using System; public class Starter { public static void Main() { MyClass obj = new MyClass(); obj.MethodA<XClass>(); } } public class MyClass { public void MethodA<T>() where T : XClass, new() { Console.WriteLine("MyClass.MethodA"); T obj = new T(); obj.MethodB(); } } public class XClass { public void MethodB() { Console.WriteLine("XClass.MethodB"); } }
Author: coder
Derivation Constraint
using System; public class Starter { public static void Main() { // good MyClass<XClass, YClass> obj = new MyClass<XClass, YClass>(); // good MyClass<XClass, WClass> obj2 = new MyClass<XClass, WClass>(); // bad MyClass<WClass, YClass> obj3 = new MyClass<WClass, YClass>(); } } public class MyClass<K, V> where K : XClass where V : YClass { } public class XClass { } public class YClass { } public class WClass : YClass { }
Demonstrate a value type constraint
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()); } }
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>(); } }