Visual Basic.NET BASICS Lesson 5 Exponentiation, Order of Operations, and Error Handling

2 Objectives Use the exponentiation operator to raise numbers to a power. Describe the order of operations. Use the Visible property to enhance output. Describe the purpose of comments in programs.

3 Objectives (continued) Handle run-time errors using the Try/Catch structure. Display messages using the MsgBox function. Control program flow using the Exit Sub statement.

4 Exponentiation Exponentiation is the process of raising a number to a power. The symbol that represents exponentiation is the caret (^). The operator raises the number to the left of the operator to the power that appears on the right of the operator.

5 Order of Operations From your math classes, you may recall the rules called the order of operations. Visual Basic.NET uses the same set of rules for its calculations. Visual Basic.NET allows you to use parentheses to override the order of operations.

6 Using the Visible Property to Enhance Output You have used the Visible property to make an image appear and disappear. Another example of a useful application of the Visible property is preventing labels from appearing until you are ready for the user to see the label. By initially setting the Visible property to False, the output will remain invisible until you make the labels visible in the code.

7 Using Comments The Properties window allows you to easily alter the properties of objects. Two of the most important properties of a form are the Text and Name properties.  The Text property specifies the text that appears in the title bar.  The Name property names the object for coding.

8 Handling Run-Time Errors Users will enter all kinds of unexpected data or fail to enter required data. These are exceptions or run-time errors. Run-time errors are not detected at the time the program is compiled because the error is caused by conditions that do not exist until the program is running. When a run-time error occurs, the system throws an exception.

9 Trapping Run-Time Errors with the Try/Catch Structure Visual Basic.NET allows you to write code that will executed when a run-time error occurs. To specify what code will execute when an error occurs, you must turn on error trapping. To turn on error trapping, place a Try statement above the code that may generate a run-time error. The code that will be executed if an error occurs is often called an error handler.

10 Using MsgBox One of the easiest ways to display a message of your own, such as an error message, is to use the MsgBox function. The MsgBox function causes a dialog box to pop up, displaying a message that you specify.

11 Using End Sub to Exit a Subroutine The Exit Sub statement forces the event procedure to end, regardless of whether there is more code in the procedure. The procedure will end when the flow of execution reaches the Exit Sub statement and can be placed anywhere in the subroutine. This allows the programmer to exit the subroutine for different reasons.

12 Summary The exponential operator (^) raises a number to a power. The rules that dictate the order that math operators are applied in a formula are called the order of operations. Parentheses can be used to override the order of operations. The Visible property can be used to hide a label until you are ready for the user to see it.

13 Summary (continued) The apostrophe is used to add comments to Visual Basic. NET code. Comments allow you to keep track of changes in code and explain the purpose of code. Comments are often called internal documentation. Errors that occur while a program is running are called run-time errors or exceptions. Visual Basic. NET allows you to trap errors, using Try/Catch, and execute special code that you specify to handle the error.

14 Summary (continued) The MsgBox function pops up a dialog box, delivering a message to the user. When you detect errors that cannot be completely handled, you should send the user a message and use the Exit Sub to end the event procedure before the error can cause additional problems.