WITH#/. NET: Why can not I postpone this number to int?

Possible duplicate:
Why can't I remove the int as a decimal number?

Ok, C # /. NET guru, can someone tell me why this works:

static void Main(string[] args)
{
    int _int = 0;
    decimal _decimal = 1;

    _int = (int)_decimal;

    Console.ReadLine();
}

... but none of them do?

static void Main(string[] args)
{
    int _int = 0;
    decimal d = 1;
    object _decimal = d;

    _int = (int)_decimal;

    Console.ReadLine();
}

static void Main(string[] args)
{
    int _int = 0;
    object _decimal = 1M;

    _int = (int)_decimal;

    Console.ReadLine();
}

I can use the decimal value for int while I am explicitly declared to be the decimal type from it, but I can not distinguish decimal from int when the decimal mark is stored in the type of the object? What's up with that?

NOTE. I know that I can probably use Convert.ToInt32 (), but I'm trying to figure out that this does not work here.