C# - Generics

Quiz

What are Generics in C#?

Generics allow you to define the specification of the data type of programming elements in a class or a method, until it is actually used in the program. In other words, generics allow you to write a class or method that can work with any data type.

Features of Generics

Generics is a technique that enriches your programs in the following ways −

It helps you to maximize code reuse, type safety, and performance.
You can create generic collection classes. The .NET Framework class library contains several new generic collection classes in the System.Collections.Generic namespace. You may use these generic collection classes instead of the collection classes in the System.Collections namespace.
You can create your own generic interfaces, classes, methods, events, and delegates.
You may create generic classes constrained to enable access to methods on particular data types.
You may get information on the types used in a generic data type at run-time by means of reflection.

Generic Classes

A generic class is defined with a type parameter which allows a class to be instantiated with different data types and it maintains the type safety also.

You write the specifications for the class or the method, with substitute parameters for data types. When the compiler encounters a constructor for the class or a function call for the method, it generates code to handle the specific data type.

Example

Following is the basic example of the Generic class, which helps us to create type-safe, reusable arrays −

using System;
using System.Collections.Generic;
namespace GenericApplication {
   public class MyGenericArray<T> {
      private T[] array;
      
      public MyGenericArray(int size) {
         array = new T[size + 1];
      }
      public T getItem(int index) {
         return array[index];
      }
      public void setItem(int index, T value) {
         array[index] = value;
      }
   }
   class Tester {
      static void Main(string[] args) {
         
         //declaring an int array
         MyGenericArray<int> intArray = new MyGenericArray<int>(5);
         
         //setting values
         for (int c = 0; c < 5; c++) {
            intArray.setItem(c, c*5);
         }
         
         //retrieving the values
         for (int c = 0; c < 5; c++) {
            Console.Write(intArray.getItem(c) + " ");
         }
         
         Console.WriteLine();
         
         //declaring a character array
         MyGenericArray<char> charArray = new MyGenericArray<char>(5);
         
         //setting values
         for (int c = 0; c < 5; c++) {
            charArray.setItem(c, (char)(c+97));
         }
         
         //retrieving the values
         for (int c = 0; c< 5; c++) {
            Console.Write(charArray.getItem(c) + " ");
         }
         Console.WriteLine();
         
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

0 5 10 15 20
a b c d e

Generic Methods

Generic methods allow you to define a method with type parameters and operate with different types of parameters without defining multiple methods.

Example

In the previous example, we used a generic class; we can declare a generic method with a type parameter. The following program illustrates this concept −

using System;
using System.Collections.Generic;
namespace GenericMethodAppl {
   class Program {
      static void Swap<T>(ref T lhs, ref T rhs) {
         T temp;
         temp = lhs;
         lhs = rhs;
         rhs = temp;
      }
      static void Main(string[] args) {
         int a, b;
         char c, d;
         a = 10;
         b = 20;
         c = 'I';
         d = 'V';
         
         //display values before swap:
         Console.WriteLine("Int values before calling swap:");
         Console.WriteLine("a = {0}, b = {1}", a, b);
         Console.WriteLine("Char values before calling swap:");
         Console.WriteLine("c = {0}, d = {1}", c, d);
         
         //call swap
         Swap<int>(ref a, ref b);
         Swap<char>(ref c, ref d);
         
         //display values after swap:
         Console.WriteLine("Int values after calling swap:");
         Console.WriteLine("a = {0}, b = {1}", a, b);
         Console.WriteLine("Char values after calling swap:");
         Console.WriteLine("c = {0}, d = {1}", c, d);
         
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Int values before calling swap:
a = 10, b = 20
Char values before calling swap:
c = I, d = V
Int values after calling swap:
a = 20, b = 10
Char values after calling swap:
c = V, d = I

Generic Delegates

You can define a generic delegate with type parameters. For example −

delegate T NumberChanger<T>(T n);

Example

This is another example of a generic class. Here, we demonstrate the use of the delegates −

using System;
using System.Collections.Generic;

delegate T NumberChanger<T>(T n);
namespace GenericDelegateAppl {
   class TestDelegate {
      static int num = 10;
      
      public static int AddNum(int p) {
         num += p;
         return num;
      }
      public static int MultNum(int q) {
         num *= q;
         return num;
      }
      public static int getNum() {
         return num;
      }
      static void Main(string[] args) {
         //create delegate instances
         NumberChanger<int> nc1 = new NumberChanger<int>(AddNum);
         NumberChanger<int> nc2 = new NumberChanger<int>(MultNum);
         
         //calling the methods using the delegate objects
         nc1(25);
         Console.WriteLine("Value of Num: {0}", getNum());
         
         nc2(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Value of Num: 35
Value of Num: 175

Generic Interfaces

Generic interfaces allow you to implement classes to specify the actual data types.

Consider the following syntax to declare a generic interface −

public class Repository<T> : IRepository<T>

Example

The following example demonstrates the use of a generic interface and its implementation in a generic class to manage a collection of items:

using System;
using System.Collections.Generic;

public interface IRepository<T>
{
    void Add(T item);
    IEnumerable<T> GetAll();
}

public class Repository<T> : IRepository<T>
{
    private List<T> _items = new List<T>();

    public void Add(T item)
    {
        _items.Add(item);
    }

    public IEnumerable<T> GetAll()
    {
        return _items;
    }
}

class Program
{
    static void Main()
    {
        IRepository<string> stringRepo = new Repository<string>();
        stringRepo.Add("Item 1");
        stringRepo.Add("Item 2");

        foreach (var item in stringRepo.GetAll())
        {
            Console.WriteLine(item);
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Item 1
Item 2

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