Why must write O AS INT and can't write (INT)O

Question

I've found in the Jon Skeet's book an example of using as operator with the types, which allows null value.

using System;

class A
{
    static void PrintValueAsInt32(object o)
    {
        int? nullable = o as int?; // can't write int? nullable = (int?)o
        Console.WriteLine(nullable.HasValue ?
                          nullable.Value.ToString() :
                          "null");
    }

    static void Main()
    {
        PrintValueAsInt32(5);
        PrintValueAsInt32("some string");
    }
}

I can't understand, why I can't write int? nullable = (int?)o? When I try to do that, I'm getting an exception.

Answer 1

Because the as operator is performing a check before cast.If types are not convertible to each other then it just returns null and avoids the InvalidCastException.

You are getting an exception when you try to perfom explicit cast because in the second call you are passing a string to the method which is not convertible to int?