Collection Defined

I am processing a form and I want to double check that a field is defined so that no error occurs. How do I check if a key exists in a collection (the Request.Form collection)?I do not want to process all of the fields in the form, so I do not want to use "For Each x In Request.Form". I process a certain list of fields and just in case the field was left out, I want to prevent an error from occurring from trying to use a key that does not exist.

Code:

For Each x In arrayFields 'arrayFields contains certain field names
'verify that field actually defined
If [field is a key in the Request.Form collection] Then
stringTemp = Request.Form(x)
'do stuff
End If
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