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Introduction to Generics (C# Programming Guide) 

Generic classes and methods combine reusability, type safety and efficiency in a way that their non-generic counterparts cannot. Generics are most commonly used with collections and the methods that operate on them. Version 2.0 of the .NET Framework class library provides a new namespace, System.Collections.Generic, which contains several new generic-based collection classes. It is recommended that all applications that target Version 2.0 use the new generic collection classes instead of the older non-generic counterparts such as ArrayList. For more information, see Generics in the .NET Framework Class Library (C# Programming Guide).

Of course, you can also create custom generic types and methods to provide your own generalized solutions and design patterns that are type-safe and efficient. The following code example shows a simple generic linked-list class for demonstration purposes. (In most cases, it is recommended that you use the List<T> class provided by the .NET Framework class library, rather than create your own.) The type parameter T is used in several places where a concrete type would normally be used to indicate the type of the item stored in the list. It is used in the following ways:

  • As the type of a method parameter in the AddHead method.

  • As the return type of the public method GetNext and the Data property in the nested Node class.

  • As the type of the private member data in the nested class.

Note that T is available to the nested Node class. When GenericList<T> is instantiated with a concrete type — for example as a GenericList<int> — each occurrence of T will be replaced with int.

// type parameter T in angle brackets
public class GenericList<T> 
{
    // The nested class is also generic on T
    private class Node
    {
        // T used in non-generic constructor
        public Node(T t)
        {
            next = null;
            data = t;
        }

        private Node next;
        public Node Next
        {
            get { return next; }
            set { next = value; }
        }
        
        // T as private member data type
        private T data;

        // T as return type of property
        public T Data  
        {
            get { return data; }
            set { data = value; }
        }
    }

    private Node head;
    
    // constructor
    public GenericList() 
    {
        head = null;
    }

    // T as method parameter type:
    public void AddHead(T t) 
    {
        Node n = new Node(t);
        n.Next = head;
        head = n;
    }

    public IEnumerator<T> GetEnumerator()
    {
        Node current = head;

        while (current != null)
        {
            yield return current.Data;
            current = current.Next;
        }
    }
}

The following code example shows how client code uses the generic GenericList<T> class to create a list of integers. Simply by changing the type argument, the code below could easily be modified to create lists of strings or any other custom type:

class TestGenericList
{
    static void Main()
    {
        // int is the type argument
        GenericList<int> list = new GenericList<int>();

        for (int x = 0; x < 10; x++)
        {
            list.AddHead(x);
        }

        foreach (int i in list)
        {
            System.Console.Write(i + " ");
        }
        System.Console.WriteLine("\nDone");
    }
}

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