Scope of Variables and Constants A Variable or Constant may exist and be Visible for an entire project, for only one form, or for only one procedure Therefore, the visibility of a variable or constant is referred to as its Scope Visibility means that “can this variable be seen” in this location The scope of a variable or constant is said to be Global, Module (Form) Level, or Local

Module Level variables or constants are accessible from all procedures of a Form A Local variable or constant may be used only within the procedure in which it is declared The Scope of a Variable declared with a DIM statement is determined by where the declaration statement is made The Lifetime of a Variable is the period of time that the variable exists The Lifetime of a Local variable is normally one execution of the procedure The Lifetime of a Module Level variable is the entire time the Form is Loaded (generally the lifetime of the entire project)

Local Declarations Any variable you declare inside a procedure The lifetime of the variable is one execution of the procedure Each time you execute (e.g. click on a command button) a procedure, the local DIM statements are executed. Each variable is created as a “fresh” new one with an initial value of 0 for numeric datatypes or “” for string When the procedure finishes, its variables disappear, their memory locations are released

Local Declarations

Module-Level Declarations When you need to use a variable or constant in more than one procedure of a form Declare at module-level [General] [Declarations]

Module-Level Declarations

Global Variables Though you can declare global variables anywhere, convention dictates that public variables be placed only in the standard code module. Examples of global and module level declarations in a SCM: –Public gcurTotalSalary as Currency 'global variable –Public Const gcurTAX_RATE as Single =.082 'global constant –Dim mcurMySalary as Currecy'module-only variable If a procedure were to contain a variable declared locally whose name is the same as a global variable (a violation of unwritten convention for variable naming), then the local variable takes precedence. A local variable, when in scope, always preempts use of a global variable. The Private statement is rarely used, because that is the default (for “Dim”)

Static Variables Static variables are local to a procedure but retain their value for the life of a project. They are often used inside a procedure that counts things and must maintain that count. Static variables are initialized once only. Thereafter they are not initialized during the project: –Private Sub Something() – Static intCount as Integer 'initialized to 0 once – intCount = intCount + 1 'remembered across invocations – ··· –End Sub

Standard Code Modules Public procedures are“visible” to all forms Public variables are visible to all forms SCM has the extension.BAS Create SCM: Project, Add Module DIM variables in the code module are visible to all procedures in the module, but not to procedures in the form modules. SCM modules do not have any event procedures because SCM has no events to which it can react. The SCM only has code and procedures.

Identifiers When you use variables and constants, it is important that you know their scope. Good programming practice to include scope information in naming conventions Use ‘m’ as a prefix to identify module-level declarations Use ‘g’ to identify variables with global scope

Multi-Form Projects Working with more than one form in a project The first form that VB displays is called the STARTUP FORM To add another form; Project Menu/Add Form or select the icon from the shortcut toolbar This form will be added to the project, and cam be viewed from the Project Explorer Window

Each form is a separate entity, and the code exists and is related only to the specific form To move between each form: FORMNAME.SHOW FORMNAME.HIDE Form style can be 1 (modal) or the default value 0 (nonmodal)

Variables & Constants in Multi- form Projects Scope of variables: –Local: available inside a procedure –Static: inside procedure, but remembered –Module level: available anywhere in a form –Global: available across forms--anywhere Global variables declared with Public Variable prefix naming conventions: m for module, g for global Scope a variable as narrowly as possible

Referring to Other Forms’ Objects You can refer to txtName in another form called frmSummary this way: frmSummary!txtName = … or frmSummary!txtName.Font.Name =... This implies that control names are unique within a form but need not be unique across forms.

An About Box Acts like a Windows Help|About box Often displays information about the programmers, designers, and so on An about box is simply a modal form with an OK button and label boxes displaying information You can use VB’s About Dialog template

A Splash Screen Splash screen displays while product loads Create: Project, Add Form, then select Splash Screen Splash screen loads first instead of main form Place splash screen load statement in Sub Main procedure in Standard Code Module