The Basics in VS .NET

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The Basics
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A consequence of the value type nature of Structure variables is that the actual data is copied when you assign a structure variable to another variable, whereas only a pointer to data is copied when you assign a reference value to a variable. Also note that the equality operator isn t supported for structures. This code summarizes the differ ences between classes and structures:
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This code assumes you have a PersonClass class, with the same members as the PersonStruct structure.
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Creation is similar, but structures
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don t require New. Dim aPersonObject As New Person()
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Dim aPersonStruct As PersonStruct
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New is optional. Assignment to members is identical.
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aPersonObject.FirstName = Joe"
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aPersonObject.LastName = Doe"
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aPersonStruct.FirstName = Joe"
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aPersonStruct.LastName = Doe"
Method and property invocation is also identical.
Console.WriteLine(aPersonObject.CompleteName()) Console.WriteLine(aPersonStruct.CompleteName())
=> Joe Doe
=> Joe Doe
Assignment to a variable of the same type has different effects.
Dim aPersonObject2 As Person = aPersonObject
Classes are reference types; hence, the new variable receives
a pointer to the original object.
aPersonObject2.FirstName = Ann"
The original object has been affected.
Console.WriteLine(aPersonObject.FirstName) => Ann
Dim aPersonStruct2 As PersonStruct = aPersonStruct
Structures are value types; hence, the new variable receives
a copy of the original structure.
aPersonStruct2.FirstName = Ann"
The original structure hasn t been affected.
Console.WriteLine(aPersonStruct.FirstName) => Joe
A few other features of classes aren t supported by structures in Visual Basic .NET. For example, structures implicitly inherit all the methods of the Object class, but they can neither explicitly inherit from another structure nor can they be inherited from. After this first exposure to the most important syntax changes in how modules, classes, and variables are declared and used, you re ready to see what has changed in the lan guage and how you can control execution flow under Visual Basic .NET.
3 Control Flow and Error Handling
Even with the many changes in the core language syntax, Visual Basic .NET code con tinues to look like Visual Basic. Most of the differences are in the details, which means sometimes you must dig far into the language specifications to find what s new in Visual Basic. That you must look carefully is especially true of statements that have to do with flow control, such as procedure definitions and execution flow statements, which I cover in the first part of this chapter. Later in this chapter, I show you which Visual Basic com mands have been preserved in the transition to Visual Basic .NET and which have been replaced by something else. Finally, the last portion of this chapter explains what exceptions are and the .NET way of handling errors.
Execution Flow Control
Visual Basic .NET has inherited the syntax of most execution flow statements, such as the If, For, and Do loops, but a few old-style instructions have been dropped, such as GoSub. Most syntax changes are related to how procedures are defined and invoked.
Procedures
As in previous versions, Visual Basic .NET supports Sub and Function procedures, which can be Private, Public, or Friend. A procedure s definition can include ByVal and ByRef parameters, Optional parameters, and ParamArray arguments. However, there are a few important differences that you must take into account when porting a legacy application to avoid subtle bugs and when building an application from scratch to avoid unnecessary performance hits. A major syntax change under Visual Basic .NET is that the list of arguments being passed to a procedure must be enclosed in brackets, whether you re calling a Sub or a Function procedure:
A call to a Sub procedure MySubProc(first, second)
Note that this rule applies to procedure calls but not to language keywords, such as Throw or AddHandler. Conveniently, the Visual Studio .NET editor puts a pair of parentheses around the argument list if you forget to add them yourself. If Option Strict is on, you can t rely on implicit narrowing conversions when you re passing arguments
Part I:
The Basics
to a procedure. For example, when passing a Double variable to a Single argument you must make the conversion explicit by using the CSng function:
Dim d As Double = 1.23 MyProc(CSng(d)) Sub MyProc(ByVal s As Single) End Sub
ByVal and ByRef Arguments
By default, Visual Basic .NET passes arguments using ByVal, not ByRef, as was the case with previous language versions up to and including Visual Basic 6. If you re manually porting a legacy application, you must add the ByRef keyword for all those arguments that don t have the explicit ByVal keyword. For example, the following Visual Basic procedure
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