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.NET Generics

Generics

• Using Generics, we can create class templates that support any type.

• Compile-time type checking– By creating a generic class, you can create a collection that

is type-safe at compile-time.– There is no need to write code to test for the correct data

type, because it is enforced at compile time– The need for type casting and the possibility of run-time

errors are reduced.• Binary code reuse– Reuse same code with multiple types– Generic code generated at run-timerun-time!

• Generics are classes, structures, interfaces, and methods that have placeholders (type parameters) for one or more of the types that they store or use.

• A generic collection class might use a type parameter as a placeholder for the type of objects that it stores

• improved performance • reduced code

simple generic class definition

• public class Generic<T> { public T Field; }

public static void Main() { Generic<string> g = new Generic<string>(); g.Field = "A string"; Console.WriteLine("Generic.Field = \"{0}\"", g.Field); Console.WriteLine("Generic.Field.GetType() = {0}", g.Field.GetType().FullName); }

Define and use a generic method

public class Method<T> { T[] item; public void Test(T item) {...} } Method<int> Test= new Method<int>();

Placeholders for Generic Types

• T…type �• K…key �• V…value�

Applying Generics

• Generics may have any # of type parametersClass Node<K, T>

{ public Node(K key, T item, Node<K, T>nextNode {….}

private K Key;

private T Item;

private Node<k, T> NextNode; }

• Must then instantiate with the # of parameters

Generic Interfaces

public interface ICalculator<T>

{

T Add(T param1, T param2);

}

public class SamCalculator : ICalculator<int>

{

public int Add(int arg1, int ar2)

{ return arg1 + arg2; }

}

• Interface declared with type parameters = a generic interface declaration

Derivation Constraints

• Where reserved word to define a constraint• Use where on generic type parameter followed by derivation

colon indicating generic type parameter implements particular interface

public class LinkedList<K, T> where K : IComparable

• Overcome by new generic interface IComparable<T> in System.Collections.Generics

• Constrain plus new interface eliminates boxing penalty:public class LinkedList<K, T> where K : IComparable<K>

Derivation Constraints

• All constraints must appear after actual derivation list of generic class

• Ex: If LinkedList<T> derives from IEnumerable<T>, put where keyword after it:

public class LinkedList<K,T> : IEnumerable<T> where K :

IComparable<K>

• Can provide constraints for every generic type parameterpublic class LinkedList<K, T> where K : IComparable<K>

where T : IClonable

Constructor Constraints

• Suppose want to instantiate new generic object inside generic class– C# compiler doesn’t know whether specific type you are going to use

has matching public default constructor– Will not compile

• Constructor constraint makes possible for generic code to create instances of type specified by constraining type args to types that implement public default constructor

• Only default or parameterless constructors supported

Using Generics in the BCL

Enhancing the Base LibraryGeneric Collections

• System.Collections.Generic classes– List<T>– LinkedList<T> // new in Beta1!– Dictionary<K, V> // equivalent to non-generic HashTable– Stack<T>– Queue<T>

• System.Collections.Generic interfaces– IList<T> // replacement for IList– IDictionary<K, V> // replacement for IDictionary– ICollection<T> // replacement for ICollection– IEnumerable<T> // replacement for IEnumerable– IEnumerator<T> // replacement for IEnumerator– IComparable<T> // replacement for IComparable– IComparer<T> // replacement for IComparer

Using Generics in the BCL

• All generic collections implement generic IEnumerable<T> interface

• IEnumerable<T> provides access to IEnumerator<T> Iterators interface, for abstracted iterations over collections (Think C++ STL Iterators)

Public interface IEnumerable<T>

{ IEnumerator<T> GetEnumerator(); }

Public interface IEnumerator<T> : IDisposable

{ T Current {get;}

bool MoveNext(); }

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