Validating user input before it is evaluated provides a better solution than complex exception-handling code that may add a great deal of resource overhead to your program.
Here are the four basic techniques used to validate user input: You can restrict the allowable values within a control by using the properties that we discussed in Chapter 2, "Controls on Forms." In addition to simply restricting input to a selection from a list of values, configuration of control properties may be used in order to further limit possible input values.
When he goes back to the website on browser A, the cookie is still valid, while his account doesn't exist anymore in the database.
If you don't care, you may think the user is logged in, whereas it shouldn't.
Validation may also be performed to include the entire value entered within a control by configuring validation code to run before focus is lost or the form is closed.
When a user enters or leaves a field, the following events occur in order: event is ideal for input value validation, as you may write code to check the value presented and display an error message to the user, or prevent the loss of focus from the current control until the value has been corrected. If you set the Cancel property to True during a Validating event when the form is being closed, this will prevent the form from closing.In this modified version, we have made use of the $# special variable.We know from an earlier article () that this variable represents the number of command-line arguments passed to an executed script.If the user provides the wrong number of inputs, or inputs of incorrect type or format, the script may generate an error, and/or behave oddly (and of course, you don’t like to see either of these results).For this reason, it is strongly recommended to spend some extra time writing code that validate user’s input. If the user enters 0 as the divisor, the division operation will be something over 0 (which we know from mathematics that it is an illegal operation).The idea is so simple, an if condition checks whether the $# variable is equal to the expected number of arguments (which is one in this example).If not, an error message is printed to the user, and the script exits with an exit code 3.Now, in this modified version, we are going to check the number of arguments to verify the script will get the single argument it expects.Now, let’s see what we will get if we enter the wrong number of arguments.This property does not restrict the length of values that may be input programmatically.Manipulating access properties such as the property of a Text Box control is set to True, it may still receive focus, allowing users to scroll through its contents while preventing changes.