Testing for an object

  • Thread starter Thread starter Lance
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L

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.InnerException.StackTrace != null)

nvc.Add("** INNER EXCEPTION", ex.InnerException.StackTrace.ToString());

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
 
Lance said:
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.InnerException.StackTrace != null)

nvc.Add("** INNER EXCEPTION", ex.InnerException.StackTrace.ToString());

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?
Click to expand...

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 NullReferenceException you got above is almost certainly
due to ex.InnerException 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.
 
Lance said:
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.InnerException.StackTrace != null)

nvc.Add("** INNER EXCEPTION", ex.InnerException.StackTrace.ToString());

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
Click to expand...

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

so it is:
if (ex.InnerException != null && ex.InnerException.StackTrace != null)
{
....
}


Hans Kesting
 
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.InnerException != null)

if(ex.InnerException.StackTrace != null)

nvc.Add("** INNER EXCEPTION", ex.InnerException.StackTrace.ToString());
 
Lance said:
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.
Click to expand...

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

if(ex.InnerException != null)

if(ex.InnerException.StackTrace != null)

nvc.Add("** INNER EXCEPTION", ex.InnerException.StackTrace.ToString());
Click to expand...

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

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?
 
Lance said:
Thanks for the quick replies.

Assume that 'ex' is already instatiated.
Click to expand...

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.InnerException==null)

shouldn't throw a NullReferenceException (assuming ex.InnerException is
reasonably implemented).
I'd like to know how to *test* for the existance of a value(or an object).
Click to expand...

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==null)

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.
 
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