In this example, we extend our calculator application to include Validation.

As we mentioned earlier, Qforms will go through a validation process just before it executes any Server-based actions, if needed. If the Control that triggers the ServerAction has its CausesValidation property set to "true", then before executing the ServerAction, the Form will go through every visible control in the entire Form and call Validate(). Only after ensuring that every control is valid, will the Form go ahead and execute the assigned ServerAction. Otherwise, every Control that had its Validate() fail will have its ValidationError property set with the appropriate error message.

What the validation checks for is dependent on the control you are using. In general, QControls that have their Required property set to "true" will check to ensure that data was at least entered or selected. Some controls have additional rules. For example, we'll use QIntegerTextBox here to have Qforms ensure that the data entered in our two textboxes are valid integers.

So we will utilize the Qforms validation in our application by doing the following:

• Set our btnCalculate button's CausesValidation property to true
• Use QIntegerTextBox classes
• For those textboxes, we will use RenderWithError() instead of Render() in the HTML template code. This is because Render() only renders the control, itself, with no other markers or placeholders. RenderWithError() will be sure to render any error/warning messages for that control if needed.
• Lastly, we will add our first "business rule": ensure that the user does not divide by 0. This rule will be implemented as an if statement in the Form_Validate method.

For more advanced users, note that CausesValidation can also be set to QCausesValidation::SiblingsAndChildren or QCausesValidation::SiblingsOnly. This functionality is geared for developers who are creating more complex QForms with child controls (either dynamically created, via custom composite controls, custom QPanels, etc.), and allows for more finely-tuned direction as to specify a specific subset of controls that should be validated, instead of validating against all controls on the form.

SiblingsAndChildren specifies to validate all sibling controls and their children of the control that is triggering the action, while SiblingsOnly specifies to validate the triggering control's siblings, only.