泛型接口（C# 编程指南） · MSDN C# 编程指南 & 参考手册 2015

# 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<T>](https://msdn.microsoft.com/zh-cn/library/4d7sx9hd.aspx) 而不使用 [IComparable](https://msdn.microsoft.com/zh-cn/library/system.icomparable.aspx)，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](https://msdn.microsoft.com/zh-cn/library/system.collections.generic.aspx) 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）](https://msdn.microsoft.com/zh-cn/library/0x6a29h6.aspx)。 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](https://msdn.microsoft.com/zh-cn/library/43hc6wht.aspx) 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，[IEnumerable<T>](https://msdn.microsoft.com/zh-cn/library/9eekhta0.aspx) 从 [IEnumerable](https://msdn.microsoft.com/zh-cn/library/system.collections.ienumerable.aspx) 继承，因为 [IEnumerable<T>](https://msdn.microsoft.com/zh-cn/library/9eekhta0.aspx) 只在 [GetEnumerator](https://msdn.microsoft.com/zh-cn/library/s793z9y2.aspx) 的返回值和 [Current](https://msdn.microsoft.com/zh-cn/library/58e146b7.aspx) 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）](https://msdn.microsoft.com/zh-cn/library/twcad0zb.aspx)。 ## 请参阅 [C# 编程指南](https://msdn.microsoft.com/zh-cn/library/67ef8sbd.aspx) [泛型介绍（C# 编程指南）](https://msdn.microsoft.com/zh-cn/library/0x6a29h6.aspx) [接口（C# 参考）](https://msdn.microsoft.com/zh-cn/library/87d83y5b.aspx) [.NET Framework 中的泛型](https://msdn.microsoft.com/zh-cn/library/ms172192.aspx)