泛型接口(C# 编程指南)

为泛型集合类或表示集合中项的泛型类定义接口通常很有用。 对于泛型类,使用泛型接口十分可取,例如使用 IComparable 而不使用 IComparable,这样可以避免值类型的装箱和取消装箱操作。 .NET Framework 类库定义了若干泛型接口,以用于 System.Collections.Generic 命名空间中的集合类。

将接口指定为类型参数的约束时,只能使用实现此接口的类型。 下面的代码示例显示从 SortedList<T> 类派生的 GenericList<T> 类。 有关更多信息,请参见 泛型介绍(C# 编程指南)。 SortedList<T> 添加约束 where T : IComparable<T>。 这将使 SortedList<T> 中的 BubbleSort 方法能够对列表元素使用泛型 CompareTo 方法。 在此示例中,列表元素为简单类,即实现 Person 的 IComparable<Person>。

//Type parameter T in angle brackets. 
public class GenericList<T> : System.Collections.Generic.IEnumerable<T>
{
    protected Node head;
    protected Node current = null;

    // Nested class is also generic on T 
    protected class Node
    {
        public Node next;
        private T data;  //T as private member datatype 

        public Node(T t)  //T used in non-generic constructor
        {
            next = null;
            data = t;
        }

        public Node Next
        {
            get { return next; }
            set { next = value; }
        }

        public T Data  //T as return type of property
        {
            get { return data; }
            set { data = value; }
        }
    }

    public GenericList()  //constructor
    {
        head = null;
    }

    public void AddHead(T t)  //T as method parameter type
    {
        Node n = new Node(t);
        n.Next = head;
        head = n;
    }

    // Implementation of the iterator 
    public System.Collections.Generic.IEnumerator<T> GetEnumerator()
    {
        Node current = head;
        while (current != null)
        {
            yield return current.Data;
            current = current.Next;
        }
    }

    // IEnumerable<T> inherits from IEnumerable, therefore this class  
    // must implement both the generic and non-generic versions of  
    // GetEnumerator. In most cases, the non-generic method can  
    // simply call the generic method.
    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

public class SortedList<T> : GenericList<T> where T : System.IComparable<T>
{
    // A simple, unoptimized sort algorithm that  
    // orders list elements from lowest to highest: 

    public void BubbleSort()
    {
        if (null == head || null == head.Next)
        {
            return;
        }
        bool swapped;

        do
        {
            Node previous = null;
            Node current = head;
            swapped = false;

            while (current.next != null)
            {
                //  Because we need to call this method, the SortedList 
                //  class is constrained on IEnumerable<T> 
                if (current.Data.CompareTo(current.next.Data) > 0)
                {
                    Node tmp = current.next;
                    current.next = current.next.next;
                    tmp.next = current;

                    if (previous == null)
                    {
                        head = tmp;
                    }
                    else
                    {
                        previous.next = tmp;
                    }
                    previous = tmp;
                    swapped = true;
                }
                else
                {
                    previous = current;
                    current = current.next;
                }
            }
        } while (swapped);
    }
}

// A simple class that implements IComparable<T> using itself as the  
// type argument. This is a common design pattern in objects that  
// are stored in generic lists. 
public class Person : System.IComparable<Person>
{
    string name;
    int age;

    public Person(string s, int i)
    {
        name = s;
        age = i;
    }

    // This will cause list elements to be sorted on age values. 
    public int CompareTo(Person p)
    {
        return age - p.age;
    }

    public override string ToString()
    {
        return name + ":" + age;
    }

    // Must implement Equals. 
    public bool Equals(Person p)
    {
        return (this.age == p.age);
    }
}

class Program
{
    static void Main()
    {
        //Declare and instantiate a new generic SortedList class. 
        //Person is the type argument.
        SortedList<Person> list = new SortedList<Person>();

        //Create name and age values to initialize Person objects. 
        string[] names = new string[] 
        { 
            "Franscoise", 
            "Bill", 
            "Li", 
            "Sandra", 
            "Gunnar", 
            "Alok", 
            "Hiroyuki", 
            "Maria", 
            "Alessandro", 
            "Raul" 
        };

        int[] ages = new int[] { 45, 19, 28, 23, 18, 9, 108, 72, 30, 35 };

        //Populate the list. 
        for (int x = 0; x < 10; x++)
        {
            list.AddHead(new Person(names[x], ages[x]));
        }

        //Print out unsorted list. 
        foreach (Person p in list)
        {
            System.Console.WriteLine(p.ToString());
        }
        System.Console.WriteLine("Done with unsorted list");

        //Sort the list.
        list.BubbleSort();

        //Print out sorted list. 
        foreach (Person p in list)
        {
            System.Console.WriteLine(p.ToString());
        }
        System.Console.WriteLine("Done with sorted list");
    }
}

可将多重接口指定为单个类型上的约束,如下所示:

class Stack<T> where T : System.IComparable<T>, IEnumerable<T>
{
}

一个接口可定义多个类型参数,如下所示:

interface IDictionary<K, V>
{
}

适用于类的继承规则同样适用于接口:

interface IMonth<T> { }

interface IJanuary     : IMonth<int> { }  //No error 
interface IFebruary<T> : IMonth<int> { }  //No error 
interface IMarch<T>    : IMonth<T> { }    //No error 
//interface IApril<T>  : IMonth<T, U> {}  //Error

如果泛型接口为逆变的,即仅使用其类型参数作为返回值,则此泛型接口可以从非泛型接口继承。 在 .NET Framework 类库中,IEnumerableIEnumerable 继承,因为 IEnumerable 只在 GetEnumerator 的返回值和 Current 属性 getter 中使用 T。

具体类可以实现已关闭的构造接口,如下所示:

interface IBaseInterface<T> { }

class SampleClass : IBaseInterface<string> { }

只要类参数列表提供了接口必需的所有参数,泛型类便可以实现泛型接口或已关闭的构造接口,如下所示:

interface IBaseInterface1<T> { }
interface IBaseInterface2<T, U> { }

class SampleClass1<T> : IBaseInterface1<T> { }          //No error 
class SampleClass2<T> : IBaseInterface2<T, string> { }  //No error

对于泛型类、泛型结构或泛型接口中的方法,控制方法重载的规则相同。 有关更多信息,请参见 泛型方法(C# 编程指南)

请参见

参考

泛型介绍(C# 编程指南)

接口(C# 参考)

概念

C# 编程指南

其他资源

.NET Framework 中的泛型