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Object-Oriented Programming with Visual Basic .NET by J.P. Hamilton

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3.5. Passing Parameters

Data types fall into two categories: values types and reference types. Examples of value types include the built-in data types like Short, Integer, and Long (System.Int16, System.Int32, and System.Int64). Value types are created on the stack, which is the area of memory that is local to a method. When the location is destroyed, the value type is destroyed—the garbage collector does not manage the stack.

Reference types are defined using classes and created on the heap, which is available for the lifetime of the object. The garbage collector manages this area of memory.

By default, both types are passed by value; a copy of the value is placed on the stack. Therefore, any changes made to a variable are local to the method. Example 3-4 demonstrates this concept.

Example 3-4. Passing parameters ByVal
Imports System
   
Public Class App
   
  Private Class Counter
   
    Public Sub Increment(ByVal x As Integer)
      x += 1
    End Sub
   
  End Class
   
  Public Shared Sub Main( )
    Dim cc As New Counter( )
    Dim x As Integer = 1
    cc.Increment(x)
    Console.WriteLine(x.ToString( ))
    Console.WriteLine( )
    Console.WriteLine("Hit ENTER to continue.")
    Console.ReadLine( )
  End Sub
   
End Class

When Example 3-4 is executed, the output will be 1 because a copy of x was passed to the Counter.Increment method. It is useful to use the ByVal keyword explicitly instead of relying on the default behavior:

Public Sub Increment(ByVal x As Integer)

Replacing ByVal with ByRef causes a reference to be passed instead of ...

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