Visual Basic.NET How to Program (PDF)

198 Procedures Chapter 6
Setting Option Strict to On in the Property Pages applies the change globally,
to the entire project. The programmer also can enable Option Strict within an individual
code file by typing Option Strict On at the start of the file above any declarations
or Imports statements.
6.8 Value Types and Reference Types
In the next section, we discuss passing arguments to procedures by value and by reference.
To understand this, we first need to make a distinction between data types in Visual Basic.
All Visual Basic data types can be categorized as either value types or reference types. A
variable of a value type contains data of that type. Normally, value types are used for a single
piece of data, such as an Integer or a Double value. By contrast, a variable of a
reference type (sometimes called a reference) contains a location in memory where data is
stored. The location in memory can contain many individual pieces of data. Collectively,
reference types are known as objects and are discussed in detail in Chapters 8, 9 and 10,
Object-Based Programming, Object-Oriented Programming: Inheritance, and Object-Oriented
Programming: Polymorphism.
Both value types and reference types include built-in types and types that the programmer
can create. The built-in value types include the integral types (Byte, Short,
Integer and Long), the floating-point types (Single and Double) and types
Boolean, Date, Decimal and Char. The built-in reference types include Object and
String (although type String often behaves more like a value type, as we discuss in
the next section). The value types that can be constructed by the programmer include
Structures and Enumerations. The reference types that can be created by the programmer
include classes, interfaces and delegates. Programmer-defined types are discussed
in greater detail in Chapter 8, Object-Based Programming, Chapter 9, Object-
Oriented Programming: Inheritance and Chapter 15, Strings, Characters and Regular
Expressions.
The table in Fig. 6.10 lists the primitive data types, which form the building blocks for
more complicated types, such as classes. If Option Explicit is set to On, all variables
must have a type before they can be used in a program. This requirement is referred to as
strong typing.
Type
Size
in bits Values Standard
Boolean 16 True or False
Char 16 One Unicode character (Unicode character set)
Byte 8 0 to 255
Date 64 1 January 0001 to 31 December 9999
0:00:00 to 23:59:59
Decimal 128 1.0E-28 to 7.9E+28
Short 16 –32,768 to 32,767
Fig. Fig. 6.10 6.10 Visual Basic primitive data types (part 1 of 2).