C# Generics

Topics
Why Generics are required?
Generic types and functions ,
Type parameters and Constraints
Difference between templates in C++ and Generics in C#
Generic Delegates and event handling
Generic Interfaces

Introduction
Generics are the most powerful feature of C# 2.0. Generics allow you to
define type-safe data structures, without committing to actual data types.
This results in a significant performance boost and higher quality code,
because you get to reuse data processing algorithms without duplicating
type-specific code.
In concept, generics are similar to C++ templates, but are drastically
different in implementation and capabilities.

Why Generics are required?
Consider an everyday data structure such as a stack, providing the classic
Push() and Pop() methods.
When developing a general-purpose stack, you would like to use it to store
instances of various types.
Under C# 1.1, you have to use an Object-based stack, meaning that the
internal data type used in the stack is an amorphous Object, and the stack
methods interact with Objects.

An Object-based stack
public class Stack
{
int m_Size;
int m_StackPointer = 0;
object[] m_Items;
public Stack():this(100)
{}
public Stack(int size)
{
m_Size = size;
m_Items = new object[m_Size];
}

public void Push(object item)
{
if(m_StackPointer >= m_Size)
throw new StackOverflowException();
m_Items[m_StackPointer] = item;
m_StackPointer++;
}

public object Pop()
{
m_StackPointer--;
if(m_StackPointer >= 0)
return m_Items[m_StackPointer];
else
{
m_StackPointer = 0;
throw new InvalidOperationException("empty stack");
}
}
}

Problems with Object-based solutions
Performance – When using value types, you have to box them in order to
push and store them, and unbox the value types when popping them off the
stack. Boxing and unboxing incurs a significant performance penalty it also
increases the pressure on the managed heap, resulting in more garbage
collections

Type safety – the compiler lets you cast anything to and from Object, you
lose compile-time type safety. Eg: the following code compiles fine, but
raises an invalid cast exception at run time:
Stack stack = new Stack();
stack.Push(1);
string number = (string)stack.Pop();

Generics
Generics allow you to define type-safe classes without compromising type safety,
performance, or productivity.
public class Stack<T>
{...}

Stack<int> stack = new Stack<int>();

Generic stack
public class Stack<T>
{
int m_Size;
int m_StackPointer = 0;
T[] m_Items;
public Stack():this(100){ }
public Stack(int size)
{
m_Size = size;
m_Items = new T[m_Size];
}

public void Push(T item)
{
if(m_StackPointer >= m_Size)
throw new StackOverflowException();
m_Items[m_StackPointer] = item;
m_StackPointer++;
}
public T Pop()
{
m_StackPointer--;
else
{
m_StackPointer = 0;
throw new InvalidOperationException("Cannot pop an empty stack");
}
}
}

default() operator
public T Pop()
{
m_StackPointer--;
else
{
m_StackPointer = 0;
return default(T);
}
}

Multiple Generic Types
class Node<K,T>

{

public K Key;

public T Item;

public Node<K,T> NextNode;

public Node()

{

Key = default(K);

Item = default(T);

NextNode = null;

}

public Node(K key,T item,Node<K,T> nextNode)

{

Key = key;

Item = item;

NextNode = nextNode;

}

}

public class LinkedList<K,T>
{
Node<K,T> m_Head;
public LinkedList()
{
m_Head = new Node<K,T>();
}
public void AddHead(K key,T item)
{
Node<K,T> newNode = new
Node<K,T>(key,item,m_Head.NextNode);
m_Head.NextNode = newNode;
}
}

LinkedList<int,string> list = new LinkedList<int,string>();
list.AddHead(123,"AAA");

Generic type aliasing
using List = LinkedList<int,string>;

class ListClient
{
static void Main(string[] args)
{
List list = new List();
list.AddHead(123,"AAA");
}
}

Generic Constraints
The compiler compiles the generic code into IL independent of any type
arguments that the clients will use. As a result, the generic code could try to
use methods, properties, or members of the generic type parameters that
are incompatible with the specific type arguments the client uses. This is
unacceptable because it amounts to lack of type safety.

There are three types of constraints.
A derivation constraint indicates to the compiler that the generic type parameter
derives from a base type such an interface or a particular base class.
A default constructor constraint indicates to the compiler that the generic type
parameter exposes a default public constructor (a public constructor with no
parameters).
A reference/value type constraint constrains the generic type parameter to be a
reference or a value type.

Derivation Constraints
The “where” keyword on the generic type parameter followed by a
derivation colon to indicate to the compiler that the generic type parameter
implements a particular interface.

public class LinkedList<K,T> where K : IComparable
{
T Find(K key)
{
Node<K,T> current = m_Head;
while(current.NextNode != null)
{
if(current.Key.CompareTo(key) == 0)
break;
else
current = current.NextNode;
}
return current.Item;
}
...
}

Multiple interfaces on the same generic type parameter, separated by a comma. For
example:
where K : IComparable<K>,Iconvertible
{...}

Constraints for every generic type parameter your class uses, for example:
where K : IComparable<K>
where T : ICloneable

base class constraint
public class MyBaseClass
{...}
public class LinkedList<K,T> where K : MyBaseClass
{...}

constrain both a base class and one or more interfaces
where K : MyBaseClass, IComparable<K>
{...}

Constructor Constraint
Suppose you want to instantiate a new generic object inside a generic class.
The problem is the C# compiler does not know whether the type argument
the client will use has a matching constructor, and it will refuse to compile
the instantiation line.
Its overcome using the “new()” constraint.

class Node<K,T> where T : new()
{
public K Key;
public T Item;
public Node<K,T> NextNode;
public Node()
{
Key = default(K);
Item = new T();
NextNode = null;
}
}

Reference/Value Type Constraint
Value Type Constraint
public class MyClass<T> where T : struct
{...}
Reference Type Constraint
public class MyClass<T> where T : class
{...}

Generic Methods
A method can define generic type parameters, specific to its execution scope.
Generic Class:
public class MyClass<T>
{
public void MyMethod<X>(X x)
{...}
}

Non Generic Class:
public class MyClass
{
public void MyMethod<T>(T t)
{...}
}

Generic Delegates
A delegate defined in a class can take advantage of the generic type parameter of that class. For
example:
public class MyClass<T>
{
public delegate void GenericDelegate(T t);
public void SomeMethod(T t)
{...}
}

MyClass<int> obj = new MyClass<int>(); MyClass<int>.GenericDelegate del;
del = obj.SomeMethod;

Generic Event Handiling
namespace Generic_Delegates_and_Events

{

public delegate void GenericEventHandler<S,A>(S sender,A args); //generic delegate

public class MyPublisher

{

public event GenericEventHandler<MyPublisher,EventArgs> MyEvent;

public void FireEvent()

{

MyEvent(this,EventArgs.Empty);

}

}

public class MySubscriber<A> //Optional: can be a specific type

{

public class MySubscriber2<A> //Optional: can be a specific type

{

public void SomeMethod2(MyPublisher sender, A args)

{

Console.WriteLine("MySubscriber2::SomeMethod2()");

}

}

class Program

{


{

MyPublisher publisher = new MyPublisher();

MySubscriber<EventArgs> subscriber = new MySubscriber<EventArgs>();

publisher.MyEvent += subscriber.SomeMethod;

MySubscriber2<EventArgs> subscriber2 = new MySubscriber2<EventArgs>();

publisher.MyEvent += subscriber2.SomeMethod2;

publisher.FireEvent();

}

}

}

Differences Between C++ Templates
and C# Generics
C# generics do not provide the same amount of flexibility as C++ templates.
C# does not allow non-type template parameters, such as template C<int i> {}.
C# does not support explicit specialization; that is, a custom implementation of a template for a specific
type.
C# does not support partial specialization: a custom implementation for a subset of the type arguments.
C# does not allow the type parameter to be used as the base class for the generic type.
C# does not allow type parameters to have default types.
In C#, a generic type parameter cannot itself be a generic, although constructed types can be used as
generics. C++ does allow template parameters.

C# Generics is more type safe than C++ templates.
C++ templates use a compile-time model. C# generics are not just a feature of the
compiler, but also a feature of the runtime.
Code Bloating is reduced in C# compared to C++.
Generics have full run time support. Which is not available to templates.

Generic Interfaces
The System.Collections.Generic namespace contains interfaces and classes that
define generic collections, which allow users to create strongly typed collections
that provide better type safety and performance than non-generic strongly typed
collections.

ICollection<T>
• Add(), Clear(), Contains(), CopyTo(), Remove(),Count, IsReadOnly
• The interface ICollection<T> is implemented by collection classes. Methods of this interface can be used to add and remove elements from
the collection. The generic interface ICollection<T> inherits from the non-generic interface IEnumerable. With this it is possible to pass
objects implementing ICollection<T> to methods that require IEnumerable objects as parameters.

IList<T>
• Insert(), RemoveAt(), IndexOf(),Item
• The interface IList<T> allows you to access a collection using an indexer. It is also possible to insert or remove elements at any position of the
collection. Similar to ICollection<T>, the interface IList<T> inherits from IEnumerable.

IEnumerable<T>
• GetEnumerator()
• The interface IEnumerable<T> is required if a foreach statement is used with the collection. This interface defines the method
GetEnumerator() that returns an enumerator implementing IEnumerator<T>.The generic interface IEnumerable<T> inherits from the non-
generic interface IEnumerable.

IEnumerator<T>
• Current
• The foreach statement uses an enumerator implementing IEnumerator<T> for accessing all
elements in a collection. The interface IEnumerator<T> inherits from the non-generic interfaces
IEnumerator and IDisposable. The interface IEnumerator defines the methods MoveNext() and
Reset(), IEnumerator<T> defines the type-safe version of the property Current.

IDictionary<TKey, TValue>
• Add(), ContainsKey(), Remove(), TryGetValue(),Item, Keys, Values
• The interface IDictionary<K, V> is implemented by collections whose elements have a key and a
value.

IComparer<T>
• Compare()
• The interface IComparer<T> is used to sort elements inside a collection with the Compare()
method.

IEqualityComparer<T>
• Equals(), GetHashCode()
• IEqualityComparer<T> is the second interface to compare objects. With this interface the objects
can be compared for equality. The method GetHashCode() should return a unique value for every
object. The method Equals() returns true if the objects are equal, false otherwise.

The ICollection<T> interface is the base interface for classes in the
System.Collections.Generic namespace.

The ICollection<T> interface extends IEnumerable<T>.

IDictionary<TKey, TValue> and IList<T> are more specialized interfaces that
extend ICollection<T>.

A IDictionary<TKey, TValue> implementation is a collection of key/value pairs, like
the Dictionary<TKey, TValue> class.

A IList<T> implementation is a collection of values, and its members can be
accessed by index, like the List<T> class.

IEnumerable is an interface that defines one method GetEnumerator which
returns an IEnumerator interface, this in turn allows readonly access to a
collection. A collection that implements IEnumerable can be used with a foreach
statement.

public interface ICustomInterface<T>
{
void ShowMethod();
void SomeMethod(T t);

}
class CustomClass<U> : ICustomInterface<U>
{
U u;
public CustomClass(U temp)
{
u = temp;
}

public void ShowMethod()
{
Console.WriteLine("Value: " + u);
}

public void SomeMethod(U t)
{
Console.WriteLine(t + " " + u);
}
}
class Program
{
{

CustomClass<int> MyClass = new CustomClass<int>(10);
MyClass.ShowMethod();
CustomClass<int> MyClass2 = new CustomClass<int>(10);
MyClass2.SomeMethod(20);
}
}

C# Generics

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