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。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：

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。