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Visual Basic 6 Validation - Writing Code that Validates User Input

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Input validation is an important part of any computer application that requires user interaction. In its broadest sense, the concept applies to anything that the application does to ensure that data entered by the user is acceptable for the purposes of the application.

Input validation can take place at various times in the data entry cycle. For example, the programmer can:

  • Constrain the user's entry of data before it begins by providing very restricted data input fields that permit only valid choices. The most common way to do this is to provide standard controls that do not permit free keyboard entry, such as drop-down lists, option buttons, and check boxes.

  • Constrain the user's entry of data at the moment that it occurs by monitoring every keystroke for validity and rejecting unwanted input as it's typed. For instance, a particular entry field might seem to the user to ignore anything but numeric characters.

  • React to the user's entry of data after the user is finished, accepting or rejecting the contents of a data field when the user attempts to leave the field or close the screen.

Input validation can also have varying degrees of user participation. For instance, the program can

  • Automatically correct a user's mistakes without asking the user's opinion.

  • Warn the user of incorrect input and prompt the user to correct the input before allowing the user to continue with other activities.

The first subobjective for this chapter, verifying data entered by a user at the field level and the form level, deals mostly with the immediate validation of user input from the keyboard. In order to cover this objective, we discuss the three main Keystroke events, KeyUp, KeyDown, and KeyPress.

These events can fire at the level of individual controls and also at the level of the form, thus allowing two levels of validation. An application also usually validates field-level data when the user finishes entry in each field and attempts to leave the field by setting focus away from it. We also discuss the Validate event and CausesValidation property (new to VB6) that you can employ in this type of validation.

The second sub-objective for this chapter, enabling or disabling controls based on input in fields, typically involves a more global type of crossvalidation between data entered in two or more controls. We discuss these techniques in the section entitled "Enabling Controls Based on Input."


  1. Keystroke Events at Field and Form Level
            The KeyPress Event
            The KeyUp and KeyDown Events
            KeyPress Versus KeyUp and KeyDown
            Enabling Two-Tier Validation With the Form's KeyPreview Property

  2. Field-Level Validation Techniques
            The Validate Event and CausesValidation Property
            The Validate Event
            The CausesValidation Property
            The Change Event and Click Events
            Validation With GotFocus and LostFocus Events

  3. Enabling Controls Based on Input

  4. Miscellaneous Properties for Validation
            Data-Bound Properties


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