C#: System.Object vs Generics

Question

I'm having a hard time understanding when to use Object (boxing/unboxing) vs when to use generics.

For example:

public class Stack 
{
    int position;
    object[] data = new object[10];
    public void Push (object o) { data[position++] = o; }
    public object Pop() { return data[--position]; }
}

VS.

public class Stack<T>
{ 
  int position; 
  T[] data = new T[100]; 
  public void Push(T obj)  {data[position++] = obj; }
  public T Pop() { return data[--position]; }
 }

Which one should I use and under what conditions? It seems like with the System.Object way I can have objects of all sorts of types currently living within my Stack. So wouldn't this be always preferable? Thanks!

Answer 1

Always use generics! Using object's results in cast operations and boxing/unboxing of value-types. Because of these reasons generics are faster and more elegant (no casting). And - the main reason - you won't get InvalidCastExceptions using generics.

So, generics are faster and errors are visible at compile-time. System.Object means runtime exceptions and casting which in general results in lower performance (sometimes MUCH lower).