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Posted 15 Nov 2005

Is it really Numeric?

, 15 Nov 2005
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A bad trap in the IsNumeric() function.


This is not a true article, it is just a tip about a "trap" that I discovered in the VB.NET IsNumeric() function and that I need to share with the VB programmers community.


If you used VB6 in the past especially, you would know that the IsNumeric() function is able to determine if a given string is convertible to a number or not. Of course, VB.NET also supports it, as it supports many other legacy VB functions (by referencing Microsoft.VisualBasic.dll). These functions should all be avoided in favor of the .NET Framework counterparts, however many VB programmers still use them. That's why I think it's important you are aware of this trap.

The trap

Let's start with this line of code:


Honestly, do you think it will ever return "False"? No, it always returns "True", doesn't it? Well, try it. Now:

  1. Close your application;
  2. Go to Control Panel / Regional and Language Options;
  3. On the "Regional Options" tab, click on the "Customize..." button;
  4. On the "Customize Regional Options" dialog box, change the "Decimal symbol" and the "Digit grouping symbol" in order to have them identical (for example, a comma in both cases);
  5. Confirm your new settings;
  6. Re-run your application: you'll get "False" as a result.

This behavior is due to the ambiguity you created with these settings in the parsing algorithm that tries to convert your string to a number. Other functions such as CDbl() will crash throwing a quite self-explaining exception in this situation, but not IsNumeric(), which simply gives a wrong result...

Let's not discuss about the logic of setting the "Decimal symbol" equal to the "Digit grouping symbol". And let's not discuss about the fact that a system dialog box allows you this kind of a setting! Also, let's not talk about the users' stupidity (yes, I found a user so stupid!). Let's talk about a workaround, to make your code "safe" in the IsNumeric() execution.

A possible workaround

A good workaround consists in explicitly setting the current thread culture, and - with the culture - programmatically setting the "Decimal symbol" and the "Digit grouping symbol", as in:

Imports System.Globalization
Imports System.Threading
Dim ci As New CultureInfo("it-IT")
Dim nfi As New NumberFormatInfo
nfi.CurrencyDecimalSeparator = ","
nfi.CurrencyGroupSeparator = "."
ci.NumberFormat = nfi
Thread.CurrentThread.CurrentCulture = ci

Of course, these settings you are "forcing" programmatically are limited to your application context and they won't influence (hopefully) anything else in the system.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Alberto Venditti
Technical Lead
Italy Italy
I was born in 1970.

My first computer experience dates back to early 80s, with a Sinclair ZX81.
From that time on, as many "friends" say, my IT-illness has increased year by year.

I graduated in Electronic Engineering and earned the following Microsoft certifications:
MCP, MCT, MCDBA, MCSD, MCAD, MCSD for .NET (early achiever).

I worked in IT as a developer, a teacher, a consultant, a technical writer, a technical leader.
IT knowledge applied to real life is my primary interest and focus.

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Comments and Discussions

GeneralCouldnt you just... Pin
il_manti24-Nov-05 1:37
memberil_manti24-Nov-05 1:37 
GeneralRe: Couldnt you just... Pin
Alberto Venditti24-Nov-05 4:34
memberAlberto Venditti24-Nov-05 4:34 
GeneralRe: Couldnt you just... Pin
il_manti24-Nov-05 5:33
memberil_manti24-Nov-05 5:33 
GeneralRe: Couldnt you just... Pin
unklegwar28-Nov-05 7:07
memberunklegwar28-Nov-05 7:07 
GeneralRe: Couldnt you just... Pin
Alberto Venditti28-Nov-05 23:22
memberAlberto Venditti28-Nov-05 23:22 

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