Testing for an object

[Lance]

Hi,

How do you test for an object existing in C#? C# seems to differentiate
between 'null' and non-existance. In other languages I could test as so:

if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION", ex.InnerExcepti on.StackTrace.T oString());

But an exception gets thrown at the if(...) statement. 'Object Reference
not set to an Instance of an Object'.

So how can I test for an object's existance?

Thanks,
Lance

[Jon Skeet [C# MVP]]

Lance <lance@[nospam]keayweb.com> wrote:

How do you test for an object existing in C#? C# seems to differentiate
between 'null' and non-existance. In other languages I could test as so:

if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION", ex.InnerExcepti on.StackTrace.T oString());

But an exception gets thrown at the if(...) statement. 'Object Reference
not set to an Instance of an Object'.

So how can I test for an object's existance?

Firstly, you're not testing for an object's existence - you're testing
whether or not the value of a variable is a reference to an object, or
whether it's null.

Secondly, the NullReferenceEx ception you got above is almost certainly
due to ex.InnerExcepti on being null - if there's no inner exception,
you have a problem trying to get at its stack trace! You'd have the
same problem in most other languages.

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

[Hans Kesting]

"Lance" <lance@[nospam]keayweb.com> wrote in message news:10******** *****@corp.supe rnews.com...

Hi,

How do you test for an object existing in C#? C# seems to differentiate
between 'null' and non-existance. In other languages I could test as so:

if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION", ex.InnerExcepti on.StackTrace.T oString());

But an exception gets thrown at the if(...) statement. 'Object Reference
not set to an Instance of an Object'.

So how can I test for an object's existance?

Thanks,
Lance

You CAN use this, but (in this case) ONLY if ex.InnerExcepti on exists ...

so it is:
if (ex.InnerExcept ion != null && ex.InnerExcepti on.StackTrace != null)
{
....
}
Hans Kesting

[Lance]

Ok, fair enough, but how do I test for it? I tried breaking up the test, as
I had a theory that perhaps if the InnerException was missing the StackTrace
would be missing in a worse(!?) way. On the following code the exception
still gets thrown at the first if(...)
if(ex.InnerExce ption != null)

if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION", ex.InnerExcepti on.StackTrace.T oString());

"Jon Skeet [C# MVP]" <sk***@pobox.co m> wrote in message
news:MP******** *************** *@msnews.micros oft.com...

Lance <lance@[nospam]keayweb.com> wrote:

How do you test for an object existing in C#? C# seems to differentiate
between 'null' and non-existance. In other languages I could test as so:
if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION", ex.InnerExcepti on.StackTrace.T oString());

But an exception gets thrown at the if(...) statement. 'Object Reference not set to an Instance of an Object'.

So how can I test for an object's existance?

Firstly, you're not testing for an object's existence - you're testing
whether or not the value of a variable is a reference to an object, or
whether it's null.

Secondly, the NullReferenceEx ception you got above is almost certainly
due to ex.InnerExcepti on being null - if there's no inner exception,
you have a problem trying to get at its stack trace! You'd have the
same problem in most other languages.

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

[Jon Skeet [C# MVP]]

Lance <lance@[nospam]keayweb.com> wrote:

Ok, fair enough, but how do I test for it? I tried breaking up the test, as
I had a theory that perhaps if the InnerException was missing the StackTrace
would be missing in a worse(!?) way.
This is why you need to review what you mean by "missing". The value of
an expression can't be missing - but it can be null, in which case, if
you try to dereference that value (eg to try to use a property) you'll
get a NullReferenceEx ception.
On the following code the exception
still gets thrown at the first if(...)

if(ex.InnerExce ption != null)

if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION", ex.InnerExcepti on.StackTrace.T oString());

If the exception is still getting thrown, that suggests that ex itself
is null.

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

[Lance]

Thanks for the quick replies.

Assume that 'ex' is already instatiated.

I'd like to know how to *test* for the existance of a value(or an object).
I understand that if the value is empty, I can't use it. So how do I *test*
for it's existance. I am hoping to use some function like:

System.Test(obj ect)

as in

if(System.Test( object) == true)...

or even

if(System.Test( object.property ) == true)...

to see if the object (or property) exists. If the object is instantiated,
it returns true, if not false. Maybe something in the Reflection namespace
will do this?
"Jon Skeet [C# MVP]" <sk***@pobox.co m> wrote in message
news:MP******** *************** *@msnews.micros oft.com...

Lance <lance@[nospam]keayweb.com> wrote:

Ok, fair enough, but how do I test for it? I tried breaking up the test, as I had a theory that perhaps if the InnerException was missing the StackTrace would be missing in a worse(!?) way.
This is why you need to review what you mean by "missing". The value of
an expression can't be missing - but it can be null, in which case, if
you try to dereference that value (eg to try to use a property) you'll
get a NullReferenceEx ception.

On the following code the exception
still gets thrown at the first if(...)

if(ex.InnerExce ption != null)

if(ex.InnerExce ption.StackTrac e != null)

nvc.Add("** INNER EXCEPTION",

ex.InnerExcepti on.StackTrace.T oString());
If the exception is still getting thrown, that suggests that ex itself
is null.

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

[Jon Skeet [C# MVP]]

Lance <lance@[nospam]keayweb.com> wrote:

Thanks for the quick replies.

Assume that 'ex' is already instatiated.
ex isn't an object, it's a variable. I find terminology terribly
important in this kind of discussion. In particular, if ex is non-null,
then

if (ex.InnerExcept ion==null)

shouldn't throw a NullReferenceEx ception (assuming ex.InnerExcepti on is
reasonably implemented).
I'd like to know how to *test* for the existance of a value(or an object).

There's no such concept as a non-existent object. What you *can* test
is whether the value of a reference type expression is null, which you
do precisely with

if (expression==nu ll)

You really need to get away from the idea that expressions *are*
objects. At most, the value of an expression is a reference to an
object.

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