Wednesday, 1 April 2009

Quick intro to generics

Recently ive been teaching some new developers on the team about generics, its not a new concept i know, and when you know about them you cant quite figure how people sometimes just dont know about them, they are so helpfull.

So tell me about them then!

Generics came in at .net 2.0 and are used extensively through out the .net framework internally, if you do much .net dev work you will come across then at some point.

What are generics

Generics allow you to define a type (class or structure) while leaving some of the details unspecified.

Quick Tip
When you see the following in code you know you are looking at code that uses generics

List myArray = new List();
List my2ndArray = new List();

Dim myArray As New List(Of integer)();
Dim my2ndArray As New List(Of myClass)();

The main benefits for using generics are:
Type safety (you can only put ints in a collection of ints for example)
Performance (boxing and unboxing of primitive types is eliminated)
Clarity (you can see in visual studio what type of class is allowed in a generic collection, and intellisence will also help you out)

The most common (and simple) usage is in the new (as of 2.0) generic collection classes.
Previous to .net 2.0 you had ArrayList, HashTable, Queue and stack (to name just a few) these classes allow you to store collections of objects, and since every class in .net derives from object you can store any object in these classes.
So if you can store any object in a collection what the point of limiting a collection to allow it to only store objects of a certain type? The main reason is for type safety and clarity. This is best discussed by example:

ArrayList myArray = new ArrayList();

int counter = 0;
foreach(Object o in myArray)
int myInt = (int) o;
counter += myInt;

This code will compile fine, but will throw a runtime exception when trying to cast "3" (a string) to an int. you have to be careful when adding things to your collection and you have to make sure you handle exceptions that may arise from incorrect use. I often see code testing for the type of an object before doing a cast and if it is not of the correct type ignoring it, its bad practice, error prone, and is just unnecessary when using generics.

So if we rewrite the above example using generics:

List myArray = new List();
//myArray.Add("3"); //if uncommented will not compile

int counter = 0;
foreach(int myInt in myArray)
counter += myInt;

The change to use generic list of type int has made the code type safe (you cant add anything other than an int to myArray, if you do you will get a compile time error. Also the foreach loop is much simpler, you know that all the objects in myArray are ints, so no need for explicit type casting. All in all generic collections make the code more robust and just plain easier to read and hence maintain.

Now generics goes much deeper than just generic collections but in general you wont need to know how to create your own generic types and generic methods, but i would recommend every .net developer should know, its used all over the place. But i have not created any in any of my professional work projects.

Generics on MSDN
Use MSDN documentation on your machine
Google for Generics and .net

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