How should variables be declared and referenced in both the base and
derived form so they can be accessed?
derived form so they can be accessed?
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RE: Accessing inherited variables
Hi,
You should be able to access stuff in a base class with
mybase.Variable Name
Ken
---------------------
"MrJim" wrote:
[color=blue]
> How should variables be declared and referenced in both the base and
> derived form so they can be accessed?
>[/color]
-
Re: Accessing inherited variables
MrJim wrote:[color=blue]
> How should variables be declared and referenced in both the base and
> derived form so they can be accessed?[/color]
You might try it this way
Declare all variables in a class as private.
If the variable needs to be access in derived classes, then provide a
protected property
e.g.
dim _conn as Connection
Protected Property Connection as Connection
...
end property
If the variable needs to be access outside the class and its
descendents then create a public property.
NOTE: In both of the above cases, you will probably want to access the
variable through the property even in the base class. Although you
have direct access to the variable it is useful to chokepoint all
access, e.g. if you want to raise an event when the value is changed.
Form specific:
By default, the IDE makes all control variables Friend.
This translates as 'public to other classes in the same assembly' which
can cause problems if you inherit from them in another assembly.
there are a number of choices here
1) make them protected
2) make them protected friend
3) Add wrapper functions/properties to access them
e.g.
if you have a listbox in the base class to which you want to add
items, then instead of directly access the listbox in the derived
class [mybase.listbox1 .items.add(xxx)], in the base class create a
protected sub to do it for you
Protected Sub AddItem(w as widget)
Listbox1.Items. add(w)
end sub
why? Say, halfway through you decide that you want to use a treeview
instead of a listview, all you need change is the base class.
hth,
Alan.
Comment
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Accessing inherited variables
Ken Tucker [MVP] wrote:[color=blue]
> Hi,
>
> You should be able to access stuff in a base class with
> mybase.Variable Name
>
> Ken
> ---------------------
>
> "MrJim" wrote:
>[color=green]
>> How should variables be declared and referenced in both the base and
>> derived form so they can be accessed?
>>[/color][/color]
I've tried this method, but all I got was the variable is not a member
of deviceapp1.etc.
Both the base and derived sub are declared as protected but still no luck.
Comment
-
Re: Accessing inherited variables
[color=blue]
> I've tried this method, but all I got was the variable is not a member
> of deviceapp1.etc.
> Both the base and derived sub are declared as protected but still no luck.[/color]
The important point is not whether or not how the subs are declared but
how the variable is declared.
[color=blue]
>From the description it sounds like you have something along the lines[/color]
of
BaseClass
Protected overridable Sub Foo()
dim bar as integer = 19
End Sub
DerivedClass
Protected Overrides Sub Foo()
bar = 23
End Sub
If so, you cannot access bar within the derived sub. it is not a
instance variable, it is local to the Base:Foo() sub and can only be
seen within that sub. If you want a variable that can be accessed in
both Base and Derived classes it needs to be a member of the class
e.g.
BaseClass
protected _bar as integer
Protected overridable Sub Foo()
_bar = 19
End Sub
DerivedClass
Protected Overrides Sub Foo()
_bar = 23
End Sub
or,
BaseClass
Private _bar as integer
Protected Property Bar as integer
get
return _bar
end get
set (value as integer)
_bar = value
end set
end property
Protected overridable Sub Foo()
Bar = 19
End Sub
DerivedClass
Protected Overrides Sub Foo()
Bar = 23
End Sub
hth,
Alan.
Comment
-
Re: Accessing inherited variables
AlanT wrote:[color=blue][color=green]
>> I've tried this method, but all I got was the variable is not a member
>> of deviceapp1.etc.
>> Both the base and derived sub are declared as protected but still no luck.[/color]
>
> The important point is not whether or not how the subs are declared but
> how the variable is declared.
>[color=green]
>>From the description it sounds like you have something along the lines[/color]
> of
>
> BaseClass
>
> Protected overridable Sub Foo()
>
> dim bar as integer = 19
>
> End Sub
>
>
> DerivedClass
>
> Protected Overrides Sub Foo()
>
> bar = 23
>
> End Sub
>
>
> If so, you cannot access bar within the derived sub. it is not a
> instance variable, it is local to the Base:Foo() sub and can only be
> seen within that sub. If you want a variable that can be accessed in
> both Base and Derived classes it needs to be a member of the class
>
> e.g.
>
>
> BaseClass
>
> protected _bar as integer
>
> Protected overridable Sub Foo()
> _bar = 19
> End Sub
>
>
> DerivedClass
>
> Protected Overrides Sub Foo()
> _bar = 23
> End Sub
>
> or,
>
> BaseClass
>
> Private _bar as integer
>
> Protected Property Bar as integer
> get
> return _bar
> end get
> set (value as integer)
> _bar = value
> end set
> end property
>
> Protected overridable Sub Foo()
> Bar = 19
> End Sub
>
>
> DerivedClass
>
> Protected Overrides Sub Foo()
> Bar = 23
> End Sub
>
>
> hth,
> Alan.
>[/color]
Thanks a lot for your help, that makes things clear
Comment
Comment