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From the March 2016 Issue of Prestwood eMag

ASP Classic Empty String Check (Len(s&vbNullString))

By Mike Prestwood

Posted 15 years ago on 9/17/2009 and updated 9/21/2010

ASP Classic Code Snippet:

To/From Code Guide: ASP Classic Empty St... vs. Other Languages

A flashcard from our ASP Classic Flashcards Library

A code snippet from our ASP Classic Code Snippets Page

Tags: ASP Classic , Empty String Check

KB102039

Languages Focus: Empty String Check

An empty string is a zero length string, a string that is equal to null (""), or not assigned. In some languages, you can check if a string is empty by comparing it to an empty string (""). Some languages distinguish between nil and null ("") so checking if the length is 0 is easier.

ASP Classic Empty String Check

In ASP Classic, you have to add an empty string to the value being compared in order to get consistent results. For example, add &"" to your string varilable or it's code equivalent &vbNullString. Then compare to an empty string or verify it's length to 0 with Len.

Syntax Example:

All these will work for variables unassigned, set to "", or set to Null:

If s&"" = "" Then
  Response.Write("<br>Quotes with &'' say null is empty")
End If

 
If Len(s&"") = 0 Then
  Response.Write("<br>Len with &'' says null is empty")
End If

If Len(s&vbNullString) = 0 Then
  Response.Write("<br>Using vbNullString also works!")
End If

Complete Example

In the following code, notice techniques notice techniques 2a and 2c fail with Null values.

<%@LANGUAGE='VbScript'%>
<%Option Explicit%>

<html>

<body>
<h1>Empty String Check</h1>
<%
Dim s


''''''''''''''''''''''''''''
''' 1. Unassigned value. '''
''''''''''''''''''''''''''''
''Technique A
'''''''''''''
If s = "" Then
  Response.Write("<br>1a. Quotes say unassigned empty")
End If

''Technique B
'''''''''''''
If s&"" = "" Then
  Response.Write("<br>1b. Quotes say unassigned empty")
End If

''Technique C
'''''''''''''
If Len(s) = 0 Then
  Response.Write("<br>1c. Len says unassigned is empty")
End If

''Technique D
'''''''''''''
If Len(s&"") = 0 Then
  Response.Write("<br>1d. Len with &'' says unassigned is empty")
End If


''''''''''''''''''''''
''' 2. Null value. '''
''''''''''''''''''''''
s = Null

''Technique A
'''''''''''''
If s = "" Then
  Response.Write("<br>2a. Quotes say null is empty") ''Fails!!!
End If

''Technique B
'''''''''''''
If s&"" = "" Then
  Response.Write("<br>2b. Quotes with &'' say null is empty")
End If

''Technique C
'''''''''''''
If Len(s) = 0 Then
  Response.Write("<br>2c. Len says null is empty") ''Fails!!!
End If

''Technique D
'''''''''''''
If Len(s&"") = 0 Then
  Response.Write("<br>2d. Len with &'' says null is empty")
End If


''''''''''''''''''''
''' 3. "" value. '''
''''''''''''''''''''
s = ""

''Technique A
'''''''''''''
If s = "" Then
  Response.Write("<br>3a. Quotes say '' is empty") ''Fails!!!
End If

''Technique B
'''''''''''''
If s&"" = "" Then
  Response.Write("<br>3b. Quotes with &'' say '' is empty")
End If

 
''Technique C
'''''''''''''
If Len(s) = 0 Then
  Response.Write("<br>3c. Len says '' is empty") ''Fails!!!
End If

''Technique D
'''''''''''''
If Len(s&"") = 0 Then
  Response.Write("<br>3d. Len with &'' says '' is empty")
End If

 
''vbNullString is the code version of ""
''''''''''''''''''''''''''''''''''''''''
If Len(s&vbNullString) = 0 Then
  Response.Write("<br>Using vbNullString also works!")
End If
%>
</body>

</html>

More Info


Comments 1 Comments. Share a thought or comment...

Anonymous

Comment 1 of 3

Thank you so much! Beautiful and profound explanation!

---

Atos

Posted 14 years ago

Anonymous

Comment 2 of 3

Thanks a lot. Find the solution after my half day analysis. Good Tip. Sensible.

---

Maheswaran

Posted 14 years ago

Anonymous

Comment 3 of 3

thank you very much indeed, deep clear explanation

---

manuele

Posted 11 years ago

Code Contributed By Mike Prestwood:

Mike Prestwood is a drummer, an author, and creator of the PrestwoodBoards online community. He is the President & CEO of Prestwood IT Solutions. Prestwood IT provides Coding, Website, and Computer Tech services. Mike has authored 6 computer books and over 1,200 articles. As a drummer, he maintains play-drums.com and has authored 3 drum books. If you have a project you wish to discuss with Mike, you can send him a private message through his PrestwoodBoards home page or call him 9AM to 4PM PST at 916-726-5675 x205.

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Linked Certification Question(s)

The following are practice certification questions with answers highlighted. These questions were prepared by Mike Prestwood and are intended to stress an important aspect of this KB post. All our practice questions are intended to prepare you generally for passing any certification test as well as prepare you for professional work.

Advanced

1 Advanced Level Question

Question #1: Multiple Choice

Will the following code print "Hello, World!" or will it cause an error?

<%@LANGUAGE='VbScript'%>
<%Option Explicit%>

<html>

<body>
<%
Dim s

s = Null

If Len(s) = 0 Then
  Response.Write("Hello, World!")

End If

%>
</body>

</html>

Answer:

It will cause an error.

It will print "Hello, World!".

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Prestwood eMagazine

KB102039

Languages Focus: Empty String Check

ASP Classic Empty String Check

Complete Example

More Info

Comments

Linked Certification Question(s)

Advanced