Understanding C# generics much better

Question

A very simple example is the generic List<T> class. It can hold a number of objects of any type. For example, you can declare a list of strings (new List<string>()) or a list of Animals (new List<Animal>()), because it is generic.

What if you couldn't use generics? You could use the ArrayList class, but the downside is that it's containing type is an object. So when you'd iterate over the list, you'd have to cast every item to its correct type (either string or Animal) which is more code and has a performance penalty. Plus, since an ArrayList holds objects, it isn't type-safe. You could still add an Animal to an ArrayList of strings:

ArrayList arrayList = new ArrayList();
arrayList.Add(new Animal());
arrayList.Add("");

So when iterating an ArrayList you'd have to check the type to make sure the instance is of a specific type, which results in poor code:

foreach (object o in arrayList)
{
  if(o is Animal)
    ((Animal)o).Speak();
}

With a generic List<string>, this is simply not possible:

List<string> stringList = new List<String>();
stringList.Add("Hello");
stringList.Add("Second String");
stringList.Add(new Animal()); // error! Animal cannot be cast to a string

To summarize other answers with some emphasis:

1) generics enable you to write 'generic' code (i.e., it will work for multiple types). If you have 'generic' behavior you want to write, which you need to behave for differing data types, you only need to write that code once. The example of List is a great example, you can need lists of perhaps customers, products, orders, suppliers...all using the same code instantiated for each type

//  snippet
List<Customer> customers = new List<Customer>();
Customer thisCustomer = new Customer();
customers.Add(thisCustomer);

List<Order> orders = new List<Order>();
Order thatOrder = new Order();
orders.Add(thatOrder);

//  etc.

2) amazingly, generics still enable type safety! So if you try this, you will rightly get an error:

//  continued for snippet above
Order anotherOrder = new Order();
customers.Add(anotherOrder);    //  FAIL!

And you would want that to be an error, so that later on your customer processing code doesn't have to handle a bogus order showing up in the customers list.

Duplication is the root of all evil. One case of code duplication occurs when you have to perform same operation on different types of data. Generics let you avoid it by allowing you to code around a 'generic' type and later substitute it with specific types.

The other solution to this problem is to use variables of type 'System.Object' to which object of any type can be assigned. This method involves boxing and unboxing operations between value and reference types which hit performance. Also type casting keeps the code from being clean.

Generics are supported in MSIL and the CLR which makes it perform really well.

You should read these articles about generics -

http://msdn.microsoft.com/en-us/library/512aeb7t(VS.80).aspx

http://msdn.microsoft.com/en-us/library/ms379564(VS.80).aspx#csharp_generics_topic1