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> 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 2.0 类库定义了若干新的泛型接口，以用于 System.Collections.Generic 命名空间中新的集合类。

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 GenericList<T> 类派生的 SortedList<T> 类。有关更多信息，请参见泛型介绍（C# 编程指南）。SortedList<T> 添加了约束 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 IComparable<Person> 的 Person。

C#

//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：

C#

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：

C#

interface IDictionary<K, V>
{
}

类之间的继承规则同样适用于接口：

C#

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> 从 IEnumerable 继承，因为 IEnumerable<T> 仅在 GetEnumerator 的返回值和当前属性 getter 中使用 T。

具体类可以实现已关闭的构造接口，如下所示：

C#

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：

C#

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）。