Make a Parameter CONST?

[Brien King]

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?


Thanks,


Brien King

[david]

Re: Make a Parameter CONST?

On 2005-09-17, Brien King <spammehere@arc aderestoration. com> wrote:[color=blue]
> If I have a parameter that has an Object type (as opposed to something like
> a string), can I make that parameter a CONST?[/color]

That's kinda weirdly phrased, a string IS an Object, but the answer is
no.
[color=blue]
>
> 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).[/color]
[color=blue]
> 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?[/color]

Nope. This seems to be one of those things that the CLR and .Net
language designers looked at and decided they just didn't want.

[m.posseth]

Re: Make a Parameter CONST?

just pass a class or structure as the parameter and define a property as
readonly
now you can read the parameter but you can`t change it



Private Class constVal

ReadOnly Property example()

Get

Return "this is a constant value"

End Get

End Property

End Class

Private Sub test(ByVal x As constVal)

MsgBox(x.exampl e)

End Sub

Private Sub Button1_Click(B yVal sender As System.Object, ByVal e As
System.EventArg s) Handles Button1.Click

Dim y As New constVal

test(y)

End Sub


regards

Michel Posseth

"Brien King" <spammehere@arc aderestoration. com> wrote in message
news:erx4YvyuFH A.2008@TK2MSFTN GP10.phx.gbl...[color=blue]
> 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?
>
>
> Thanks,
>
>
> Brien King
>[/color]

[Jesse Liberty]

Re: Make a Parameter CONST?

"Brien King" <spammehere@arc aderestoration. com> wrote in message
news:erx4YvyuFH A.2008@TK2MSFTN GP10.phx.gbl...[color=blue]
> If I have a parameter that has an Object type ...can I make that parameter
> a CONST?
> 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?[/color]


There is no way to do so, but there may be acceptable work-around. You can
for example create a shadow struct that has members for the values you want
to pass by value, and a method in your class to generate that struct. You
would then pass the struct, which is passed by value.


--
Jesse Liberty
Author of .NET books for O'Reilly

http://www.LibertyAssociates.com

[Herfried K. Wagner [MVP]]

Re: Make a Parameter CONST?

"Brien King" <spammehere@arc aderestoration. com> schrieb:[color=blue]
> 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?[/color]

No.

--
M S Herfried K. Wagner
M V P <URL:http://dotnet.mvps.org/>
V B <URL:http://classicvb.org/petition/>

[m.posseth]

Re: Make a Parameter CONST?

after rethinking ,,, i provided a workaround but it is absolutely useless

why the $##$$# would you do that if you could just as easily use a local (
in the same class as the method ) contstant value

maybe we should ask you what is your situation in wich you think this is
needed ???

as i said i would not pass a contstant value at all to a parameter , i would
make it a class construct parameter and call it with a read only property in
the method itself

regards

Michel Posseth [MCP]




"Brien King" <spammehere@arc aderestoration. com> wrote in message
news:erx4YvyuFH A.2008@TK2MSFTN GP10.phx.gbl...[color=blue]
> 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?
>
>
> Thanks,
>
>
> Brien King
>[/color]

[david]

Re: Make a Parameter CONST?

On 2005-09-17, m.posseth <michelp@nohaus ystems.nl> wrote:[color=blue]
> after rethinking ,,, i provided a workaround but it is absolutely useless
>
> why the $##$$# would you do that if you could just as easily use a local (
> in the same class as the method ) contstant value
>
> maybe we should ask you what is your situation in wich you think this is
> needed ???
>
> as i said i would not pass a contstant value at all to a parameter , i would
> make it a class construct parameter and call it with a read only property in
> the method itself[/color]

You're missing the point. What the OP wants is to pass reference types
to a method and ensure that the method won't change the contents of the
passed object. It would look something like this...

Public Sub Foo(ByConst list As ArrayList)

Console.Writeli ne(list(0)) ' This is OK
list(0) = "Hello" ' This would be a compile error


Because the list parameter is declared const, the Foo method can't
change the contents. This is a very common construct in C++, but
isn't supported in the CLR or the Framework.






[color=blue]
>
> regards
>
> Michel Posseth [MCP]
>
>
>
>
> "Brien King" <spammehere@arc aderestoration. com> wrote in message
> news:erx4YvyuFH A.2008@TK2MSFTN GP10.phx.gbl...[color=green]
>> 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?
>>
>>
>> Thanks,
>>
>>
>> Brien King
>>[/color]
>
>[/color]

[Cor Ligthert [MVP]]

Re: Make a Parameter CONST?

David,

[color=blue]
> You're missing the point. What the OP wants is to pass reference types
> to a method and ensure that the method won't change the contents of the
> passed object.[/color]

And in my opinion very correct answered by you last week.

http://groups.google.com/group/microsoft.public.dotnet.languages.vb/msg/c108a96be66eac3c?hl=en&

:-)

Cor

[david]

Re: Make a Parameter CONST?

On 2005-09-17, Cor Ligthert [MVP] <notmyfirstname @planet.nl> wrote:[color=blue]
> David,
>
>[color=green]
>> You're missing the point. What the OP wants is to pass reference types
>> to a method and ensure that the method won't change the contents of the
>> passed object.[/color]
>
> And in my opinion very correct answered by you last week.
>
> http://groups.google.com/group/micro...66eac3c?hl=en&
>
>:-)[/color]

Yes, the same questions do seem to pop up here quite often.

[m.posseth]

Re: Make a Parameter CONST?


No i am not missing the point ,

i do understand that what he want is not possible in this way , however i
provided a workaround to acomplish something simular
however after rethinking my first solution i thought that there is a much
better way

If i rethink this in a OOP way it isn`t so difficult to acomplish at all
( define the needed business rules in some objects on demand ) ,,, however
it is still a workaround but it will do what he wants

well .... maybe i should have mentioned the impossibility in forehand , but
as i read the hole thread i base my answer on what is previously said
( sorry :-)

regards

and have a nice weekend

Michel Posseth




"david" <david@woofix.l ocal.dom> wrote in message
news:slrndio3ot .edn.david@loca lhost.localdoma in...[color=blue]
> On 2005-09-17, m.posseth <michelp@nohaus ystems.nl> wrote:[color=green]
>> after rethinking ,,, i provided a workaround but it is absolutely
>> useless
>>
>> why the $##$$# would you do that if you could just as easily use a local
>> (
>> in the same class as the method ) contstant value
>>
>> maybe we should ask you what is your situation in wich you think this is
>> needed ???
>>
>> as i said i would not pass a contstant value at all to a parameter , i
>> would
>> make it a class construct parameter and call it with a read only property
>> in
>> the method itself[/color]
>
> You're missing the point. What the OP wants is to pass reference types
> to a method and ensure that the method won't change the contents of the
> passed object. It would look something like this...
>
> Public Sub Foo(ByConst list As ArrayList)
>
> Console.Writeli ne(list(0)) ' This is OK
> list(0) = "Hello" ' This would be a compile error
>
>
> Because the list parameter is declared const, the Foo method can't
> change the contents. This is a very common construct in C++, but
> isn't supported in the CLR or the Framework.
>
>
>
>
>
>
>[color=green]
>>
>> regards
>>
>> Michel Posseth [MCP]
>>
>>
>>
>>
>> "Brien King" <spammehere@arc aderestoration. com> wrote in message
>> news:erx4YvyuFH A.2008@TK2MSFTN GP10.phx.gbl...[color=darkred]
>>> 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?
>>>
>>>
>>> Thanks,
>>>
>>>
>>> Brien King
>>>[/color]
>>
>>[/color][/color]

[DWS]

RE: Make a Parameter CONST?

Brien,
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
DWS











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.





"Brien King" wrote:
[color=blue]
> 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?
>
>
> Thanks,
>
>
> Brien King
>
>
>[/color]

[david]

Re: Make a Parameter CONST?

On 2005-09-17, m.posseth <michelp@nohaus ystems.nl> wrote:[color=blue]
>
> No i am not missing the point ,[/color]

OK, then I misunderstood this...
[color=blue][color=green]
>>why the $##$$# would you do that if you could just as easily use a local (
>>in the same class as the method ) contstant value[/color][/color]

A local (or remote) constant value has no relevance to const parameters.
[color=blue]
> i do understand that what he want is not possible in this way , however i
> provided a workaround to acomplish something simular
> however after rethinking my first solution i thought that there is a much
> better way
>
> If i rethink this in a OOP way it isn`t so difficult to acomplish at all
> ( define the needed business rules in some objects on demand ) ,,, however
> it is still a workaround but it will do what he wants[/color]

I'm curious what you mean here, but I must admit I'm not sure what
you're getting at or what it's relevance is.

[m.posseth]

Re: Make a Parameter CONST?

Hmm well we are obviously not talking the same language :-)

when i said
[color=blue][color=green][color=darkred]
>>>why the $##$$# would you do that if you could just as easily use a local
>>>(
>>>in the same class as the method ) contstant value[/color][/color][/color]

I was refering to the previous answer i had given ( as i feel that
encapsulation in a class is a much better aproach )
[color=blue]
> I'm curious what you mean here, but I must admit I'm not sure what
> you're getting at or what it's relevance is.[/color]

Please tell me what is wrong with defining a class with friend / public
readonly properties that can only be set during construction from the
outside , as a workaround for the problem


Your answer was No and i have given an alternative aproach ,,,, and after
rereading the original question i believe it is verry relevant .... if you
feel that it isn`t then let the TS decide if this might be an alternative ,
as it is obvious that he comes here for a solution of his problem .

regards

Michel Posseth







"david" <david@woofix.l ocal.dom> wrote in message
news:slrndiouap .ers.david@loca lhost.localdoma in...[color=blue]
> On 2005-09-17, m.posseth <michelp@nohaus ystems.nl> wrote:[color=green]
>>
>> No i am not missing the point ,[/color]
>
> OK, then I misunderstood this...
>[color=green][color=darkred]
>>>why the $##$$# would you do that if you could just as easily use a local
>>>(
>>>in the same class as the method ) contstant value[/color][/color]
>
> A local (or remote) constant value has no relevance to const parameters.
>[color=green]
>> i do understand that what he want is not possible in this way , however i
>> provided a workaround to acomplish something simular
>> however after rethinking my first solution i thought that there is a much
>> better way
>>
>> If i rethink this in a OOP way it isn`t so difficult to acomplish at all
>> ( define the needed business rules in some objects on demand ) ,,,
>> however
>> it is still a workaround but it will do what he wants[/color]
>
> I'm curious what you mean here, but I must admit I'm not sure what
> you're getting at or what it's relevance is.
>
>[/color]

[david]

Re: Make a Parameter CONST?

On 2005-09-18, m.posseth <michelp@nohaus ystems.nl> wrote:[color=blue]
> Hmm well we are obviously not talking the same language :-)
>
> when i said
>[color=green][color=darkred]
>>>>why the $##$$# would you do that if you could just as easily use a local
>>>>(
>>>>in the same class as the method ) contstant value[/color][/color]
>
> I was refering to the previous answer i had given ( as i feel that
> encapsulation in a class is a much better aproach )
>[color=green]
>> I'm curious what you mean here, but I must admit I'm not sure what
>> you're getting at or what it's relevance is.[/color]
>
> Please tell me what is wrong with defining a class with friend / public
> readonly properties that can only be set during construction from the
> outside , as a workaround for the problem[/color]

You're right, we're not talking the same language. The paragraph I said
I didn't understand what you meant was "If i rethink this in a OOP way
it isn`t so difficult to acomplish at all ( define the needed business
rules in some objects on demand )", which is a whole lot more obscure
than what you just said. But no matter...
[color=blue]
> Your answer was No and i have given an alternative aproach ,,,, and after
> rereading the original question i believe it is verry relevant .... if you
> feel that it isn`t then let the TS decide if this might be an alternative ,
> as it is obvious that he comes here for a solution of his problem .[/color]

No, Jesse posted more or less the same thing. Wrapping the object in
read-only class or interface is the standard response given to this,
and it's certainly relevant.

Of course, it's also quite limited and not really a reasonable replacement
at all except in very simple circumstances. All in all, I agree with
the arguments against const parameters that people like Anders Hejlberg
have made, but it's a very useful construct that isn't easily duplicated
in .Net, and I think the idea that the equivalent is somehow easy to
accomplish is simply wrong.

[Brien King]

Re: Make a Parameter CONST?

DWS,

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
Get
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.
INPUT.



Brien King


"DWS" <DWS@discussion s.microsoft.com > wrote in message
news:2F53C4B1-E2D8-4D42-8C56-E31428902882@mi crosoft.com...[color=blue]
> Brien,
> 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
> DWS
>
> 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.
>
>[/color]