* "Rigga" <s@v.c> scripsit:

Why not?  Both string objects are pointing to the same "constant".

I believe the String is a true object, but it Overloads the '=' Operator.
so when you write the code x1=x2 the and x1 and x2 are both strings, it will
actually compile the same as x1 is x2.  If your using C#, you can overload
operators for your own classes, and make them behave the same way.

Lance

Hi all,

I am wondering why string's are not true objects?.... Let me explain...

If i write the code

Dim x1 as String = "veg"
Dim x2 as String = "veg"

If x1 = x2 then
   ' i expect this code to be executed
End If

If x1 is x2 then
   ' i do not expect this code to be executed
End If

However the second lot of code is executed!

Is this correct behavior??? if so is it true then that strings are not true
objects?

Any thoughts would be appreciated!

Rigga.

Thanks all for your replies but I'm still confused.

So coding "veg" anywhere will only ever create one object?

and assigning that object to the reference variable as

x1 as string = "veg"

points to the same object as

x2 as string = "veg"

"veg" being the object?

actually, it has always been kind of strange hat you do not have to code

x1 as New String    to instantiate the object.

So we are saying that string does not behave in the same way as other
objects?

Anyway, now that I know this I'll have to code around it.. thanks all..

Rigga.

Rigga,
In addition to the other comments.

Try the following:

       Dim x1 As String = "veggie"
       Dim x2 As String = "veg"

       x1 = x1.Substring(0, 3)

       If x1 = x2 Then
           Debug.WriteLine("i expect this code to be executed")
       End If

       If x1 Is x2 Then
           Debug.WriteLine("i do not expect this code to be executed")
       End If

Both strings contain "veg", however because the second one was "calculated"
instead of being a constant it is a different string object on the heap.

You can use String.Copy to create a new instances of a string.

       Dim x1 As String = "veg"
       Dim x2 As String = String.Copy(x1)

Note String.Clone returns the same instance...

Hope this helps
Jay

The compiler recognizes you setting it to the same string and creates IL
that points to a single spot in memory. Run this test as a console
application and it will become crystal clear:

Module Module1

   Private veg As String = "veg"

   Sub Main()
       Test1()
       Test2()
       Test3()

       Console.Read()
   End Sub

   Sub Test1()
       Console.WriteLine("TEST 1")
       Console.WriteLine("-------------")

       Dim x1 As String = "veg"
       Dim x2 As String = "veg"

       If x1 = x2 Then
           Console.WriteLine("x1 = x2")
       End If

       If x1 Is x2 Then
           Console.WriteLine("x1 is x2")
       End If

       Console.WriteLine("")
   End Sub

   Sub Test2()
       Console.WriteLine("TEST 2")
       Console.WriteLine("-------------")

       Dim x1 As String = veg
       Dim x2 As String = veg

       If x1 = x2 Then
           Console.WriteLine("x1 = x2")
       End If

       If x1 Is x2 Then
           Console.WriteLine("x1 is x2")
       End If

       Console.WriteLine("")
   End Sub

   Sub Test3()
       Console.WriteLine("TEST 3")
       Console.WriteLine("-------------")

       Dim sb As New Text.StringBuilder

       sb.Append("v")
       sb.Append("e")
       sb.Append("g")

       Dim x1 As String = "veg"
       Dim x2 As String = sb.ToString()

       If x1 = x2 Then
           Console.WriteLine("x1 = x2")
       End If

       If x1 Is x2 Then
           Console.WriteLine("x1 is x2")
       End If

       Console.WriteLine("")
   End Sub

End Module

I know you are talking about VB.net but check this, in VC++ there is a compiler option called /Gf or /GF, that creates single copy of identical strings in the program image and memory during execution, resulting in smaller programs, an optimization called *string pooling*.
I think this optimization is already built in to VB.net compiler and hence you get this kind of behavior in VB apps.
Any body plz correct me if I'm wrong.

Hope that helps.

Abubakar.
http://joehacker.blogspot.com