1. Chapter 7 INTRODUCTION TO VB .NET Database-Driven Web Sites, Second Edition

2. Objectives In this chapter, you will: • Learn about the VB .NET programming language • Learn how to declare variables and assign values to variables • Use the VB .NET Debugger to monitor program execution • Learn how to create user-defined procedures Database-Driven Web Sites, Second Edition

3. Objectives In this chapter, you will: • Work with complex numeric and string expressions in VB .NET • Create decision and repetition (looping) structures • Create and reference object classes • Learn about the VB .NET collection class Database-Driven Web Sites, Second Edition

4. Introduction to VB .NET • VB .NET: • Full-featured object-oriented programming language • Strongly typed programming language • Commands are not case-sensitive • The dot notation is used to reference program objects, properties, and methods Database-Driven Web Sites, Second Edition

5. Introduction to VB .NET • VB .NET program commands end when the Enter key is pressed to move to a new line in the Code editor • To continue a VB .NET command beyond one line, use the line continuation character: a blank space followed by a single underline character (_) Database-Driven Web Sites, Second Edition

6. VB .NET Variables and Assignment Statements • Programs use variables to store and reference values such as numbers, text strings, dates, and other types of data • When a variable is declared, the system sets aside space in main memory that contains information about the variable Database-Driven Web Sites, Second Edition

7. VB .NET Data Types • Every variable has a data type that specifies the kind of data that the variable stores • Four basic non-numeric data types: Boolean, Date, Object, and String Database-Driven Web Sites, Second Edition

8. VB .NET Variable Names • Every variable must be given a name • Variable names: • Can be a maximum of 255 characters long • Can contain letters, numbers, or underscores (_) • Cannot begin with a number Database-Driven Web Sites, Second Edition

9. Assigning Values to Variables • Assignment statement: • Assigns a value to a variable • Syntax:variable_name assignment_operator expression • Simple expression: a literal value • Complex expression: • multiple literal values • variables • built-in or user-defined functions • object properties connected by arithmetic or concatenation operators Database-Driven Web Sites, Second Edition

10. VB .NET Variable Persistence and Scope • Variable’s persistence: how long a variable is available to commands that need to reference or modify it • Variable’s scope: the location of the commands that can reference and modify a variable • Local variable: declared within a procedure using the Dim declaration command Database-Driven Web Sites, Second Edition

11. VB .NET Variable Persistence and Scope • Local variable: can only be accessed by commands in the procedure that declares it • Class variable: • Declared within a code behind file’s Declarations block using the Private declaration command • Visible to all procedures within the code behind file, but not visible to procedures within other classes in the Web application project Database-Driven Web Sites, Second Edition

12. Using the VB .NET Debugger • Debugger: • helps find and correct errors • allows setting breakpoints to pause execution on specific program lines • provides multiple ways to view variable values during program execution • If the mouse pointer is placed on the variable in the Code editor, the variable’s value appears in a ScreenTip • Watch: can monitor the variable’s value during execution Database-Driven Web Sites, Second Edition

13. Creating VB .NET Procedures • Procedures: • Self-contained code blocks that commands in other programs can call and pass parameters to • Make the code more modular and easier to maintain and debug • Can be subroutines or functions Database-Driven Web Sites, Second Edition

14. Subroutines • Subroutine: a procedure that manipulates variable values but does not return a specific value to the calling program • Figure 7-6 shows the general syntax of a VB .NET subroutine Database-Driven Web Sites, Second Edition

15. Functions • Function: a procedure that manipulates variable values and returns a specific value to the calling program • Figure 7-8 shows the general syntax of a VB .NET function Database-Driven Web Sites, Second Edition

16. Scope of Procedures • Procedures have scopes that limit the location of commands that can call the procedures • Scope can be • Public: commands in other code modules can call the procedure and commands in custom classes can call the procedure • Private: only commands within the code behind file where the procedure is declared can call the procedure Database-Driven Web Sites, Second Edition

17. Scope of Variables in Procedures • Local variable: visible only within the procedure that declares it • Commands within other procedures cannot reference a procedure’s local variables • Class variable: is visible to all procedures in the Web form’s code behind file • Two or more Web form variables can have the same name as long as the scope of the variables is different Database-Driven Web Sites, Second Edition

18. Persistence of Variables in Procedures • Persistence: the time between when a variable is declared and when the system destroys the variable • Local variable persists from the time the variable is declared to the time that the procedure in which the variable is declared ends • Class variable persists as long as the form in which it was declared is running Database-Driven Web Sites, Second Edition

19. Passing Parameters to Procedures • When a procedure is created, a parameter list is specified in the procedure declaration • Parameter list: enables the command that calls the procedure to pass data values to the procedure • Parameters can be: • Literal values, such as the number 10 or the string “Paula Harris” • Variables Database-Driven Web Sites, Second Edition

20. Passing Parameters to Procedures • When a procedure is called, the parameters are passed to the procedure by listing the parameter values in the same order in which the parameters appear in the procedure declaration • Each parameter value must have the same data type as its associated parameter in the procedure declaration Database-Driven Web Sites, Second Edition

21. Working with Complex Expressions in VB .NET • Simple expression: a literal value such as the number 3, a variable, or a value returned by a function • Complex expression: combines multiple simple expressions using arithmetic or concatenation operators Database-Driven Web Sites, Second Edition

22. Creating Complex Numeric Expressions • Complex numeric expressions can be created in VB .NET using arithmetic operators • Multiple operations can be combined in a single command • Operations can be nested • Overflow error: occurs if a developer attempts to assign a numeric value to a variable but the value is too large for the variable’s data type Database-Driven Web Sites, Second Edition

23. Creating Complex String Expressions • Concatenation operator (&): creates a single string value by joining two or more string expressions • VB .NET supports several built-in string functions and methods to support string operations • ToUpper and ToLower: convert a string to all uppercase characters or to all lowercase characters Database-Driven Web Sites, Second Edition

24. Creating Complex String Expressions • Parse: to extract one or more characters from a string and represent the extracted characters as separate string variables • Plus sign (+) can be used for concatenation, but this usage should be avoided • The plus sign can cause errors or unpredictable results when concatenating string values that contain numeric expressions Database-Driven Web Sites, Second Edition

25. Converting Data Types in VB .NET • It is a good practice to convert a value to the correct data type explicitly before it is used in an expression that involves a numeric or string operation • The data types of VB .NET variables and object properties can be converted using the VB .NET data conversion functions and the ToString string conversion method Database-Driven Web Sites, Second Edition

26. Converting Data Types in VB .NET • VB .NET has several built-in functions that perform data conversion operations: new_value = Function (expression) • The ToString method can be used to convert non-string values to the String data type • The general syntax of the ToString method is:value.ToString Database-Driven Web Sites, Second Edition

27. VB .NET Decision Control and Repetition (Looping) Structures • Decision control structure: enables a program to execute alternate statements based on a condition that is either true or false • Repetition (looping) structure: processes multiple values the same way until an exit condition is true • The general syntax of the VB .NET If control structure is:If condition Thenprogram_statementsEnd If Database-Driven Web Sites, Second Edition

28. VB .NET Decision Control and Repetition (Looping) Structures • Sometimes in a decision control structure, the program needs to execute one set of commands if the condition is true and an alternate set of commands if the condition is false • To create this kind of a decision control structure, the If/Else form of the If decision control structure is used Database-Driven Web Sites, Second Edition

29. VB .NET Decision Control and Repetition (Looping) Structures • If/ElseIf form: allows you to create an If decision control structure that evaluates multiple different conditions and executes different commands for each condition • Select Case control structure: tests multiple conditions that compare the same variable value and then performs associated programming commands Database-Driven Web Sites, Second Edition

30. VB .NET Decision Control and Repetition (Looping) Structures • AND operator: both conditions must be true for the overall condition to be true • OR operator: either condition must be true for the overall condition to be true • NOT operator: used to execute commands if a condition is not true Database-Driven Web Sites, Second Edition

31. Creating Looping Structures • A loop • Repeatedly processes statements the same way • Executes a series of program statements and periodically evaluates an exit condition that determines whether the loop repeats or exits • Pretest loop: evaluates the exit condition before any program commands execute • Posttest loop: executes one or more program commands before evaluating the exit condition the first time Database-Driven Web Sites, Second Edition

32. Creating Custom Classes in VB .NET • Object: • Abstract representation of something in the real world • Has properties and methods that specify its behavior • Object class: defines the properties and actions of similar objects • Custom classes: programmers define specific properties and methods that are not built-in • All of the project’s Web forms can use the custom class’s methods Database-Driven Web Sites, Second Edition

33. Defining Class Properties and Methods • Class definition file: where variables that specify the class’s properties are declared • Procedures that specify the class’s methods also need to be created Database-Driven Web Sites, Second Edition

34. Creating Class Instances and Calling Class Methods • The next steps in using a custom class are to create a new object instance of the class in a Web form and use the object to call the class methods • Instantiating the class: creating an object that is a member of a class • Local object: visible only within the procedure that creates it • Module-level object: visible to all procedures within the Web form that creates it Database-Driven Web Sites, Second Edition

35. Creating Class Instances and Calling Class Methods • To create local and module-level objects:Dim object_name As New project_name.class_namePrivate object_name As New project_name.class_name • To call a class method from a Web form, the following general syntax is used:object_name.method_name (parameter1, parameter2, ...) Database-Driven Web Sites, Second Edition

36. Class Inheritance • In object-oriented programming, a class can inherit properties and methods from a base, or parent, class • In VB .NET, a developer can create a custom class that inherits properties and methods from a base class • To reference a property or method in a parent class, the property or method is prefaced with the MyBase keyword, using the following dot syntax:MyBase.name Database-Driven Web Sites, Second Edition

37. The VB .NET Collection Class • Collection class: an object class with methods that can be used to manipulate class objects in VB .NET programs • A developer can instantiate a collection and then add objects to the new collection • Collection element: each individual object within a collection Database-Driven Web Sites, Second Edition

38. The VB .NET Collection Class • To instantiate a collection:Private collection_name As New Collection() • To access individual objects within a collection, use a For Each/Next loop • For Each/Next loop: sequentially steps through the objects in a collection and allows the developer to manipulate them Database-Driven Web Sites, Second Edition

39. Summary • VB .NET • Full-featured object-oriented programming language • Supports several numeric and non-numeric data types • Assignment statements: assign values of simple or complex expressions to variables • A variable’s persistence: how long the variable remains in memory and when commands can reference the variable and modify its value Database-Driven Web Sites, Second Edition

40. Summary • Variable’s scope: the location of the commands that can reference and modify the variable • Decision control structures can be created using the If, If/ElseIf, and Select Case decision structures • Looping structures can be created using Do loops, Do While loops, and For/Next loops • VB .NET allows programmers to create custom object classes, add object instances to these classes, and create associated object methods Database-Driven Web Sites, Second Edition