Accessing inherited variables

  • Thread starter Thread starter MrJim
  • Start date Start date
M

MrJim

How should variables be declared and referenced in both the base and
derived form so they can be accessed?
 
MrJim said:
How should variables be declared and referenced in both the base and
derived form so they can be accessed?
Click to expand...


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.
 
Ken said:
Hi,

You should be able to access stuff in a base class with
mybase.VariableName

Ken
Click to expand...
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.
 
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.
Click to expand...

The important point is not whether or not how the subs are declared but
how the variable is declared.
From the description it sounds like you have something along the lines
Click to expand...
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.
 
AlanT said:
The important point is not whether or not how the subs are declared but
how the variable is declared.

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.
Click to expand...
Thanks a lot for your help, that makes things clear
 
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