The is keyword

[Tony Johansson]

Hello!

I just want to check with you the usage of the is keyword is only relevant
when having reference type.

Is that correct?

//Tony

[Jon Skeet [C# MVP]]

Re: The is keyword

On Jul 31, 4:51 pm, "Tony Johansson" <johansson.ande rs...@telia.com >
wrote:

I just want to check with you the usage of the is keyword is only relevant
when having reference type.
>
Is that correct?

Well, the expression on the left has to be a reference type, but the
type name on the right could be a value type when it comes to boxing:

object o = 10;
if (o is int)
{
// Yup
}

Jon

[Ignacio Machin ( .NET/ C# MVP )]

Re: The is keyword

On Jul 31, 11:51 am, "Tony Johansson" <johansson.ande rs...@telia.com >
wrote:

Hello!
>
I just want to check with you the usage of the is keyword is only relevant
when having reference type.
>
Is that correct?
>
//Tony

the left side at least, you can do object1 is Int32.

You will fnid a different situation with the "as" operator, there the
second operator should be a reference type.
so this expression is valid:

int i = 23;
object e = i as object;

[Marc Gravell]

Re: The is keyword

Well, the expression on the left has to be a reference type

It doesn't have to be... of course the compiler knows that structs can't
be subclassed, so will raise warnings about unnecessary usage, but
consider generics:

static int? CompareToDefaul t<T>(T value) where T : struct
{
if (value is IComparable<T>)
{
return Comparer<T>.Def ault.Compare(de fault(T), value);
}
else
{
return null;
}
}

The left hand operand is definitely a value-type...

Marc

[Jon Skeet [C# MVP]]

Re: The is keyword

On Aug 1, 10:11 am, Marc Gravell <marc.grav...@g mail.comwrote:

Well, the expression on the left has to be a reference type

>
It doesn't have to be... of course the compiler knows that structs can't
be subclassed, so will raise warnings about unnecessary usage, but
consider generics:

Ooh, very true. Interesting example :)

Can you think of any non-generic value type examples? I suspect
*those* are impossible, because other than generic cases, if the
compiler knows it's a value type, it knows the exact type. I could be
missing something though.

Jon

[Marc Gravell]

Re: The is keyword

Well, you can still *use* is with known value-types; but the compiler
will ignore you ;-p But it is a warning, not an error...

But no, I can't think of any useful, non-generic value-type examples of
"is".

Marc

[Hans Kesting]

Re: The is keyword

It happens that Ignacio Machin ( .NET/ C# MVP ) formulated :

On Jul 31, 11:51 am, "Tony Johansson" <johansson.ande rs...@telia.com >
wrote:

>Hello!
>>
>I just want to check with you the usage of the is keyword is only relevant
>when having reference type.
>>
>Is that correct?
>>
>//Tony

>
the left side at least, you can do object1 is Int32.
>
You will fnid a different situation with the "as" operator, there the
second operator should be a reference type.
so this expression is valid:
>
int i = 23;
object e = i as object;

Not quite, a nullable type will work as well. The main point is that
"null" should be an acceptable value for the receiving type.

object o = "nop";
int? i = o as int?; // i becomes null

or you could use
int i2 = o as int? ?? 2; // i2 becomes 2

Hans Kesting