using System; struct MyStruct { } class MyClass { } class Test<T> where T : struct { T obj; public Test(T x) { obj = x; } } class Test { public static void Main() { Test<MyStruct> x = new Test<MyStruct>(new MyStruct()); Test<int> y = new Test<int>(10); // But, the following declaration is illegal! // Test<MyClass> z = new Test<MyClass>(new MyClass()); } }
Generics
Demonstrate a reference constraint
using System; class MyClass { } class Test<T> where T : class { T obj; public Test() { // The following statement is legal only // because T is guaranteed to be a reference // type, which can be assigned the value null. obj = null; } public void print(){ Console.WriteLine(obj); } } class ClassConstraintDemo { public static void Main() { Test<MyClass> x = new Test<MyClass>(); // The next line is in error because int is // a value type. // Test<int> y = new Test<int>(); } }
Reference Type Constraint: A reference type constraint restricts a type parameter to a reference type.
using System; using System.Collections; public class Starter { public static void Main() { // MyClass<int> obj1=new MyClass<int>(); [illegal] MyClass<XClass> obj2 = new MyClass<XClass>(); } } public class MyClass<T> where T : class { public void Iterate(T data) { } } public class XClass { }
A new() constructor constraint
using System; class MyClass { public MyClass() { } } class Test<T> where T : new() { T obj; public Test() { // This works because of the new() constraint. obj = new T(); // create a T object } } class ConsConstraintDemo { public static void Main() { Test<MyClass> x = new Test<MyClass>(); } }
Use an interface constraint
using System; class NotFoundException : ApplicationException { } public interface IPhoneNumber { string Number { get; set; } string Name { get; set; } } class Friend : IPhoneNumber { string name; string number; public Friend(string n, string num) { name = n; number = num; } public string Number { get { return number; } set { number = value; } } public string Name { get { return name; } set { name = value; } } } class Supplier : IPhoneNumber { string name; string number; public Supplier(string n, string num) { name = n; number = num; } public string Number { get { return number; } set { number = value; } } public string Name { get { return name; } set { name = value; } } } class EmailFriend { } class PhoneList<T> where T : IPhoneNumber { T[] phList; int end; public PhoneList() { phList = new T[10]; end = 0; } public bool add(T newEntry) { if(end == 10) return false; phList[end] = newEntry; end++; return true; } public T findByName(string name) { for(int i=0; i<end; i++) { if(phList[i].Name == name) return phList[i]; } throw new NotFoundException(); } public T findByNumber(string number) { for(int i=0; i<end; i++) { if(phList[i].Number == number) return phList[i]; } throw new NotFoundException(); } } class Test { public static void Main() { PhoneList<Friend> plist = new PhoneList<Friend>(); plist.add(new Friend("A", "555-1111")); plist.add(new Friend("B", "555-6666")); plist.add(new Friend("C", "555-9999")); try { Friend frnd = plist.findByName("B"); Console.Write(frnd.Name + ": " + frnd.Number); } catch(NotFoundException) { Console.WriteLine("Not Found"); } Console.WriteLine(); PhoneList<Supplier> plist2 = new PhoneList<Supplier>(); plist2.add(new Supplier("D", "555-4444")); plist2.add(new Supplier("E", "555-3333")); plist2.add(new Supplier("F", "555-2222")); try { Supplier sp = plist2.findByNumber("555-2222"); Console.WriteLine(sp.Name + ": " + sp.Number); } catch(NotFoundException) { Console.WriteLine("Not Found"); } //PhoneList<EmailFriend> plist3 = new PhoneList<EmailFriend>(); } }
A base class constraint
using System; class MyBase { public void hello() { Console.WriteLine("Hello"); } } class B : MyBase { } class C { } class Test<T> where T : MyBase { T obj; public Test(T o) { obj = o; } public void sayHello() { obj.hello(); } } class BaseClassConstraintDemo { public static void Main() { MyBase a = new MyBase(); B b = new B(); C c = new C(); Test<MyBase> t1 = new Test<MyBase>(a); t1.sayHello(); Test<B> t2 = new Test<B>(b); t2.sayHello(); // The following is invalid because // C does not inherit MyBase. // Test<C> t3 = new Test<C>(c); // Error! } }
Combination of Overriding Generic Methods
/* Base Method Derived Method Comments Nongeneric Generic (open) Permitted Nongeneric Generic (closed) Permitted Generic (open) Nongeneric Not permitted Generic (open) Generic (open) Permitted; must use the same type parameters Generic (open) Generic (closed) Not permitted Generic (closed) Nongeneric Permitted Generic (closed) Generic (closed) Permitted Generic (closed) Generic (open) Not permitted */