Music and Videos. with Exception Handling

Transcription

1 VISUAL BASIC LAB with Exception Handling Copyright 2015 Dan McElroy - Lab Assignment Develop a Visual Basic application program that is used to input the number of music songs and the number of videos sold. The songs sell for $0.99 each and the videos sell for $4.95 each. The store keeps a 20% commission for each song and a 25% commission for each video. The program is to display the total number of items sold, the gross sales (total price), the amount of commission collected and the net amount given to the sellers. The program needs to reject invalid inputs and negative numbers. A button needs to be provided to clear the inputs and display, and a button needs to be provided to exit the program. 1

2 Define the Inputs and Outputs Music Sold Videos Sold INPUT PROCESSING OUTPUT Compute button Input sold Compute gross sales Compute commission Compute net to sellers Total items sold Total gross sales Total commission Net to sellers Clear button Exit button Clear inputs and outputs Exit program Refer to the previous lab assignments on how to create a Visual Basic project. 1. Create the Form Set the form's Text property to. This text will be displayed on the forms Title Bar. 2

3 Use a Label to put a title on the form. Set the Text property to display your name and "Online Store" 2. Put a Title on the Form Set the Name property to lbltitle (You may need to scroll to the top of the properties to see the Name property) 3. Set the Title Font to 16pt, Bold Make sure that the Label is selected, then click the square button with the inside to open the font dialog box. Adjust the size of the form if the label is too big for the form. 3

4 4. Music & Videos Sold GroupBox Find GroupBox in the Containers section of the ToolBox. Click the arrow to expand the list. Put a GroupBox on the form. Set the Text property to Music && Video Sales Because the ampersand & is a control character, use two ampersands && to display one &. 5. Labels and TextBoxes for Input Make sure that the GroupBox is still selected then add a Label and a TextBox to be used for inputting the number of music songs and videos sold. Set the Text property of the labels to Name these controls: lblmusic txtmusic lblvideos txtvideos 4

5 Make sure that the GroupBox is still selected then add a Labels for outputting the results. Set the Name and Text properties as follows: Name lblitemssold lblgrosssales lblcommission Text 6. Labels for Output Items Sold Gross Sales Commission lblnettosellers Net to Sellers Adjust the size of the form if needed. 7. Buttons for Compute, Clear and Exit Make sure that the GroupBox is still selected then add the buttons. Adjust the size of the form if needed. Name btncompute btnclear btnexit The ampersand & defines a hot key: Alt-C = Compute Alt-L = Clear Alt-X = Exit Text &Compute C&lear E&xit 5

6 8. Set the Form's Accept and Escape Set the Form's AcceptButton to btncompute. The Enter key will cause the Event Handler for btncompute to be executed. Set the Form's CancelButton to btnclear. The Esc key will cause the Event Handler for btnclear to be executed. 9. Change the Background Color Change the background color so that the GroupBox, Textboxes and Buttons stand out. Pick colors that are pleasing to the eye. Change the background color of the form first. You will then need to change the background color of the lbltitle and the GroupBox. 6

7 Part-1 of Compute Button Code 8/8/ Enter the Title Block and Constants Double-click the Compute button and add the following code. Note that the constants are defined outside of any subroutine. 11. Code for the Compute Button 7

8 Part-2 of Compute Button Code 8/8/ Code for the Compute Button 13. Code for Clear and Exit Buttons 8

9 14. Test the Program Music = 5 Videos = 10 The program works correctly The program crashes If either textbox is empty or has non-numeric data, the program crashes because the CInt routine has a data type conversion error when trying to convert the text from the TextBox into an Integer value. 15. Exception Handling (Try/Catch) Put all of the Input/Process/Output code inside a Try/Catch block. Any runtime errors detected by VB while in the Try block will automatically move execution of the code to the Catch block instead of crashing the program. If the CInt is not able to convert the text in txtmusic or txtvideo to an integer, the rest of the code in the Try block is skipped and the code in the Catch block is executed instead. A Try block is also a good idea to use when doing math calculations where there is a possibility of dividing by zero. These errors can also be handled by the Catch block. 9

10 16. Test and Fix the Program Again The program no longer crashes if a TextBox is empty or has non-numeric data, but it would be nice if an empty TextBox could be treated as a zero, meaning no music or videos were sold. The specification for the project also indicates that negative input values are to be rejected. The.Trim on txtmusic.text.trim deletes leading and trailing spaces so that MusicSold will still be a zero even if spaces are entered into the TextBox and nothing else. 17. Test Everything Again Make sure that the Alt-C Alt-L Alt-X each activate the proper buttons. Make sure that the [Enter] and [Esc] keys activate their proper buttons. Test the program with all positive numbers, empty TextBoxes, negative values, text input instead of numeric (try entering "five" instead of the number "5". 10