Section 3.5. Validate Input While the User Types

3.5. Validate Input While the User Types

Visual Basic 6 and Access both provide developers with masked editing controls: text input controls that automatically format your input as you type it in based on a specific mask. For example, if you type 1234567890 into a masked input control that uses a telephone-number mask, the number is displayed as the string (123) 456-7890.

Note: VB 6 programmers accustomed to the ActiveX MaskedEdit control were disappointed to find . NET did not include a replacement. In . NET 2.0, the new MaskedTextBox fills the gap.

Masked input controls not only improve the presentation of certain valuesthey also prevent errors. Choosing the right mask ensures that certain characters will be rejected outright (for example, a telephone- number mask will not accept letters). Masked input controls also neatly avoid canonicalization errors, which occur when there is more than one way of representing the same information. For example, with the telephone number mask, the user will immediately realize that an area code is required, even if you don't specifically explain this requirement.

3.5.1. How do I do that?

.NET 2.0 includes a new control named MaskedTextBox that extends the TextBox control. Once you've added a MaskedTextBox to a form, you can set the mask in two ways:

  • You can choose one of the prebuilt masks.

  • You can define your own custom mask.

To set a mask, click the MaskedTextBox smart tag and select Set Mask. The Input Mask dialog box appears, with a list of commonly used masks, including masks for phone numbers, zip codes, dates, and so on. When you select a mask from the list, the mask is displayed in the Mask text box. You can now customize the mask. You can also try the mask out using the Try It text box, as shown in Figure 3-7.

Figure 3-7. Selecting a mask for the MaskedTextBox

Note: Thanks to the wonders of COM Interop, it's still possible to use the VB 6 MaskedEdit control in . NET. However, the . NET MaskedTextBox control improves on several limitations and quirks in the MaskedEdit control, so it's still superior.

The mask you choose will be stored in the MaskTextBox.Mask property. Once you've chosen a mask, it will be applied whenever the user types in the MaskedTextBox. If you want to respond to user mistakes (like invalid characters) to provide more information, you can respond to the MaskInputRejected event.

If you want to build a custom mask, you need to understand a little more about how masking works. Essentially, a mask is built out of two types of characters: placeholders, which designate where the user must supply a character; and literals, which are used to format the value. For example, in the phone number mask (999)-000-000, the hyphens and brackets are literals. These characters are always present and can't be deleted, modified, or moved by the user. The number 0 is a placeholder that represents any number character, while the number 9 is a placeholder that represents an optional numeric character.

Table 3-1 lists and explains all the characters you can use to create a mask. You can use this as a reference to build your own masks.

Table 3-1. Mask characters




Required digit (0-9).


Optional digit or space. If left blank, a space is inserted automatically.


Optional digit, space, or plus/minus symbol. If left blank, a space is inserted automatically.


Required ASCII letter (a-z or A-Z).


Optional ASCII letter.


Required Unicode character. Allows anything that isn't a control key, including punctuation and symbols.


Optional Unicode character.


Required alphanumeric character (allows letter or number but not punctuation or symbols).


Optional alphanumeric character.


Decimal placeholder.


Thousands placeholder.


Time separator.


Date separator.


Currency symbol.


All the characters that follow will be converted automatically to lowercase as the user types them in. (There is no way to switch a subsequent portion of the text back to mixed-case entry mode once you use this character.)


All the characters that follow will be converted automatically to uppercase as the user types them in.


Escapes a masked character, turning it into a literal. Thus, if you use \& it is interpreted as a literal character &, which will be inserted in the text box.

All other characters

All other characters are treated as literals, and are shown in the text box.

Finally, there are a few more properties that the MaskedTextBox provides (and you might want to take advantage of). These include:


If the user inputs an invalid character and BeepOnError is TRue, the MaskedTextBox will play the standard error chime.


When the text box is empty, every required value is replaced with a prompt character. By default, the prompt character is the underscore (_), so a mask for a telephone number will display (_ _ _)-_ _ _-_ _ _ _ while empty.


Returns TRue if there are no empty characters in the text box (meaning the user has entered the required value).


InputText returns the data in the MaskedTextBox without any literal characters. For example, in a MaskedTextBox that allows the user to enter a telephone number, the Text property will return the fully formatted number, like (123)-456-7890, while InputText returns just the numeric content, or 1234567890.

3.5.2. What about...

...using masked editing in other input controls? It is possible, but not easy. The MaskedTextBox relies on a special MaskedEditProvider class in the System.ComponentModel namespace.

To create a different type of masked control, you need to create a custom control that uses the MaskedEditProvider internally. When your control receives a key press, you need to determine the attempted action and pass it on to the MaskedEditProvider using methods like Add( ), Insert( ), Remove( ), and Replace( ). Then, you can retrieve the new display value by calling MaskedEditProvider.ToDisplayString( ), and refresh your custom control appropriately. The hard part is handling all of this low-level editing without causing flicker or losing the user's place in the input string. For more information, you can refer to the full example that's included with the downloadable code in the MaskedEditing project.

Visual Basic 2005(c) A Developer's Notebook
Visual Basic 2005: A Developers Notebook
ISBN: 0596007264
EAN: 2147483647
Year: 2006
Pages: 123

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