C# 關於變體

關於協變和逆變
協變和逆變統稱為變體，這是用於數組類型，委托類型，泛型參數類型間進行隱式引用轉換用的語法規則，有點類似多態。


泛型接口中的變體
<1>協變

接口中聲明的方法的泛型返回類型，它可以接受派程度更大的返回類型

interface ICovariant
{
    R GetSomething();
    // The following statement generates a compiler error.
    // void SetSometing(R sampleArg);
}

<2>逆變
接口中聲明的方法的泛型參數類型，它可以接受派生程度更小的參數類型

interface IContravariant
{
    void SetSomething(A sampleArg);
    void DoSomething() where T : A;
    // The following statement generates a compiler error.
    // A GetSomething();            
}

<3> 協變和抗變的同時實現

interface IVariant
{
    R GetSomething();
    void SetSomething(A sampleArg);
    R GetSetSometings(A sampleArg);
}

<4>關於變體的一些理解
變體包括抗變，協變，它是為了處理泛型，委托中基類與派生類賦值問題而出現的，因此類似於多態。
關於協變我們很容易理解，它是實現派生級別高賦給派生級別低的。在泛型接口中，協變的標示是out，並且它表示的也是函數的返


回值，就根據這一點說明它就很類似。
關於抗變，接口中標示的是in，並且它表示的是傳入的函數參數，所以抗變中表象是派生級別低的賦給派生級別高的，實質上還是把


用派生級別高的來做輸入，輸入到派生級別低的函數參數中。


關於抗變的例子：

namespace ConsoleApplication24
{
    // Simple hierarchy of classes.
    public class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }


    public class Employee : Person { }


    // The custom comparer for the Person type
    // with standard implementations of Equals()
    // and GetHashCode() methods.
    class PersonComparer : IEqualityComparer
    {
        public bool Equals(Person x, Person y)
        {
            if (Object.ReferenceEquals(x, y)) return true;
            if (Object.ReferenceEquals(x, null) ||
                Object.ReferenceEquals(y, null))
                return false;
            return x.FirstName == y.FirstName && x.LastName == y.LastName;
        }
        public int GetHashCode(Person person)
        {
            if (Object.ReferenceEquals(person, null)) return 0;
            int hashFirstName = person.FirstName == null
                ? 0 : person.FirstName.GetHashCode();
            int hashLastName = person.LastName.GetHashCode();
            return hashFirstName ^ hashLastName;
        }
    }


    class Program
    {


        public static void Test()
        {
            List employees = new List {
               new Employee() {FirstName = "Michael", LastName = "Alexander"},
               new Employee() {FirstName = "Jeff", LastName = "Price"}
            };


            // You can pass PersonComparer, 
            // which implements IEqualityComparer,
            // although the method expects IEqualityComparer.


            IEnumerable noduplicates =
                employees.Distinct(new PersonComparer());


            foreach (var employee in noduplicates)
                Console.WriteLine(employee.FirstName + " " + employee.LastName);
        }


        public static void Main()
        {
            Test();
        }
    }


}

泛型類型參數中的變體
<1>
如果要啟用隱式轉換，必須使用in或者out關鍵字

public delegate T SampleGenericDelegate ();

public static void Test()
{
    SampleGenericDelegate  dString = () => " ";


    // You can assign delegates to each other,
    // because the type T is declared covariant.
    SampleGenericDelegate