Direct casting vs 'as' operator?

Question

Consider the following code:

void Handler(object o, EventArgs e)
{
   // I swear o is a string
   string s = (string)o; // 1
   //-OR-
   string s = o as str         


        

Answer 1

'as' is based on 'is', which is a keyword that checks at runtime if the object is polimorphycally compatible (basically if a cast can be made) and returns null if the check fails.

These two are equivalent:

Using 'as':

string s = o as string;

Using 'is':

if(o is string) 
    s = o;
else
    s = null;

On the contrary, the c-style cast is made also at runtime, but throws an exception if the cast cannot be made.

Just to add an important fact:

The 'as' keyword only works with reference types. You cannot do:

// I swear i is an int
int number = i as int;

In those cases you have to use casting.