Hi, and welcome to another Tech Bite!

When including forms in applications, developers often wonder if they need to re-validate data on the server side after having checked it on the client. Why go to the trouble of checking the data twice?

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Even using mandatory fields and validating the data on the button's "client action," what happens on the device is never safe and ready to use.

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That's why the OutSystems Platform automatically reminds us to validate all data on the server side before saving!

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We should add an "if node" in our application and validate the Price input parameter. Drag the IF node to your logic and write the condition.

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If the validation fails, the logic should raise an exception. Pick a Raise Exception Node and select a new User Exception for the first validation. 

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Later, you will be reusing this exception, so let’s rename it to something meaningful like Invalid Parameters. 

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Now, return to your Exception node and configure the Exception Message field with the message you will display to the user.

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That raised exception could be handled on the Screen's Client Action with an Exception Handler node, but it is unnecessary. Because, for all user exceptions without a handler, the Global Exception Handler of your module will do the work. 

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You can check it on the On Exception action of the "Common" UI Flow. This will trigger an error message to be shown to the end user.

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Let's simulate an issue in the data typed by a user. To be able to test the exception in runtime, we will temporarily disable the client-side validation.

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Now, if we try to input invalid data, such as a negative value in the Price field, an error message will be displayed! Meaning the application raised an exception because the data was not valid.

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Thanks for watching.

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