Data Access Class programming

[ZmrAbdulla]

Can anyone explain please..

If I have a Class like below

Imports System.Data
Imports System.Data.OleDb
Public Class Categories
#Region "Declarations"
	Private _CategoryID As Integer
	Private _CategoryName As String
	Private _Description As String	
#End Region
#Region "Properties"
	Public Property CategoryID() As Integer
		Get
			Return _CategoryID
		End Get
		Set(ByVal Value as Integer)
			_CategoryID = Value
		End Set
	End Property
	Public Property CategoryName() As String
		Get
			Return _CategoryName
		End Get
		Set(ByVal Value as String)
			_CategoryName = Value
		End Set
	End Property
	Public Property Description() As String
		Get
			Return _Description
		End Get
		Set(ByVal Value as String)
			_Description = Value
		End Set
	End Property

#End Region
#Region "NewClass"
Public Sub New(	ByVal _CategoryID As Integer, _
		ByVal _CategoryName As String, _
		ByVal _Description As String)
			CategoryID = _CategoryID
			CategoryName = _CategoryName
			Description = _Description
			
End Sub


'...............
'...............
'...............
#End Region
End Class

When I create a new instance of the class I have to use like

Dim newcls As New Categories(1, "Cars", "Car Description")

What is the difference if I declare the New Instance in the Class without any "ByVal" clause?

Public Sub New()
	CategoryID = _CategoryID
	CategoryName = _CategoryName
	Description = _Description	
End Sub

declaring the vaiables where I instaniate a new class like

    Dim newcls As New Categories()

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim CName As String = "Cars"
        With newcls
            .CategoryName = CName
        End With

        MessageBox.Show(newcls.CategoryName)
    End Sub

In this approach I don't have to give all the data..
Which one is correct?
I am missing something here..

Any place where I can read more on this?



________________________________________________________
Zameer Abdulla
Help to find Missing people

 

[chrissie1]

First of all naming a class should always be singular so category and not categories. Except if you make it a collectionclass.

This

Public Sub New()
    CategoryID = _CategoryID
    CategoryName = _CategoryName
    Description = _Description    
End Sub

Will not really work since you are putting in the values from the local/private variables which are empty in the beginning.

As seen here

Private _CategoryID As Integer
    Private _CategoryName As String
    Private _Description As String

Better would it be to write this.

#Region "NewClass"
Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

public sub new()
  Categoryid = ""
  categoryname = ""
  description = ""
end sub
#end region

This way you have an empty constructor and a normal constructor with all the fields which should be the normal way of working, this is the minimum.

The following will also work and is shorter.

#Region "NewClass"
Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

public sub new()
  me.new("","","")
end sub
#end region

You could of course write more then two constructors.

like so

#Region "NewClass"
Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

public sub new()
  me.new("","","")
end sub

category and not categories. Except if you make it a collectionclass.

This

[code]
Public Sub New()
    CategoryID = _CategoryID
    CategoryName = _CategoryName
    Description = _Description    
End Sub

Will not really work since you are putting in the values from the local/private variables which are empty in the beginning.

As seen here

Private _CategoryID As Integer
    Private _CategoryName As String
    Private _Description As String

Better would it be to write this.

#Region "NewClass"
Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

public sub new()
  Categoryid = ""
  categoryname = ""
  description = ""
end sub
#end region

This way you have an empty constructor and a normal constructor with all the fields which should be the normal way of working, this is the minimum.

The following will also work and is shorter.

#Region "NewClass"
Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

public sub new()
  me.new("","","")
end sub
#end region

You could of course write more then two constructors.

like so

#Region "NewClass"
Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

public sub new()
  me.new("","","")
end sub

Public Sub New(ByVal _CategoryName As String)
  me.new("",_categoryname,"")            
End Sub
#end region

I hope that clears things up

BTW the variabels used int the constructor with the same name as the private members are not the same.




Christiaan Baes
Belgium

"In a system where you can define a factor as part of a third factor, you need another layer to check the main layer in case the second layer is not the base unit." - jrbarnett

 

[ZmrAbdulla]

Thanks Chrissie..
>>First of all naming a class should always be singular so category and not categories. Except if you make it a collectionclass.

Actually I am working on a Class generator that will produce data access classes from a selected Access.mdb. So the Class Name is taken as the selected table/Query Name.

>>.You could of course write more then two constructors.

So when you create more constructors don't you need a "Shared" or "Overridable" key word ? I know "Sub New()" won't allow these key words. How the app recognize it or how will I call them ?

>>BTW the variabels used int the constructor with the same name as the private members are not the same.

Can you please explain ?

________________________________________________________
Zameer Abdulla
Help to find Missing people

 

[chrissie1]

<< Actually I am working on a Class generator that will produce data access classes from a selected Access.mdb. So the Class Name is taken as the selected table/Query Name.

Ok, not good but if you can live with it then I can.

<< So when you create more constructors don't you need a "Shared" or "Overridable" key word ? I know "Sub New()" won't allow these key words. How the app recognize it or how will I call them ?

They get overloaded there is an overloads keyword but you don't have to type it. As long as the parameter-signature is different it knows that you mean overloading. Overloading is The same method with a different parameter-signature. I'm sure you have seen that thousands of times in the framework. Like in dim _con as new sqlconnection(...) you have different option there of different constructors.

So this won't work

Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

Public Sub New(    ByVal _CategoryID2 As Integer, _
        ByVal _CategoryName2 As String, _
        ByVal _Description2 As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

<< Can you please explain ?

I had the expression that yuo think that the members used here

#Region "Declarations"
    Private _CategoryID As Integer
    Private _CategoryName As String
    Private _Description As String    
#End Region

are the same as the ones with the same name that you use here.

Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

try this

#Region "Declarations"
    Private _CategoryID As Integer = 10
    Private _CategoryName As String = "string1"
    Private _Description As String = "description"
#End Region

with this constructor

Public Sub New()
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

So

dim _categories as new Categories()

What do you think the values for categories.categoryname will be?

and then with this constructor

Public Sub New(    ByVal _CategoryID2 As Integer, _
        ByVal _CategoryName2 As String, _
        ByVal _Description2 As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

So

dim _categories as new Categories(20,"","test")

What do you think the values categories.categoryname will be?

and then with this constructor

Public Sub New(    ByVal _CategoryID As Integer, _
        ByVal _CategoryName As String, _
        ByVal _Description As String)
            CategoryID = _CategoryID
            CategoryName = _CategoryName
            Description = _Description
            
End Sub

So

dim _categories as new Categories(20,"","test")

What do you think the values categories.categoryname will be?

Do you understand?




Christiaan Baes
Belgium

"In a system where you can define a factor as part of a third factor, you need another layer to check the main layer in case the second layer is not the base unit." - jrbarnett

 

[ZmrAbdulla]

>>They get overloaded there is an overloads keyword but you don't have to type it.

This is what I was confused. It is clear now. Thanks.



________________________________________________________
Zameer Abdulla
Help to find Missing people