Last Modified:
9-15-2010

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Invoice Total Calculate Button Event Code

Now that we have some familiarity with integrating Visual Basic code with .NET Framwork Forms, we are ready to expand the requirements of pressing the button. Pressing the Calculate button should give us the following procedure:

• Value entered into Subtotal TextBox is transferred to the variable Subtotal.
• double Subtotal=Convert.ToDouble(txtSubtotal.Text);


• Determine DiscountPercent from the following requirements:
• If Subtotal >= 500 then DiscountPercent = 20%.
• If Subtotal < 500 and Subtotal >= 250 then DiscountPercent = 15%.
• If Subtotal < 250 and Subtotal >= 100 then DiscountPercent = 10%.
• Otherwise, DiscountPercent = 0%.


• Multiply the Subtotal by the DiscountPercent to get a DiscountAmount.
• DiscountAmount = Subtotal * DiscountPercent;


• Subtract the DiscountAmount from the Subtotal to get the InvoiceAmount.
• InvoiceAmount = Subtotal - DiscountAmount.


• Display the DiscountPercent in the DiscountPercent TextBox.
• txtDiscountPercent.Text = DiscountPercent.ToString("p1");


• Display the DiscountAmount in the DiscountAmount TextBox.
• txtDiscountAmount = DiscoutAmount.ToString("c");


• Display the InvoiceTotal in the InvoiceTotal TextBox.
• txtInvoiceTotal = InvoiceTotal.ToString("c");


• Set the Focus back to the Subtotal TextBox.
• txtSubtotal.Focus();

Visual Basic If/Then/Else Conditional Statements

By far the most common decision-making construct in programming is the if construct. The If Construct uses Boolean logic to evaluate an expression to either true or false. If the expression evaluates to true, then the block of statements after the If/Then statement is evaluated. If the statement is false, Visual Basic does not execute the statement block for the If Construct. The coding rules for Visual Basic If/Then/Else Constructs are as follows:

	If BooleanExpression Then
		Statements
	[Else If BooleanExpression Then
		Statements]
	[Else
		Statements]
	End If
						

	If Subtotal < 200 Then
		DiscountPercent = .15
	End If
						

	If Subtotal < 200 Then
		DiscountPercent = .15
	Else
		DiscountPercent = .20
	End If
						

	If Subtotal < 200 Then
		DiscountPercent = .15
	ElseIf Subtotal >= 200 And Subtotal < 300 Then
		DiscountPercent = .20
	ElseIf Subtotal >= 300 And Subtotal < 400 Then
		DiscountPercent = .25
	Else
		DiscountPercent = .30
	End If
						

	If CustomerType = "A" Then
		If Subtotal < 200 Then
			DiscountPercent = .15
		Else
			DiscountPercent = .20
		End If
	Else
		DiscountPercent = .10
	End If
						
					

Visual Basic Coding Recommendations

• Align statements with indentation and extra spaces to reflect the structure of the program.
• Place spaces between words, operators and values to make them more readable.
• Use blank lines before and after groups of related statements.
• Use the line-continuation character '_' to break long lines into shorter more readable lines.

With that background it is time to develop the code for the requirements stated above. Change the code in the Calculate Button event procedure as follows:

Figure VB-32: Visual Basic Invoice Total Calculate Button Event Handler ver 2.0

We are now going to test our code by following the steps below:

• Press the Start Debugging symbol.


• Enter the value 600 in the Subtotal TextBox:


• Press the <ENTER> key on your keyboard or the button.


• Repeat Subtotal values of 400, 200 and 50 to validate each of the test cases and percentages.

Below is the illustration for sample output with Subtotal input = 600:

Figure VB-33: Visual Basic Invoice Total Form At Runtime with Subtotal = 600

Code Editor and Error List Windows with syntax errors displayed.

• Visual Studio checks the syntax of your Visual Basic code as you enter it. if a syntax error occurs it is highlighted with a wavy underline in the code editor, and you can place the mouse pointer over it to display a description of the error.
• If the Error List window is open, all of the build errors are listed in that window. Then, you can double-click on any error in the list to take you to its location in the Code Editor. When you correct the error, it is removed from the error list.
• If the Error List window is closed, you can display it by selecting the Error List command from the View menu. Then, if you want to hide this window, you can click on its Auto-Hide button.
• Visul Studio doesn't detect some errors until the project is built. As a result, you may encounter more syntax errors when you build and run the project.

Visual Basic Comments Description and Syntax

• Comments: are used to help document what a progam does and what the code within it does.
• Single-line Comment: Type // before the comment.
• Use to add a comment as its own line or
• Use to add a comment at the end of a line.

The Visual Basic Help Window

• You can display a Help window be selecting an object in the Form Designer or positioning the insertion point in a keyword in the Code Editor and pressing F1.
• You can also display a Help Window by selecting a command (such as Index, Contents or Search) from Visual Studio's Help menu.
• The Help window works like a web browser and can display help topics that are available from your computer or from the Internet. You can use the buttons in its toolbar to navigate between help topics to your list of favorite topics.
• The Help Window is divided into two panes. The left pane displays the Index, Content, and Help Favorites tabs that let you locate the help topics you want to display. The right pane displays each help topic in a separate window.
• If you click on the Search button, the right pane will display a Search tab that lets you search for help topics by entering a word or phrase.
• If you click on the How Do I button, the right pane will display a How Do I tab that lets you go to a topic by clicking on a link.
• To close a tab, click on the Close button when the tab is active. To display a tab, click on the tab or select it from the Active Files drop-down list that's next to the Close button.

Visual Basic Language Essentials

At this point, we begin to delve into some details for coding Visual Basic statements. These will include coding arithmetic operations, selction and iteration statements, methods, data validation, arrays, collections dates and strings.

Visual Basic Built-In Data Types

Summary

• The built-in data types are actually aliases for the data types defined by the Common Type System of the .NET Framework
• All of the data types shown in this figure are value types, which means that they store their own data. In contrast, reference types store a reference to the area of memory where the data is stored.
• A bit is a binary digit that can have a value of one or zero. A byte is a group of 8 bits. As a result, the number of bits for each data type is the number of bytes multiplied by 8.
• Integers are whole numbers, and the first 8 data types above provide for signed and unsigned integers of various sizes.
• Since the Decimal type is the most accurate non-integer data type, it is typically used to store monetary values.
• The Unicode character set provides for over 65,000 characters, with two bytes used for each character. Each character maps to an integer value.
• The older ASCII character set that's used by most operating systems provides for 256 characters with one byte used for each character. In the Unicode character set, the first 256 characters correspond to the 256 ASCII characters.
• A Boolean data type stores a Boolean value that is either true or false.