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The different types of Visual Basic variables and constants was were described in detail in the previous chapter ([[Understanding Visual Basic Variable and Constant Types]]). In this chapter we will learn how to declare and initialize variables and constants in Visual Basic.
Before learning how to declare variables and constants it is worth a quick reminder of the difference between variables and constants in Visual Basic. Both constants and variables provide a way to reserve memory locations for the storage of values of various types (such as numbers, characters and strings). These memory locations are assigned names which can be used in Visual Basic code to reference these memory locations. For example, you might want to store the interest rate for a banking application. To do so you would declare a variable with a name of your choice (such as interestRate) and specify the variable as an ''Integer'' type (since it will be storing a number). Having declared the variable you can assign a value to it and read that value anywhere in your application code.
'''Dim''' ''variableName'' '''As''' ''variableType''
In the above outline, ''Dim'' is the keyword which indicates to Visual Basic that a variable is being declared. ''variableName'' is the name assigned to the variable. Try to use a descriptive variable name and prefix the name with something which indicates the variable type. For example, when declaring a String variable prefix the name with ''str'' (e.g . strFirstName). The ''As'' keyword precedes the declaration of the variable type (String, Date, Integer etc). For a complete list of Visual Basic variable types see [[Understanding Visual Basic Variable & Constant Types]].
To declare an Integer value named intInterestRate for example:
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It is also possible to declare multiple variables on the same line by separating each variable with a comma. When the variable variables are of the same type, the type declaration only needs to be made once at the end of the declaration:
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== Understanding Variable and Constant Scope ==
There are four levels of scope in a Visual Basic application:
=== Procedure Level Scope ===
Variables and constants declared within a Visual Basic procedure (i.e . a Function or Subroutine) are only visible and accessible to code within that procedure (for details of Visual Basic procedures see [[Visual Basic Modules and Procedures]]. For example, the variable ''intResult'' in the following code is only visible to code within the function:
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=== Module Level Scope ===
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When a variable or constant is declared outside of any procedures or code structures in a Module it is deemed to have module level scope. This means that the variable or constant is visible to all Visual basic Basic code contained in the same module, regardless of whether that code is located in a procedure or not. There is a section at the top of each module called the ''Declarations Section'' for this purpose. The Declaration section is located above all procedures and code in a module. For more information about modules see the [[Visual Basic Modules and Procedures]] chapter of this book.
=== Global Scope ===
When a variable or constant is declared as ''global'' it is visible to all procedures and modules that comprise the application. Global variables and constants, as with Module level variables, must be declared outside of any procedures in teh the Declarations section and must use the ''Public'' keyword. The syntax for declaring a global variable is as follows:
'''Public''' ''variableName'' '''As''' ''dataType''
'''Public Const''' ''constName'' '''As''' ''dataType''
== Static Variables in Visual Basic ==
When a variable is declared with Procedure level scope it only exists while the code in the corresponding procedure is executing. Once the procedure completes, the variable and the variable assigned to it are destroyed. Under certain circumstance it may be necessary for the variable to and the current value assign to it to persist beyond the life of the procedure. Next time the procedure is called, therefore, the variable still holds the value it held on the previous invocation of the procedure. The syntax to declare a static variable is as follows:
'''Static''' ''variableName'' '''As''' ''dataType''
