OOP Question in vb.net ?

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Hi all,

I have 2 simple classes : Events and BMD. And here is the code :

Events:

VB

Public Class Events
    Implements IEvents

    Private bmdd As BMD

    Public Sub New(ByRef master As Object)
        Me.bmdd = master
    End Sub

    Public Function getevents1() As String Implements IEvents.getevents
        Return ""
    End Function
End Class

in the BMD i have this property where _eventfield is object of type Events :

VB

Public Property _eventproperty() As IEvents Implements IBMD._eventproperty
       Get
           If _eventfield Is Nothing Then
               _eventfield = New Events(Me)
           End If
           Return _eventfield
       End Get
       Set(ByVal value As IEvents)
           If _eventfield Is Nothing Then
               _eventfield = New Events(Me)
           End If
       End Set
   End Property

So when i do :

VB

_eventfield = New Events(Me)

It is calling the constructor of Events.

So my question is what does

VB

Me.BMD= master

means ?

Thanks.

Posted 11-Feb-12 4:05am

mhamad zarif

Updated 11-Feb-12 4:08am

v2

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Sander Rossel · Answer 1 · 2012-02-11T04:15:00

Solution 1

It assigns an Object to a variable... However, the variable is never used, the variable is of type BMD while the object assigned to it is of the type Object (which may or may not be of the type BMD). I suggest turning Option Strict On[^] and see the error appearing! A GetEvents that returns a String is peculiar to say the least. This piece of code is just strange and has actually nothing to do with OOP... Assigning values to variables is the absolute basics. For more information on OOP I suggest reading this: Introduction to Object Oriented Programming Concepts (OOP) and More[^]
But perhaps you'd be more helped by grabbing some beginner books or articles on programming in VB.NET... Such as this one[^].

Posted 11-Feb-12 4:15am

Sander Rossel

Updated 11-Feb-12 4:16am

v2

Comments

mhamad zarif 11-Feb-12 10:17am

thanks, but my problem is that how it is assigning and object of type Events to BMD ? knowing that when i debug the code, the master is appearing to be of type BMD which is confusing me.

Sander Rossel 11-Feb-12 10:28am

You are missing the point that BMD is ALSO an Object.
So you are saying the constructor needs an Object, and you are passing in a BMD, which is a perfectly valid Object. However, EVERYTHING in .NET is an Object. So this might go very wrong if you are passing in, for example, a String to the constructor. A String is an Object, so the compiler won't complain. Until your String is assigned to a variable that holds a BMD during runtime.

Imagine this: You are asking a friend if he likes a cup of tea. He says sure and you give him a cup of green tea. When taking his first sip he spits it out and gets mad at you because he really hates green tea! Despite green tea being a tea it's not the kind of tea your friend was expecting. Had you given him a lemon tea he might have been happy.
The thing with your code is that BMD is an Object, but the master variable is not necissarily holding the correct Object.

mhamad zarif 11-Feb-12 11:22am

But it is not giving me any error

mhamad zarif 11-Feb-12 11:25am

Also the master object is of type Events.

Sergey Alexandrovich Kryukov 11-Feb-12 14:59pm

Then why did not you use the parameter type as Events? And what would be the sense of this parameter?

All wrong. You've just totally lost. This cannot be fixed. As Naering advised, read the book/manual.

You need to understand the very elementary basics: types (run-time and compile-time), methods, assignment, parameters, types and instances, static and instance methods, etc. You need to start from scratch.

--SA

Sergey Alexandrovich Kryukov 11-Feb-12 14:52pm

My 5. Maybe this "code" does not deserve detail analysis, but I added my explanations.
--SA

Sander Rossel 11-Feb-12 17:33pm

Thanks SA :)

Sergey Alexandrovich Kryukov · Answer 2 · 2012-02-11T08:51:00

First of all, Me means the reference to the object implicitly passed as a hidden parameter to an instance (non-static) method in order to allow this method to assess the instance. In other words, suppose you have this:

VB

myInstance.MyMethod(someParameter);

This is equivalent to this pseudo-code:

VB

TheClassOfMyInstance.MyMethod(myInstance, someParameter);

This pseudo-code could be used as a real code demonstrating simulation of an instant method by a static version of the method MyMethod.

Now, the assignment me.bmdd = master is wrong, because master is Object, but bmdd is BMD. You cannot assign more specialized instance to less specialized, because BMD has more members then Object and an attempt to access some of those extra members would cause a fatal error. Fortunately, the system won't let you go that far. If you force this assignment by casting to BMD, the cast itself will trow exception in all cases where the instance of master is not of the run-time type BMD or derived from this type.

Note that it is important to distinguish compile-type name and run-time name.

I agree with Naerling about learning some programming and language.

It's very bad to hard-code any immediate constants, especially strings; even instead of "" should be string.Empty.

Actually, the signature of the constructor New makes no sense due to the reason explained above. You could use Public Sub New(ByRef master As BMD). In comments you say that the actual parameter is of the type Events. If so, the constructor would never work at all. And if you used the signature Public Sub New(ByRef master As Events) it would make no sense.

A couple of notes:

The following names violate very useful Microsoft naming conventions: Events (class name can not be plural), _eventfield (don't use underscore), getevents1 (you should use camel case for method names, not numerals).

—SA