temp to the appropriate // default value for type T. The default value is returned if the // list is empty. T temp = default(T); Node current = head; while (current != null) { temp = current.Data; current = current.Next; } return temp; } } }