Help with the 'is' command...

[curious1]

Can someone tell me how the 'is' command works in relation to derived classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

[Jeff Gaines]

On Wed, 19 Nov 2003 08:01:04 -0800, "curious1"
<an*******@discussions.microsoft.com> wrote:

Can someone tell me how the 'is' command works in relation to derived classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

My vote would be false, it's not A - it's B.

Simple enough to try though, what results do you get if you put a
small test app together?

--
Jeff Gaines Damerham Hampshire UK

[bonehead1]

Sorry all,

My own fault for not actually deriving the class from the original (forgot the :A in the definition of B).

Thanx again.

[Jon Skeet [C# MVP]]

Jeff Gaines <je**@jgaines.co.uk> wrote:

On Wed, 19 Nov 2003 08:01:04 -0800, "curious1"
<an*******@discussions.microsoft.com> wrote:

Can someone tell me how the 'is' command works in relation to derived classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

My vote would be false, it's not A - it's B.

No, the correct answer is true, because it *is* an A in the same way
that a String is also an Object.

See section 14.9.9 of the ECMA C# spec for more details.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

[Mythran]

"Jeff Gaines" <je**@jgaines.co.uk> wrote in message
news:b5********************************@4ax.com...

On Wed, 19 Nov 2003 08:01:04 -0800, "curious1"
<an*******@discussions.microsoft.com> wrote:

Can someone tell me how the 'is' command works in relation to derived classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

My vote would be false, it's not A - it's B.

Simple enough to try though, what results do you get if you put a
small test app together?

--
Jeff Gaines Damerham Hampshire UK

Actually, it is not B. It is not A either. It is an instance of B which derives
from A so "o = A" and "o = B". BUT, if you have:

Class A {}
Class B:A{}

object a = new A();
object b = new B();

then

a = A;
a != B;
b = A;
b = B;

get it?

Off top of my head using my own thinking process, which is 50% correct .. or not.

Mythran

[100]

Hi curious1,

Answer is *true*
o is B and B is A so o is A as well

You can try it.

B\rgds
100
"curious1" <an*******@discussions.microsoft.com> wrote in message
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived classes?
For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

[Paul Robson]

Jeff Gaines wrote:

On Wed, 19 Nov 2003 08:01:04 -0800, "curious1"
<an*******@discussions.microsoft.com> wrote:

Can someone tell me how the 'is' command works in relation to derived classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

My vote would be false, it's not A - it's B.

Simple enough to try though, what results do you get if you put a
small test app together?

I'm pretty sure it is true. One would expect A is System.Object to be
true for example..... because it is :)

[Bruno Jouhier [MVP]]

Of course it will return true.

Why don't you try it live? I don't really understand people who post
questions that can be tested with a 2 line program (that they actually write
when they post their questions!). Also, looking at the reference manual for
the "is keyword" will give you the answer (and a small source example with
the output that it produces, so you don't even have to compile and run it).

Bruno
"curious1" <an*******@discussions.microsoft.com> a écrit dans le message de
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived classes?
For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

[mikeb]

Bruno Jouhier [MVP] wrote:

Of course it will return true.

Why don't you try it live? I don't really understand people who post
questions that can be tested with a 2 line program (that they actually write
when they post their questions!). Also, looking at the reference manual for
the "is keyword" will give you the answer (and a small source example with
the output that it produces, so you don't even have to compile and run it).

Bruno

Amen to that - and even worse, you get several posts from people
*guessing* the answer!


"curious1" <an*******@discussions.microsoft.com> a écrit dans le message de
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived

classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

--
mikeb

[J.Marsch]

Related to subject:

I was reading an MS developer's blog a little while back, and they state
that if you are going to use "is" in perparation to do a cast, it's better
performance and better form to use the "as" operator and test for null.

So, they're saying that instead of doing:
if (someObject is A)
{
A anA = (A)someObject;
...
}

That we should do this:
A anA = someObject as A;
if(anA != null)
{
}

The reasont that the blogger gave is that the first example, using "is"
actually performs 2 casts, one for the is and then the "real" cast.

The "as" example only performs one cast, and by using "as" instead of "()",
no exceptions are thrown.

"curious1" <an*******@discussions.microsoft.com> wrote in message
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived classes?
For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

[J.Marsch]

Hi again. I dug up the actual blog post. The post was made by Brad Abrams:
(watch for linewrap)
http://blogs.gotdotnet.com/BradA/per...7-da18088cd8f0

"J.Marsch" <je****@ctcdeveloper.com> wrote in message
news:O4**************@TK2MSFTNGP09.phx.gbl...

Related to subject:

I was reading an MS developer's blog a little while back, and they state
that if you are going to use "is" in perparation to do a cast, it's better
performance and better form to use the "as" operator and test for null.

So, they're saying that instead of doing:
if (someObject is A)
{
A anA = (A)someObject;
...
}

That we should do this:
A anA = someObject as A;
if(anA != null)
{
}

The reasont that the blogger gave is that the first example, using "is"
actually performs 2 casts, one for the is and then the "real" cast.

The "as" example only performs one cast, and by using "as" instead of "()", no exceptions are thrown.

"curious1" <an*******@discussions.microsoft.com> wrote in message
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived

classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

[100]

Hi J.Marsch,
Both operator *as* and *is* are translated to the same IL instruction
*isinst*.
*isinst* instruction pushes the reference to the requested type or null of
the cast is incorrect on the top of the evaluation stack.
Here comes the diference between *as* and *is* instructions. Code generated
for the *as* operator just saves the reference in a variable; Code for the
*is* instruction compares the result with *null*

What I want ot say is that *is* instruction actually does the casting.
So doing
if(a is B)
{
b = (B)a;
}

Actually you do casting two times. For b = (B) the compiler emits
*castclass* instruction which is the same as *isinst* with the only
difference that the exception is thrown if the cast is imposible instead of
pushing *null* in the evaluation stack.

IMHO *is* should be used in the rare cases (I think) where you won't do
casting afterwards.

B\rgds
100

"J.Marsch" <je****@ctcdeveloper.com> wrote in message
news:%2****************@TK2MSFTNGP09.phx.gbl...

Hi again. I dug up the actual blog post. The post was made by Brad Abrams: (watch for linewrap)
http://blogs.gotdotnet.com/BradA/per...7-da18088cd8f0
"J.Marsch" <je****@ctcdeveloper.com> wrote in message
news:O4**************@TK2MSFTNGP09.phx.gbl...

Related to subject:

I was reading an MS developer's blog a little while back, and they state
that if you are going to use "is" in perparation to do a cast, it's better performance and better form to use the "as" operator and test for null.

So, they're saying that instead of doing:
if (someObject is A)
{
A anA = (A)someObject;
...
}

That we should do this:
A anA = someObject as A;
if(anA != null)
{
}

The reasont that the blogger gave is that the first example, using "is"
actually performs 2 casts, one for the is and then the "real" cast.

The "as" example only performs one cast, and by using "as" instead of

"()",

no exceptions are thrown.

"curious1" <an*******@discussions.microsoft.com> wrote in message
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived

classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.

[Bruno Jouhier [MVP]]

Makes perfect sense: the first form will test the cast twice, the second one
will test it only once. This is the costly part. Testing against null is
almost no cost.

Bruno.

"J.Marsch" <je****@ctcdeveloper.com> a écrit dans le message de
news:O4**************@TK2MSFTNGP09.phx.gbl...

Related to subject:

I was reading an MS developer's blog a little while back, and they state
that if you are going to use "is" in perparation to do a cast, it's better
performance and better form to use the "as" operator and test for null.

So, they're saying that instead of doing:
if (someObject is A)
{
A anA = (A)someObject;
...
}

That we should do this:
A anA = someObject as A;
if(anA != null)
{
}

The reasont that the blogger gave is that the first example, using "is"
actually performs 2 casts, one for the is and then the "real" cast.

The "as" example only performs one cast, and by using "as" instead of "()", no exceptions are thrown.

"curious1" <an*******@discussions.microsoft.com> wrote in message
news:0B**********************************@microsof t.com...

Can someone tell me how the 'is' command works in relation to derived

classes?

For example -
Class A {}
Class B : A {}

object o = new B();

if (o is A) {...}

Should the above if statement return true or false?

Thanx.