為泛型集合類或表示集合中項的泛型類定義接口通常很有用。對於泛型類，使用泛型接口十分可取，例如使用 IComparable<T> 而不使用 IComparable，這樣可以避免值類型的裝箱和取消裝箱操作。.NET Framework 2.0 類庫定義了若干新的泛型接口，以用於 System.Collections.Generic 命名空間中新的集合類。
將接口指定為類型參數的約束時，只能使用實現此接口的類型。下面的代碼示例顯示從 GenericList<T> 類派生的 SortedList<T> 類。SortedList<T> 添加了約束 where T : IComparable<T>。這將使SortedList<T> 中的 BubbleSort 方法能夠對列表元素使用泛型 CompareTo 方法。在此示例中，列表元素為簡單類，即實現 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");
}
}
可將多重接口指定為單個類型上的約束，如下所示：
C#
class Stack<T> where T : System.IComparable<T>, IEnumerable<T>
{
}
一個接口可定義多個類型參數，如下所示：
C#
復制代碼interface IDictionary<K, V>
{
}
類之間的繼承規則同樣適用於接口：
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
如果泛型接口為逆變的，即僅使用其類型參數作為返回值，則此泛型接口可以從非泛型接口繼承。在 .NET Framework 類庫中，IEnumerable<T> 從 IEnumerable 繼承，因為 IEnumerable<T> 僅在 GetEnumerator 的返回值和當前屬性 getter 中使用 T。
具體類可以實現已關閉的構造接口，如下所示：
C#
interface IBaseInterface<T> { }
class SampleClass : IBaseInterface<string> { }
只要類參數列表提供了接口必需的所有參數，泛型類便可以實現泛型接口或已關閉的構造接口，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
對於泛型類、泛型結構或泛型接口中的方法，控制方法重載的規則相同。