Deriving from Abstract Classes with Enums and Varying Object Types

  • Thread starter Thread starter Confused Newbie
  • Start date Start date
C

Confused Newbie

I'm converting an app written in VB 2003 to 2005 and need advice for how to
deal with this situation:

The app has a number of "manager" classes that handle the data access. They
all have several routines that are identical, except for the object type and
database action specific to that particular class, such as "Public ReadOnly
Property Item(ByVal SystemID As Integer) As CEmployee" or "Public Shared
Function Save(ByVal TheItem As CEmployee) As Boolean". Each of the object
classes also have a bunch of common properties and methods that are
inherited from an abstract base class.

What I want to do with the manager classes is rewrite them using an abstract
class with MustOverride properties and methods. Where I'm hung up is the
existing implementation uses an Enum that defines the database field names
and ordinal positions in the SELECT statement used to create the datareader.
For example:

Public Class EmployeeManager

Public Enum MyFields
NameSystemID = 0
NameFirst
NameMiddle
NameLast
DateHired
RecStatus
RecDateAdd
RecDateRev
RecAddBy
RecRevBy
End Enum

Private Shared _FieldList As ArrayList = Nothing
Public Shared ReadOnly Property FieldList() As ArrayList
Get
If _FieldList Is Nothing Then
_FieldList = New ArrayList
Dim arNames() As String
arNames = MyFields.GetNames(GetType(MyFields))
For x As Integer = 0 To arNames.GetUpperBound(0)
_FieldList.Add(New VisarEnumBindingItem(EnumValue:=x,
EnumLabel:=arNames(x))
Next
End If
Return _FieldList
End Get
End Property

Private Shared _FieldListString As String = ""
Public Shared ReadOnly Property FieldListString() As String
Get
If _FieldListString = "" Then
Dim ThisItem As VisarEnumBindingItem = Nothing
For x As Integer = 0 To FieldList.Count -1
ThisItem = CType(_FieldList(x), VisarEnumBindingItem)
_FieldListString += ThisItem.StringValue & ", "
Next
_FieldListString = Left(_FieldListString,
_FieldListString.LastIndexOf(", "))
End If
Return _FieldListString
End Get
End Property

' Overloaded Item property creates a command to retrieve by PK or AK,
passes command to ItemFetch, which in turn calls ItemFill to unpack the
datareader into the instance object and pass it back up the chain.

Private Shared Function ItemFill(ByVal ItemReader As OleDbDataReader) As
CEmployee
With ItemReader
Return New CEmployee(RecSystemID:=.GetInt32(MyFields.NameSystemID),
RecStatus:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecStatus)),
RecDateAdd:=VisarGoodies.MakeDate(.GetValue(MyFields.RecDateAdd)),
RecDateRev:=VisarGoodies.MakeDate(.GetValue(MyFields.RecDateRev)),
RecAddBy:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecAddBy)),
RecRevBy:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecRevBy)),
NameFirst:=VisarGoodies.MakeString(.GetValue(MyFields.NameFirst)),
NameMiddle:=VisarGoodies.MakeString(.GetValue(MyFields.NameMiddle)),
NameLast:=VisarGoodies.MakeString(.GetValue(MyFields.NameLast)))
End With
End Function

End Class

The way it's designed, changes to the order of fields in the select
statement or even actual field names only need to be made in the Enum. The
select, insert, and update statements are dynamically constructed in each of
the managers from the FieldList arraylist.

How can I create the identical functionality using an abstract class in
VB2005, also placing the code that constructs the "FieldList" from the Enum
into a shared method in a separate utility class? I tried using a
placeholder enum and the FieldList property in the base class, and a
shadowed enum in a derived class, but when executed, it uses the base enum,
not the shadowed one. It does work when place the enum and override
FieldList in each of the derived classes but I don't want to implement
identical code over and over.

Thanks,

Gino
 
Well I'm confused.....

Is all this just for the purpose of selecting, updating, inserting
records to and from a database?

If so then I'd say this is in major need of a rewrite. Why make things
harder than they need to be?
 
The manager classes I'm rewriting perform numerous other tasks and all of
our in-house apps are built around them. They're quite simple and elegant
considering all the heavy lifting they have to do. If you think this design
is wrong, then please post an exact example of how you would rewrite them.
Just saying it's harder than it need be and needs a major rewrite is not in
the least bit helpful.
 
Confused Newbie,
I would consider using a Generic class, passing the Enum & object type as
type parameters

Something like:

Dim employeManager as New Manager(Of Employee, Employee.MyFields)

The "trick" is the implementation of ItemFill, as you need the constructor
to Employee, generally I make Manager a base class...

Alternatively you could define "Manager" as a shared member of Employee
itself:

Public Class Employee

Public Shared ReadOnly Manager As New EmployeeManager()

Public Enum MyFields
NameSystemID = 0
NameFirst
NameMiddle
NameLast
DateHired
RecStatus
RecDateAdd
RecDateRev
RecAddBy
RecRevBy
End Enum

End Class

Public Class EmployeeManager
Inherits Manager(Of Employee, Employee.MyFields)

End Class

Where Manager is defined something like:

Public MustInherit Class Manager(Of T As Class, F)

Private _FieldList As ArrayList = Nothing
Public ReadOnly Property FieldList() As ArrayList
Get
If _FieldList Is Nothing Then
_FieldList = New ArrayList
Dim arNames() As String
arNames = [Enum].GetNames(GetType(F))
For x As Integer = 0 To arNames.GetUpperBound(0)
_FieldList.Add(New
VisarEnumBindingItem(EnumValue:=x, EnumLabel:=arNames(x)))
Next
End If
Return _FieldList
End Get
End Property

Private _FieldListString As String = ""
Public ReadOnly Property FieldListString() As String
Get
If _FieldListString = "" Then
Dim ThisItem As VisarEnumBindingItem = Nothing
For x As Integer = 0 To FieldList.Count - 1
ThisItem = CType(_FieldList(x),
VisarEnumBindingItem)
_FieldListString += ThisItem.StringValue & ", "
Next
_FieldListString = Left(_FieldListString,
_FieldListString.LastIndexOf(", "))
End If
Return _FieldListString
End Get
End Property

Public MustOverride Function ItemFill(ByVal ItemReader As
OleDbDataReader) As T

End Class

--
Hope this helps
Jay B. Harlow [MVP - Outlook]
..NET Application Architect, Enthusiast, & Evangelist
T.S. Bradley - http://www.tsbradley.net


| I'm converting an app written in VB 2003 to 2005 and need advice for how
to
| deal with this situation:
|
| The app has a number of "manager" classes that handle the data access.
They
| all have several routines that are identical, except for the object type
and
| database action specific to that particular class, such as "Public
ReadOnly
| Property Item(ByVal SystemID As Integer) As CEmployee" or "Public Shared
| Function Save(ByVal TheItem As CEmployee) As Boolean". Each of the object
| classes also have a bunch of common properties and methods that are
| inherited from an abstract base class.
|
| What I want to do with the manager classes is rewrite them using an
abstract
| class with MustOverride properties and methods. Where I'm hung up is the
| existing implementation uses an Enum that defines the database field names
| and ordinal positions in the SELECT statement used to create the
datareader.
| For example:
|
| Public Class EmployeeManager
|
| Public Enum MyFields
| NameSystemID = 0
| NameFirst
| NameMiddle
| NameLast
| DateHired
| RecStatus
| RecDateAdd
| RecDateRev
| RecAddBy
| RecRevBy
| End Enum
|
| Private Shared _FieldList As ArrayList = Nothing
| Public Shared ReadOnly Property FieldList() As ArrayList
| Get
| If _FieldList Is Nothing Then
| _FieldList = New ArrayList
| Dim arNames() As String
| arNames = MyFields.GetNames(GetType(MyFields))
| For x As Integer = 0 To arNames.GetUpperBound(0)
| _FieldList.Add(New VisarEnumBindingItem(EnumValue:=x,
| EnumLabel:=arNames(x))
| Next
| End If
| Return _FieldList
| End Get
| End Property
|
| Private Shared _FieldListString As String = ""
| Public Shared ReadOnly Property FieldListString() As String
| Get
| If _FieldListString = "" Then
| Dim ThisItem As VisarEnumBindingItem = Nothing
| For x As Integer = 0 To FieldList.Count -1
| ThisItem = CType(_FieldList(x), VisarEnumBindingItem)
| _FieldListString += ThisItem.StringValue & ", "
| Next
| _FieldListString = Left(_FieldListString,
| _FieldListString.LastIndexOf(", "))
| End If
| Return _FieldListString
| End Get
| End Property
|
| ' Overloaded Item property creates a command to retrieve by PK or AK,
| passes command to ItemFetch, which in turn calls ItemFill to unpack the
| datareader into the instance object and pass it back up the chain.
|
| Private Shared Function ItemFill(ByVal ItemReader As OleDbDataReader) As
| CEmployee
| With ItemReader
| Return New CEmployee(RecSystemID:=.GetInt32(MyFields.NameSystemID),
| RecStatus:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecStatus)),
| RecDateAdd:=VisarGoodies.MakeDate(.GetValue(MyFields.RecDateAdd)),
| RecDateRev:=VisarGoodies.MakeDate(.GetValue(MyFields.RecDateRev)),
| RecAddBy:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecAddBy)),
| RecRevBy:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecRevBy)),
| NameFirst:=VisarGoodies.MakeString(.GetValue(MyFields.NameFirst)),
| NameMiddle:=VisarGoodies.MakeString(.GetValue(MyFields.NameMiddle)),
| NameLast:=VisarGoodies.MakeString(.GetValue(MyFields.NameLast)))
| End With
| End Function
|
| End Class
|
| The way it's designed, changes to the order of fields in the select
| statement or even actual field names only need to be made in the Enum.
The
| select, insert, and update statements are dynamically constructed in each
of
| the managers from the FieldList arraylist.
|
| How can I create the identical functionality using an abstract class in
| VB2005, also placing the code that constructs the "FieldList" from the
Enum
| into a shared method in a separate utility class? I tried using a
| placeholder enum and the FieldList property in the base class, and a
| shadowed enum in a derived class, but when executed, it uses the base
enum,
| not the shadowed one. It does work when place the enum and override
| FieldList in each of the derived classes but I don't want to implement
| identical code over and over.
|
| Thanks,
|
| Gino
|
|
 
Thanks so much, Jay! This is what I was looking for. I couldn't for the
life of me remember generics classes yesterday, even with BindingList
staring me right in the face.

Gino


Jay B. Harlow said:
Confused Newbie,
I would consider using a Generic class, passing the Enum & object type as
type parameters

Something like:

Dim employeManager as New Manager(Of Employee, Employee.MyFields)

The "trick" is the implementation of ItemFill, as you need the constructor
to Employee, generally I make Manager a base class...

Alternatively you could define "Manager" as a shared member of Employee
itself:

Public Class Employee

Public Shared ReadOnly Manager As New EmployeeManager()

Public Enum MyFields
NameSystemID = 0
NameFirst
NameMiddle
NameLast
DateHired
RecStatus
RecDateAdd
RecDateRev
RecAddBy
RecRevBy
End Enum

End Class

Public Class EmployeeManager
Inherits Manager(Of Employee, Employee.MyFields)

End Class

Where Manager is defined something like:

Public MustInherit Class Manager(Of T As Class, F)

Private _FieldList As ArrayList = Nothing
Public ReadOnly Property FieldList() As ArrayList
Get
If _FieldList Is Nothing Then
_FieldList = New ArrayList
Dim arNames() As String
arNames = [Enum].GetNames(GetType(F))
For x As Integer = 0 To arNames.GetUpperBound(0)
_FieldList.Add(New
VisarEnumBindingItem(EnumValue:=x, EnumLabel:=arNames(x)))
Next
End If
Return _FieldList
End Get
End Property

Private _FieldListString As String = ""
Public ReadOnly Property FieldListString() As String
Get
If _FieldListString = "" Then
Dim ThisItem As VisarEnumBindingItem = Nothing
For x As Integer = 0 To FieldList.Count - 1
ThisItem = CType(_FieldList(x),
VisarEnumBindingItem)
_FieldListString += ThisItem.StringValue & ", "
Next
_FieldListString = Left(_FieldListString,
_FieldListString.LastIndexOf(", "))
End If
Return _FieldListString
End Get
End Property

Public MustOverride Function ItemFill(ByVal ItemReader As
OleDbDataReader) As T

End Class

--
Hope this helps
Jay B. Harlow [MVP - Outlook]
.NET Application Architect, Enthusiast, & Evangelist
T.S. Bradley - http://www.tsbradley.net


| I'm converting an app written in VB 2003 to 2005 and need advice for how
to
| deal with this situation:
|
| The app has a number of "manager" classes that handle the data access.
They
| all have several routines that are identical, except for the object type
and
| database action specific to that particular class, such as "Public
ReadOnly
| Property Item(ByVal SystemID As Integer) As CEmployee" or "Public Shared
| Function Save(ByVal TheItem As CEmployee) As Boolean". Each of the
object
| classes also have a bunch of common properties and methods that are
| inherited from an abstract base class.
|
| What I want to do with the manager classes is rewrite them using an
abstract
| class with MustOverride properties and methods. Where I'm hung up is
the
| existing implementation uses an Enum that defines the database field
names
| and ordinal positions in the SELECT statement used to create the
datareader.
| For example:
|
| Public Class EmployeeManager
|
| Public Enum MyFields
| NameSystemID = 0
| NameFirst
| NameMiddle
| NameLast
| DateHired
| RecStatus
| RecDateAdd
| RecDateRev
| RecAddBy
| RecRevBy
| End Enum
|
| Private Shared _FieldList As ArrayList = Nothing
| Public Shared ReadOnly Property FieldList() As ArrayList
| Get
| If _FieldList Is Nothing Then
| _FieldList = New ArrayList
| Dim arNames() As String
| arNames = MyFields.GetNames(GetType(MyFields))
| For x As Integer = 0 To arNames.GetUpperBound(0)
| _FieldList.Add(New VisarEnumBindingItem(EnumValue:=x,
| EnumLabel:=arNames(x))
| Next
| End If
| Return _FieldList
| End Get
| End Property
|
| Private Shared _FieldListString As String = ""
| Public Shared ReadOnly Property FieldListString() As String
| Get
| If _FieldListString = "" Then
| Dim ThisItem As VisarEnumBindingItem = Nothing
| For x As Integer = 0 To FieldList.Count -1
| ThisItem = CType(_FieldList(x), VisarEnumBindingItem)
| _FieldListString += ThisItem.StringValue & ", "
| Next
| _FieldListString = Left(_FieldListString,
| _FieldListString.LastIndexOf(", "))
| End If
| Return _FieldListString
| End Get
| End Property
|
| ' Overloaded Item property creates a command to retrieve by PK or AK,
| passes command to ItemFetch, which in turn calls ItemFill to unpack the
| datareader into the instance object and pass it back up the chain.
|
| Private Shared Function ItemFill(ByVal ItemReader As OleDbDataReader)
As
| CEmployee
| With ItemReader
| Return New
CEmployee(RecSystemID:=.GetInt32(MyFields.NameSystemID),
| RecStatus:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecStatus)),
| RecDateAdd:=VisarGoodies.MakeDate(.GetValue(MyFields.RecDateAdd)),
| RecDateRev:=VisarGoodies.MakeDate(.GetValue(MyFields.RecDateRev)),
| RecAddBy:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecAddBy)),
| RecRevBy:=VisarGoodies.MakeInt32(.GetValue(MyFields.RecRevBy)),
| NameFirst:=VisarGoodies.MakeString(.GetValue(MyFields.NameFirst)),
| NameMiddle:=VisarGoodies.MakeString(.GetValue(MyFields.NameMiddle)),
| NameLast:=VisarGoodies.MakeString(.GetValue(MyFields.NameLast)))
| End With
| End Function
|
| End Class
|
| The way it's designed, changes to the order of fields in the select
| statement or even actual field names only need to be made in the Enum.
The
| select, insert, and update statements are dynamically constructed in
each
of
| the managers from the FieldList arraylist.
|
| How can I create the identical functionality using an abstract class in
| VB2005, also placing the code that constructs the "FieldList" from the
Enum
| into a shared method in a separate utility class? I tried using a
| placeholder enum and the FieldList property in the base class, and a
| shadowed enum in a derived class, but when executed, it uses the base
enum,
| not the shadowed one. It does work when place the enum and override
| FieldList in each of the derived classes but I don't want to implement
| identical code over and over.
|
| Thanks,
|
| Gino
|
|
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