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Make a Parameter CONST?

If I have a parameter that has an Object type (as opposed to something like
a string), can I make that parameter a CONST?

Right now, if you pass an object into a sub/function, that sub/function can
modify the object no matter how it's defined (ByVal or ByRef).

In some cases, I want to make sure that you cannot modify the object. Is
there a way to do that in VS2003 or the up comming VS2005?
Brien King
Nov 21 '05
20 5066
Thus your query

"Brien King" wrote:

Sorry, but that is not what happens, and thus my query. In .NET, if you
pass an object ByVal, it does NOT copy the object itself and the original
object DOES get modified. I have tested this, and it's easy to demonstrate.

Here is a quick example:

Public Class TTestClass
Private m_myVar As String
Property MyVar() As String
Return m_myVar
End Get
Set(ByVal Value As String)
m_myVar = Value
End Set
End Property
End Class

Private Sub Button4_Click(B yVal sender As System.Object, ByVal e As
System.EventArg s) Handles Button4.Click
Dim testClass As New TTestClass
Dim testString As String

ModifyMyTestCla ss(testClass)
MessageBox.Show (testClass.MyVa r)

ModifyMyTestStr ingByVal(testSt ring)
MessageBox.Show (testString)

ModifyMyTestStr ingByRef(testSt ring)
MessageBox.Show (testString)

End Sub

Private Sub ModifyMyTestCla ss(ByVal P_testClass As TTestClass)
P_testClass.MyV ar = "This shouldn't leave this method... but it does!"
End Sub

Private Sub ModifyMyTestStr ingByVal(ByVal P_testString As String)
P_testString = "This won't leave this method..."
End Sub

Private Sub ModifyMyTestStr ingByRef(ByRef P_testString As String)
P_testString = "This WILL leave this method..."
End Sub

When you run that code, you'll see the message "This shouldn't leave this
method... but it does!".

Technically, a String is an Object in .NET so you would expect the same
behaviour, but it behaves as I would expect it to. The first Message box is
Empty, the second has a value.

Now I had made the wrong assumption up to this point, that it actually made
a copy of the object that is being passed in and that it wouldn't modify the
original object. If I wanted to modify the original object I would have had
to specify ByRef. To me is was the same as passing in someting in C++ as &
or passing in as const &. Apparently I was wrong.

I don't need to work around it, I just needed to be aware that is the way it
works. Because it behaves this way, it would be EASY to introduce subtle
bugs into the code due to the fact that it DOES modify the original object.

It would be Nice to be able to specify that a Parameter is CONST so that the
compiler would catch an assignment to that parameter. The main reason for
me to use that is to ensure that every INPUT parameter is exactly that.

Brien King
"DWS" <DW*@discussion s.microsoft.com > wrote in message
news:2F******** *************** ***********@mic rosoft.com...
Your right the sub or function can modify the objects you pass to em!

Byval the sub can modify a temporary copy of the object.
Byref the sub can modify the object.

Good Luck

Byval = a copy of the object. its just a copy so when you return from the
sub or function the original object is not changed.

ByRef = the actual object.

Byref = the function can actually change the object.

Nov 21 '05 #21

This thread has been closed and replies have been disabled. Please start a new discussion.

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