Visual Basic .NET Data Types and Features

This example has two value variables ( iInteger1 and iInteger2 ) and two reference variables ( newEmp1 and newEmp2 ). Both iInteger1 and iInteger2 are assigned the value 12. Obviously, these variables are equal because their data is equal, and the message box will display True. However, the two variables are separate and distinct because they occupy different memory locations. If we assigned 42 to iInteger1 , the value of iInteger2 would remain 12.

Now examine the statements concerning the reference variables newEmp1 and newEmp2 . These variables are initially dimensioned to point to different instances of myClass , and the first message box will print False. Next, newEmp1 is assigned to newEmp2 . Because these variables are reference variables, they now point to the same object in the same memory location, as shown in Figure 4-1. If any data fields in newEmp1 are changed or modified, they are also modified in newEmp2 because both variables reference the same memory location.

Figure 4-1

Reference variables point to memory locations.

When two reference variables are checked for equality, the check is really to see whether both variables reference the same memory location. If they do, as shown in Figure 4-1, they are considered equal. However, if each points to a different object of the same type—even if both objects have the same byte-for-byte values—the variables compare as unequal .

Value Types

A value type is also considered a primitive type in the .NET Framework. (A primitive type is a data type that is natively supported by the compiler. All primitive types are value types.) Value types are always accessed directly. In fact, you can't create a reference to a value type. And unlike reference types, setting a value type to Null is not possible. Value types always hold a value, even if you haven't yet assigned one. When a value type variable is dimensioned, it's initialized to a value representative of its type. If you dimension an Integer variable and don't assign a value to it, the Visual Basic .NET compiler automatically initializes the variable to 0.

The Visual Basic primitive types are identified through keywords, which are really aliases for predefined structure types in the System namespace. This fact means that a primitive type and the structure type for which it's an alias are completely indistinguishable. The Visual Basic keyword Byte is exactly the same as System.Byte .

Value types are sealed , meaning that no other type can be derived from them. Each value type is allocated a specific, fixed- size block of memory and—as I mentioned previously—is automatically initialized to a specific value by the compiler. Table 4-1 lists the Visual Basic .NET value types.

	The Currency data type in classic Visual Basic is obsolete in the .NET Framework. When dealing with monetary transactions, be sure to use the Decimal data type with two decimal places in order to prevent rounding errors.

The Structure data type is a legacy data type; it's the Visual Basic .NET name for the user -defined type (UDT) in previous versions of Visual Basic. A Structure is a concatenation of one or more members of data types, as shown here:

Structure DriveInfo DriveNumber() As Short DriveType As String DriveSpace as Long AvailableSpace as Long End Structure

A Structure is treated as a single unit, although its members can be accessed individually. Structures have two limitations: they cannot explicitly inherit from another type (although they implicitly inherit the methods of the System.Object class), and a class cannot inherit from a Structure .

Structures are frowned upon in object-oriented programming. A class can do everything that a Structure can do—and more—so you should use classes instead of Structures .

Reference Types

As I mentioned earlier, a reference variable points to the memory location of the variable's data. This arrangement allows the garbage collector to track an object flagged for deletion and free the object's memory when it's no longer needed.

A reference variable always contains a reference to an object of its type, or null ( Nothing ) if you haven't explicitly initialized the variable. (A null reference points to nothing. It's illegal to do anything with a null reference except assign it a legitimate value.) Table 4-2 lists the Visual Basic .NET reference types.

As you learned in Chapter 2, "Object-Oriented Programming in Visual Basic .NET," an object is an instantiation of a class, and an Object variable is a reference variable that holds a pointer to the actual object. The Object variable itself always consumes exactly 4 bytes because it's simply a pointer to the memory location where the object it points to is located.

Recall that when a reference variable is created, it's initialized to null. Because of this arrangement, you must be sure to assign an actual object to the variable before you try to use it. Consider the following example:

Sub main() Dim oMyObject As Object MessageBox.Show(oMyObject.GetType.ToString) End Sub

The compiler will not object (no pun intended) to code like this. However, when this code executes and innocently tries to print out the variable's type, the program crashes with the error shown in Figure 4-2.

Figure 4-2

Accessing a null reference variable causes a run-time error.