C# 关于变体

接口中声明的方法的泛型返回类型，它可以接受派程度更大的返回类型

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

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

<3> 协变和抗变的同时实现

interface IVariant<out R, in A>
{
    R GetSomething();
    void SetSomething(A sampleArg);
    R GetSetSometings(A sampleArg);
}

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<Person>
    {
        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<Employee> employees = new List<Employee> {
               new Employee() {FirstName = "Michael", LastName = "Alexander"},
               new Employee() {FirstName = "Jeff", LastName = "Price"}
            };


            // You can pass PersonComparer, 
            // which implements IEqualityComparer<Person>,
            // although the method expects IEqualityComparer<Employee>.


            IEnumerable<Employee> noduplicates =
                employees.Distinct<Employee>(new PersonComparer());


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


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


}

public delegate T SampleGenericDelegate <out T>();

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


    // You can assign delegates to each other,
    // because the type T is declared covariant.
    SampleGenericDelegate <Object> dObject = dString;           
}

public static void Test()
{
    SampleGenericDelegate<String> dString = () => " ";
    SampleGenericDelegate<Object> dObject = () => " ";
    // SampleGenericDelegate <Object> dObject = dString;	//error
}

C# 关于变体,布布扣,bubuko.com

C# 关于变体

原文：http://blog.csdn.net/ddupd/article/details/22698871