जगदीश खोलिया: Generics in C#

Monday, June 4, 2012

Generics in C#

Parametric Polymorphism is a well-established programming language feature. Generics offers this feature to C#.
The best way to understand generics is to study some C# code that would benefit from generics. The code stated below is about a simple Stack class with two methods: Push () and Pop (). First, without using generics example you can get a clear idea about two issues: a) Boxing and unboxing overhead and b) No strong type information at compile type. After that the same Stack class with the use of generics explains how these two issues are solved.
Example Code
Code without using generics:
public class Stack
object[] store; int size; public void Push(object x) {...} public object Pop() {...}

Boxing and unboxing overhead:
You can push a value of any type onto a stack. To retrieve, the result of the Pop method must be explicitly cast back. For example if an integer passed to the Push method, it is automatically boxed. While retrieving, it must be unboxed with an explicit type cast.
Stack stack = new Stack();
int i = (int)stack.Pop(); //unboxing with explicit int casting
Such boxing and unboxing operations add performance overhead since they involve dynamic memory allocations and run-time type checks.
No strong Type information at Compile Time
Another issue with the Stack class: It is not possible to enforce the kind of data placed on a stack. For example, a string can be pushed on a stack and then accidentally cast to the wrong type like integer after it is retrieved:
Stack stack = new Stack();
//pushing the stringint i = (int)stack.Pop(); //run-time exception will be thrown at this point
The above code is technically correct and you will not get any compile time error. The problem does not become visible until the code is executed; at that point an InvalidCastException is thrown.
Code with generics
In C# with generics, you declare class Stack <T> {...}, where T is the type parameter. Within class Stack <T> you can use T as if it were a type. You can create a Stack as Integer by declaring Stack <int> or Stack as Customer object by declaring Stack<Customer>. Simply your type arguments get substituted for the type parameter. All of the Ts become ints or Customers, you don't have to downcast, and there is strong type checking everywhere.
public class Stack<T>
// items are of type T, which is kown when you create the objectT[] items; int count; public void Push(T item) {...}//type of method pop will be decided when you creat the object public T Pop()
In the following example, int is given as the type argument for T:
Stack<int> stack = new Stack<int>();
int i = stack.Pop();
The Stack<int> type is called a constructed type. In the Stack<int> type, every occurrence of T is replaced with the type argument int. The Push and Pop methods of a Stack<int> operate on int values, making it a compile-time error to push values of other types onto the stack, and eliminating the need to explicitly cast values back to their original type when they are retrieved.
You can use parameterization not only for classes but also for interfaces, structs, methods and delegates.
//For Interfaces interface IComparable <T>//for structs struct HashBucket <K,D>//for methods static void Reverse <T> (T[] arr)//for delegates delegate void Action <T> (T arg)

What you can get with Generics
Generics can make the C# code more efficient, type-safe and maintainable.
Efficiency: Following points states that how performance is boosted.
  1. Instantiations of parameterized classes are loaded dynamically and the code for their methods is generated on demand [Just in Time].
  2. Where ever possible, compiled code and data representations are shared between different instantiations.
  3. Due to type specialization, the implementation never needs to box values of primitive types.
Safety: Strong type checking at compile time, hence more bugs caught at compile time itself.
Maintainability: Maintainability is achieved with fewer explicit conversions between data types and code with generics improves clarity and expressively.
Generics gives better performance, type safety and clarity to the C# programs. Generics will increase program reliability by adding strong type checking. Learning how to use generics is straightforward, hopefully this article has inspired you to look deeper into how you can use them.
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