Check if an Object exists in VBScript

I'm m开发者_StackOverflow中文版aintaining a Classic ASP app written in VB Script by an outside company long, long ago.

I have an array of imagefile paths, like so:

dim banners, arrKeys, i
set banners=CreateObject("Scripting.Dictionary")
banners.Add "banner1.jpg", "http://www.somelink.com"
banners.Add "banner2.jpg", "http://www.somelink.com"
banners.Add "banner3.jpg", "http://www.somelink.com"

This will exist ONLY on pages that have banner ads. There is some standard code that iterates through this list in an include file (common to all pages).

If Not banners Is Nothing then 
  ' then loop through the Dictionary and make a list of image links
End if

The problem is that if banners is not instantiated on the page (it's not on all pages), I get a Can't find object error

What's the proper way to check if an object exists in VB Script?

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@Atømix: Replace

If Not banners Is Nothing then 

and use

If IsObject(banners) Then 

Your other code you can then place into an include file and use it at the top of your pages to avoid unnecessary duplication.

@Cheran S: I tested my snippets above with Option Explicit on/off and didn't encounter errors for either version, regardless of whether Dim banners was there or not. :-)

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IsObject could work, but IsEmpty might be a better option - it is specifically intended to check if a variable exists or has been initialised.

To summarize:

• IsEmpty(var) will test if a variable exists (without Object Explicit), or is initialised
• IsNull(var) will test if a variable has been assigned to Null
• var Is Nothing will test if a variable has been Set to Nothing, but will throw an error if you try it on something that isn't an object
• IsObject(var) will test if a variable is an object (and will apparently still return False if var is Empty).

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If a variable is declared, but not initialized, its value will be Empty, which you can check for with the IsEmpty() function:

Dim banners
If IsEmpty(banners) Then
    Response.Write "Yes"
Else
    Response.Write "No"
End If
' Should result in "Yes" being written

banners will only be equal to Nothing if you explicitly assign it that value with Set banners = Nothing.

You will have problems, though, with this technique if you have Option Explicit turned on (which is the recommendation, but isn't always the case). In that case, if banners hasn't been Dimed and you try to test IsEmpty(banners), you will get a runtime error. If you don't have Option Explicit on, you shouldn't have any problems.

edit: I just saw this related question and answer which might help, too.

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Somewhat related is IsMissing() to test if an optional parameter was passed, in this case an object, like this:

Sub FooBar(Optional oDoc As Object)

    'if parameter is missing then simulate it
    If IsMissing(oDoc) Then Dim oDoc as Object: oDoc = something

...

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You need to have at least dim banners on every page.

Don't you have a head.asp or something included on every page?

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Neither of IsEmpty, Is Object, IsNull work with the "Option Explicit" Setting, as stealthyninja above has misleadingly answered. The single way i know is to 'hack' the 'Option Explicit' with the 'On Error Resume Next' setting, as Tristan Havelick nicely does it here: Is there any way to check to see if a VBScript function is defined?