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Date) You can override this zero value by initializing a variable as you declare it, as with the Integer i in Figure 43 However, other variable types you declare will not behave in such a straightforward way If you declare a variable of type Object and immediately try to reference its GetType method, your program will crash unceremoniously The rst code snippet shown in Listing 415 illustrates this Sub Main() Dim i As Integer Dim o As Object ConsoleWriteLine(iGetType()) ConsoleWriteLine(oGetType())'This line causes an error End Sub Listing 415: Wrong use of an Object type
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When the console program that contains the above code is executed, the error message shown in Figure 44 is the result Note that before the error message, the data type of the integer i is properly reported (SystemInt32 is the data type in the Common Type System that Visual Basic calls Integer)
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The result of executing Listing 415
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The error occurs because the variable o is an Object type This is the base type of all classes in VB NET, and the problem occurs because it is necessary to create a new instance of the class before we can access its methods or properties The cure
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42 Object-Orientation and Variables
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Sub Main() Dim i As Integer
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Nothing
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Dim a As BankAccount a = New BankAccount End Sub
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A value object (i) and a reference object (a)
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for this error is to assign a new object to the variable o before trying to access any of its properties or methods, as shown in Listing 416
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Sub Main() Dim i As Integer Dim o As Object = New Object() ConsoleWriteLine(iGetType()) ConsoleWriteLine(oGetType()) 'No error; the object exists End Sub Listing 416: Proper use of an Object type
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The difference is that simple variables (sometimes referred to as the primitive types or simply primitives) are implemented in such a way that a variable directly accesses the memory locations at which it is stored, while other variable types (classes) are used to refer to objects stored separately in an area of memory the CLR reserves for objects The rst type is called a value type object, while the second is a reference type Value type objects inherit from the SystemValueType class The key feature of a reference type is that the variable does not contain the object, but simply knows where to nd it (as shown in Figure 45) if one exists If a reference variable does not refer to an object, it refers to Nothing, a valid Visual Basic keyword Any attempt to access a property or method of a reference variable that refers to Nothing will cause a NullReferenceException as described in the error report in Figure 44
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Exercise 45
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Using the Person class you created in previous exercises, amend the code in Sub Main() so that the keyword New is not used to create a Person object Observe the results of this when you attempt to run the code (you should be expecting an error)
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421 Value Types and Reference Types
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Every class in the Common Type System (and every new class created in a NET program) is derived from the class Object This class is the fundamental base class from which all other NET classes and types inherit The one method it provides is GetType(), a function which makes it possible to determine the underlying type of any variable in a program Directly below the Object class in the inheritance hierarchy come the two classes ValueType and Reference These provide core de nitions of how value and reference object variables are created and used A ValueType variable contains data The implications of this are profound: every ValueType variable is independent of every other one, and since the data is tightly bound to the variable, ValueType variables can be operated on very ef ciently When a ValueType is created by a Dim statement within a sub de nition, the memory that stores the variable s value can be accessed directly by the code in the sub (see Figure 46)