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Validation With GotFocus and LostFocus Events in Visual Basic 6

The technique discussed in this section is no longer recommended with VB6. Here we outline the technique without going into details of implementation.

In order to "pull back" the focus to a control that has a validation problem, VB6 uses the Validate event and the CausesValidate property as discussed in previous sections.

However, in versions of VB before VB6 the Validate event and CausesValidate property did not exist for standard controls such as the Textbox. In earlier versions of VB, a technique for retaining focus on a control was:

  1. Perform a validation check in the LostFocus event procedure.

  2. If the validation fails, call the control's SetFocus method to keep focus on the control.

Although the technique sounds simple enough, there were a number of complicating factors that the programmer had to take into account: Because the system was between two controls, calling the SetFocus method of the control being validated could cause the LostFocus event of a second control to fire. What if the second control in turn had similar validation code in its LostFocus event procedure? If there were a validation failure in the second control as well, then the second control would also try to "grab" focus back from the first control.

The first control in turn would fire its LostFocus event a second time, and so on, in an infinite loop. To handle this problem of the dueling LostFocus event procedures, it was necessary to put a global flag variable in the application that would be set during a LostFocus event procedure's validation. In that way, other controls' validation code could check this flag and would not run if another control's validation were already pending.

A second problem was caused by the fact that there were some controls (such as Cancel CommandButtons, for example) that should always be allowed to receive focus from another control on the form even if there were a validation problem. Thus, it would be necessary to put more logic in validation routines to ignore the validation logic if the proposed target control were always allowed to receive focus.

If the above seems less than robust or overly complex to you, you are not alone in your opinion. In the release of VB6, Microsoft heeded the desires of many VB programmers and provided the Validate event and the CausesValidation property (see the previous sections in this chapter) to put a stop to control validation madness.

The GotFocus/LostFocus technique just discussed in this section is therefore no longer necessary or recommended.

Other Field-Level Validation Techniques topics

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