Richard,
As the others have suggested, you are misunderstandin g Value & Reference
Parameters when used with Value & Reference Types!
A Class is a Reference Type, only a single instance of an object exists on
the heap, when pass a Class variable ByVal a copy of the reference is made,
hence the variable & parameter both refer to the same object, allowing your
routine to make changes to the object itself.
Remember there are two types of Parameters (ByRef & ByVal) and there are two
types of variables (Reference Types & Value Types).
So you can have:
ByRef - Reference Type
ByRef - Value Type
ByVal - Reference Type
ByVal - Value Type
Lets look at types of variables:
When you define a Class you are defining a Reference Type. Which means that
a variable of this Class holds a reference to the object, the object itself
exists on the heap. If I assign this variable to a second variable a copy of
this reference is made and I now have two references to the same object on
the heap. There is still only one object on the heap. If you define a
Structure you are defining a Value Type. The variable itself holds the value
of the structure. If I assign this variable to a second variable a copy of
the entire structure is made. I now have two copies of the same structure.
Value Types include:
Boolean, Byte, Short, Integer, Long, Char, Single, Double, Decimal, along
with anything defined with the Structure or Enum keyword.
Value Types all derive directly or indirectly from System.ValueTyp e
Reference Types include:
Object, and anything defined with the Class, Interface or Delegate keyword
are reference types.
Reference Types all derive from System.Object excluding types that inherit
from System.ValueTyp e
Interface is a reference type, even if defined in a Structure. The structure
itself is a value type, however if you assign the structure to a Interface
variable, it will be Boxed, boxing places the value on the heap in a new
object (effectively making it a reference type)
Lets look at types of parameters:
Now when you define a parameter to be ByVal a copy of the variable is
passed. Remember Reference types hold a reference to the object, so passing
a Reference Type ByVal causes a copy of this reference to be passed as a
parameter, the single copy of the object itself is still on the heap. The
variable & parameter both have references to this single object. Passing a
Value Type ByVal causes a complete copy of the value to be passed as a
parameter. Now passing a Reference Type ByRef, causes a reference to the
variable to be passed, the variable has a reference to the object. Passing a
Value Type ByRef also causes a reference to the variable to be passed, the
variable has a copy of the Value.
Remember ByVal & ByRef are how parameters are passed. Reference & Value
Types are how quantities are stored.
Although the following is in C# the concepts apply equally to VB.NET:
http://www.yoda.arachsys.com/csharp/parameters.html
Hope this helps
Jay
"Richard" <ra*******@hotm ail.com> wrote in message
news:c5******** ***@otis.netspa ce.net.au...
My apologies, it is a CLASS not a structure that I am passing over.
Still - VB .NET should not change it if it has been passed over as ByVal, true?
Richard
"Richard" <ra*******@hotm ail.com> wrote in message
news:c5******** ***@otis.netspa ce.net.au...
Hi,
I am passing a structure to a subroutine where the passed parameter has
been declared as ByVal.
However, changes made to the passed variable inside the subroutine flow
through to the actual variable that has been passed over, even though
with ByVal this should not happen.
Has anybody else discovered this or am I doing completely wrong /
misunderstandin g the ByVal?
Richard
