Boxing m.m

[Tony Johansson]

Hello!

I have three questions.
1. When is it useful to use boxing for value types.
2. I have earlier been using C and C++ for many years but the last year I
have been using C#.
In C# you can use pointer if you use unsafe directive but I can't see any
purpose to use the unsafe directive
with pointer. Is it right that the use of unsafe code with pointer is of no
use?
3.When you use the unboxing is it always nessesary that the type in the box
is always the same as the cast type.
An example
int i = 23;
object o = i;
i = (int)o;
//Tony

[Ben Voigt [C++ MVP]]

"Tony Johansson" <jo*****************@telia.comwrote in message
news:g9*****************@newsb.telia.net...

Hello!

I have three questions.
1. When is it useful to use boxing for value types.

Pretty much never, now that we have type-safe generic collections.

2. I have earlier been using C and C++ for many years but the last year I
have been using C#.
In C# you can use pointer if you use unsafe directive but I can't see any
purpose to use the unsafe directive
with pointer. Is it right that the use of unsafe code with pointer is of
no use?

No, but it is never necessary. You may get better performance with
pointers, then unsafe code would be useful.

3.When you use the unboxing is it always nessesary that the type in the
box is always the same as the cast type.
An example
int i = 23;
object o = i;
i = (int)o;

That's right, you must first unbox by casting to the original type, then you
can convert.

int i = 23;
object o = i;
short s = (short)(int)o; // ok
s = (short)o; // runtime error

>

//Tony

[Jon Skeet [C# MVP]]

Ben Voigt [C++ MVP] <rb*@nospam.nospamwrote:

<snip>

3.When you use the unboxing is it always nessesary that the type in the
box is always the same as the cast type.
An example
int i = 23;
object o = i;
i = (int)o;

That's right, you must first unbox by casting to the original
type, then you can convert.

Note that there are a few oddities around this in reality, which are
due to the semantics of the CLR. I can't remember many off the top of
my head, other than enums:

using System;

class Test
{
enum Fred : int
{
Foo,
Bar
}

static void Main()
{
Fred x = Fred.Bar;
object o = x;
int i = (int)o; // This works

Console.WriteLine(i);
}
}

But yes, it's a good idea to always unbox to the original type - that
should *always* work :)

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk