References in VB.NET
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23-08-2019 - |
Question
Somewhat unclear to me are references (pointers?) to classes in VB.NET. The question I am about to ask can be answered by a little bit of testing, but I was wondering if anybody could post a decent explanation (or links, too).
If you create a class:
Public Class ReferenceClass
Private myBooleanValue As Boolean = False
Public Property BooleanValue As Boolean
Get
Return myBooleanValue
End Get
Set(value As Boolean)
myBooleanValue = value
End Set
End Property
End Class
And then a class which actually uses this class as a property:
Public Class UsingClass
Private myReference As ReferenceClass
Public Property Reference As ReferenceClass
Get
return myReference
End Get
Set(value As ReferenceClass)
myReference = value
End Set
End Property
Public Sub New(ByVal Reference As ReferenceClass)
myReference = Reference
End Sub
End Class
And then use it like this:
Public Class RuntimeOrSomething
Public Shared myReference As ReferenceClass
Public Shared ReadOnly Property Reference As ReferenceClass
Get
If myReference Is Nothing Then myReference = new ReferenceClass()
return myReference
End Get
End Property
Public Shared Function BooleanCheck() As Boolean
Reference.BooleanValue = True
Dim tempClass As New UsingClass(Reference)
tempClass.Reference.BooleanValue = False
Return (tempClass.Reference.BooleanValue = Reference.BooleanValue)
End Sub
Public Shared Sub DoNothing()
Reference.BooleanValue = True
Dim someBoolean As Boolean = BooleanCheck
' Now Reference.Booleanvalue is "False"
End Sub
End Class
Now the function BooleanCheck
will always return true
, even though the reference is passed to the new class UsingClass
"by value", not by reference. So a copy of the class is not made, but the local variable myReference
in UsingClass
still references/points to the property Reference
in RuntimeOrSomething
.
How can this be explained elegantly?
Solution
A reference points to an instance of an object, it is not an instance of an object. Making a copy of the directions to the object does not create another object, it creates another reference that also points to the same object.
OTHER TIPS
I can't put much time into this answer now - typing on a train with a toddler on my knee - but I have a couple of articles which might help. They're written about C#, but the same applies to VB.NET:
From MSDN:
If you pass a variable argument by value using the ByVal keyword, the procedure cannot modify the variable itself. However, if the argument is a reference type, you can modify the members of the object to which it points, even though you cannot replace the object itself. In particular, you can modify the members of the object. For example, if the argument is an array variable, you cannot assign a new array to it, but you can change one or more of its elements. The changed elements are reflected in the underlying array variable in the calling code.
Since your ReferenceClass is a reference type, if you pass it ByVal, you can't replace it with a new object (which you don't), but you can muck around with its innards (which you do). Whether you pass ByRef or ByVal, mucking around with its innards will still "affect" the original object (since there is only ever one object in memory).
In this line:
Dim tempClass as New UsingClass(Reference)
the object referred to by the Reference
property is passed "by value", but it's not the object that is copied, it's the reference to that object that is copied (i.e. myReference and tempClass.Reference are two distinct "pointers" to the same object. You could then do tempClass.Reference = new ReferenceClass
and then myReference and tempClass.Reference are still two distinct "pointers", but now they each point to two different objects.
When passing classes byval/byref in VB.NET it is possible to think of it in terms of C programming and pointers such that -
ByVal = passing arguments via - a pointer ByRef = passing arguments via - a pointer to a pointer
Take strings as an example
' ByRef - modify str pointer to "point" to a new string
Sub Test_Ref(ByRef str as string)
str = "New String ByRef"
End Sub
' ByVal - can't modify str pointer must return a (pointer to) new string
Function Test_Val(ByVal str as String) as String
Return "New String ByVal"
End Sub
Sub Main()
Dim strTest as String = "Hello World!"
Console.WriteLine(strTest)
Test_Ref(strTest)
Console.WriteLine(strTest)
Test_Val(strTest)
Console.WriteLine(strTest) ' oops still pointing to same string
strTest = Test_Val(strTest)
Console.WriteLine(strTest) ' that's better :)
End Sub
Like said above, setting a object variable equal to another just sets the "pointer" to an instance of the same object in memory, if you want to clone the object look at implementing the iCloneable interface with something like
Public Function Clone() As Object Implements ICloneable.Clone
Return Me.MemberwiseClone()
End Function
With and when you allocation the object use the clone method
Dim tempClass as ReferenceClass = Reference.Clone