question about creating a class?

[wmosley2]

When creating a class, the attributes (member variables/instance variables) were declared as Private, which requires writing two special procedures to access the data. Why go to this extra trouble? Why not just declare these as Public?

[crptcblade]

Because the Property Let/Get/Set accessors allow you to control what you deem as valid data.

'Public member
Public SomeProp As Integer
 
'In some other project
mClass.SomeProp = (-1)

If you only want a number between 1 and 10, then there is no way to let the user of the class know. But if you use accessors...

Private mSomeProp As Integer
 
Public Property Let SomeProp(ByVal i As Integer)
  
    If i < 1 or i > 10 Then Err.Raise 5 'Raise some error
 
    mSomeProp = i 'Otherwise set the local value
 
End Property

[crptcblade]

It also gives you the ability to perform other tasks internally based on the data.

Public Event NumberChanged()
 
Private mSomeProp As Integer
 
 
Public Property Let SomeProp(ByVal i As Integer)
  
    If i < 1 or i > 10 Then Err.Raise 5 'Raise some error
 
    mSomeProp = i 'Otherwise set the local value
    RaiseEvent NumberChanged()
 
End Property

[Joacim Andersson]

Using Public variables is generally considered bad practice. There's a couple of reasons for this, first of all if you use a Public variable you will not know when the value has been changed, however if you use a Property procedure the Let/Set procedure will be called when the property is changing value. This allows you to check if a proper value has been passed. Let's say that only a value between 1 and 10 is allowed, how can you then check if the new value is correct if you have declared a public Integer variable?

In some cases certain requirements might also be filled before the user of your class may change one property value. For example if you build a class that allows the user to upload/download files from a FTP server you should not be able to pass a file name before a connection to the server has been made.

Cheers,

Joacim

[Hu Flung Dung]

Is it still considered bad practice if it doesn't matter what data is put into that variable? I know such situations aren't common, but I've seen it occur a few times before.

[crptcblade]

Originally posted by Hu Flung Dung
Is it still considered bad practice if it doesn't matter what data is put into that variable? I know such situations aren't common, but I've seen it occur a few times before.

If the data has no restrictions, then you might just want to use a UDT.

[SkiNLaB]

i need some examples of how to deal with invalid data....

any ideas?