Ajax

Allowing Only Certain Characters
in a Text Box
Christian Wenz

Overview
ASP.NET validation controls can ensure that only certain characters are allowed in user
input. Howeverthis still does not prevent users from typing invalid characters and trying
to submit the form.

Steps
The ASP.NET AJAX Control Toolkit contains the FilteredTextBox control whichextends a
text box. Once activated, only a certain set of characters may be entered into the field.
For this to work, we first need—as usual—the ASP.NET AJAX ScriptManager which loads
theJavaScript libraries which are also used by the ASP.NET AJAX Control Toolkit :

Then, we need a text box:
Numbers only:
Finally, the FilteredTextBoxExtender control takes care ofrestricting the characters
the user is allowed to type. First, set the TargetControlID attribute to the ID of the
TextBox control. Then, choose one of the available FilterType values:Custom—default; you have to provide a list of valid chars
LowercaseLetters—lowercase letters only
Numbers—digits only
UppercaseLetters—uppercase letters only
If the CustomFilterType is used, the ValidChars property must be set and provide a
list of characters that may be typed. By the way: if you try to paste text into the text
box, all invalid chars areremoved.
Here is the markup for the FilteredTextBoxExtender control that only allows digits
(something that would also have been possible with FilterType="Numbers"):

Run the page andtry to enter a letter—if JavaScript is enabled, it will not work; digits
however appear on the page. However note that the prot ection FilteredTextBox
provides is not bullet-proof: IfJavaScript is enabled, any data may be entered in the text
box, so you have to use additional validation means, i.e. ASP.NET’s validation controls.

Only digits may be entered