Click here to Skip to main content
15,309,768 members
Articles / Programming Languages / Visual Basic
Posted 15 Nov 2005


27 bookmarked

Is it really Numeric?

Rate me:
Please Sign up or sign in to vote.
4.52/5 (24 votes)
15 Nov 20052 min read
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.

A list of licenses authors might use can be found here


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.

Comments and Discussions

GeneralMy vote of 4 Pin
MarqW16-Feb-11 4:41
MemberMarqW16-Feb-11 4:41 
GeneralAnother way to skin this kind of cat [modified] Pin
PeterBiddlecombe22-Nov-07 22:32
MemberPeterBiddlecombe22-Nov-07 22:32 
GeneralRe: Another way to skin this kind of cat Pin
Alberto Venditti23-Nov-07 6:02
MemberAlberto Venditti23-Nov-07 6:02 
QuestionHow to fix this IsNumberic's error in VB6.0? Pin
Tiger.xing14-Mar-06 20:03
MemberTiger.xing14-Mar-06 20:03 
AnswerRe: How to fix this IsNumberic's error in VB6.0? Pin
Alberto Venditti23-Mar-06 21:10
MemberAlberto Venditti23-Mar-06 21:10 
GeneralCouldnt you just... Pin
il_manti24-Nov-05 0:37
Memberil_manti24-Nov-05 0:37 
GeneralRe: Couldnt you just... Pin
Alberto Venditti24-Nov-05 3:34
MemberAlberto Venditti24-Nov-05 3:34 
GeneralRe: Couldnt you just... Pin
il_manti24-Nov-05 4:33
Memberil_manti24-Nov-05 4:33 
GeneralRe: Couldnt you just... Pin
unklegwar28-Nov-05 6:07
Memberunklegwar28-Nov-05 6:07 
GeneralRe: Couldnt you just... Pin
Alberto Venditti28-Nov-05 22:22
MemberAlberto Venditti28-Nov-05 22:22 
QuestionAnd Regular Expression? Pin
Walter Dias23-Nov-05 0:25
professionalWalter Dias23-Nov-05 0:25 
AnswerRe: And Regular Expression? Pin
Alberto Venditti23-Mar-06 21:13
MemberAlberto Venditti23-Mar-06 21:13 
Generalalternative methods to IsNumeric, IsDate, InStr Pin
Alexis Rzewski22-Nov-05 2:31
MemberAlexis Rzewski22-Nov-05 2:31 
GeneralRe: alternative methods to IsNumeric, IsDate, InStr Pin
Alberto Venditti22-Nov-05 23:59
MemberAlberto Venditti22-Nov-05 23:59 
GeneralRe: alternative methods to IsNumeric, IsDate, InStr Pin
Alberto Venditti23-Nov-05 0:11
MemberAlberto Venditti23-Nov-05 0:11 
GeneralNaive IsNumeric() inC# Pin
Guido_d16-Nov-05 1:57
MemberGuido_d16-Nov-05 1:57 

Here's a rather naive implementation of IsNumeric in c#. Support for fractional numbers should be easy to add:

public static bool IsNumeric(string str) <br />
{<br />
    if (str==null || str.Length==0)<br />
        return false;<br />
    foreach(char c in str) <br />
    {<br />
        if (!Char.IsNumber(c)) <br />
        {<br />
            return false;<br />
        }<br />
    }<br />
    return true;<br />

GeneralRe: Naive IsNumeric() inC# Pin
Michael Potter16-Nov-05 3:38
MemberMichael Potter16-Nov-05 3:38 
GeneralRe: Naive IsNumeric() inC# Pin
J Sullivan26-Oct-07 9:36
MemberJ Sullivan26-Oct-07 9:36 
GeneralRe: Naive IsNumeric() inC# Pin
xfx19-Jan-09 21:53
Memberxfx19-Jan-09 21:53 
GeneralRe: Naive IsNumeric() inC# Pin
nikoniko23-Nov-05 20:49
Membernikoniko23-Nov-05 20:49 
GeneralRe: Naive IsNumeric() inC# Pin
percyboy19-Dec-05 14:13
Memberpercyboy19-Dec-05 14:13 
GeneralImplement Visual Basic .NET IsNumeric Functionality by Using Visual C# .NET Pin
Stan Angeloff15-Nov-05 21:22
MemberStan Angeloff15-Nov-05 21:22 
GeneralRe: Implement Visual Basic .NET IsNumeric Functionality by Using Visual C# .NET Pin
Alberto Venditti15-Nov-05 22:20
MemberAlberto Venditti15-Nov-05 22:20 
GeneralBetter way IMO Pin
mav.northwind15-Nov-05 19:30
Membermav.northwind15-Nov-05 19:30 
GeneralRe: Better way IMO Pin
Alberto Venditti15-Nov-05 22:16
MemberAlberto Venditti15-Nov-05 22:16 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.