Class Scoping Confusion

[Grok]

In writing a class library, one of the classes should only ever be
instantiated once, and its object should be accessible to every other
class in the library. How can I do that?

To provide some actual names for discussion, I'm trying to do this
with a custom log-writing class, named LogWriter.vb. So I would like
to declare ..

Class library project:
LogWriter.vb
Drives.vb
Folders.vb
Files.vb

Friend Logger As LogWriter = new LogWriter(LogFi lename)

and have Drivers, Folders, and Files classes all be able to use the
sole instantiation "Logger" in their code.

Hints on how to organize this properly?

[Phill W.]

Re: Class Scoping Confusion

Grok wrote:

In writing a class library, one of the classes should only ever be
instantiated once, and its object should be accessible to every other
class in the library. How can I do that?

To provide some actual names for discussion, I'm trying to do this
with a custom log-writing class, named LogWriter.vb. So I would like
to declare ..
>
Class library project:
LogWriter.vb
Drives.vb
Folders.vb
Files.vb
>
Friend Logger As LogWriter = new LogWriter(LogFi lename)
>
and have Drivers, Folders, and Files classes all be able to use the
sole instantiation "Logger" in their code.
>
Hints on how to organize this properly?

Properly? Don't know, but here's how I'd throw it together:

Public Class LogWriter
Public ReadOnly Property Drives() as Drives
Get
Return New Drives( Me )
End Get
End Property
End Class

Public Class Drives
Private Sub New()
End Sub

Friend Sub New( ByVal parent as LogWriter )
m_parent = parent
End Sub

Private m_parent as LogWriter = Nothing

Private ReadOnly Property Parent() as LogWriter
Get
return m_parent
End Get
End Property

End Class

Note the accessors on the Classes and Constructors.

Anyone "out there" can create an instance of LogWriter - it's
constructor is Public. They can't, however, create an instance of
Drives /directly/ - it's constructor is only accessible /inside/ your
library [assembly].

To get one, they have to use the Drives property on the LogWriter class,
which means that your code gets a look in and can deal with linking the
two objects together.
Of course, the Drives property doesn't /have/ to be Public...

HTH,
Phill W.

[Cor Ligthert[MVP]]

Re: Class Scoping Confusion

Grok,

Normally it is just setting a reference to your library (dll).
This can using project or by right clicking on the reference tab in your
solution explorer

Cor


"Grok" <grok@valhallal egends.comschre ef in bericht
news:qepfm3pqdi 9a2u1ichfmnmsih ubsrdvddf@4ax.c om...

In writing a class library, one of the classes should only ever be
instantiated once, and its object should be accessible to every other
class in the library. How can I do that?
>
To provide some actual names for discussion, I'm trying to do this
with a custom log-writing class, named LogWriter.vb. So I would like
to declare ..
>
Class library project:
LogWriter.vb
Drives.vb
Folders.vb
Files.vb
>
Friend Logger As LogWriter = new LogWriter(LogFi lename)
>
and have Drivers, Folders, and Files classes all be able to use the
sole instantiation "Logger" in their code.
>
Hints on how to organize this properly?

[=?Utf-8?B?RmFtaWx5IFRyZWUgTWlrZQ==?=]

RE: Class Scoping Confusion



"Grok" wrote:

In writing a class library, one of the classes should only ever be
instantiated once, and its object should be accessible to every other
class in the library. How can I do that?
>
To provide some actual names for discussion, I'm trying to do this
with a custom log-writing class, named LogWriter.vb. So I would like
to declare ..
>
Class library project:
LogWriter.vb
Drives.vb
Folders.vb
Files.vb
>
Friend Logger As LogWriter = new LogWriter(LogFi lename)
>
and have Drivers, Folders, and Files classes all be able to use the
sole instantiation "Logger" in their code.
>
Hints on how to organize this properly?
>

I think your subject through the others off the track. You seem to want to
create a singleton class. One way that I use is to create a private
constructor, and public shared instance method. The instance method checks
to see if a copy exists.

public class Logger
private mLogger as Logger = Nothing

private property Instance as Logger
get
if (mLogger is Nothing) then
mLogger = new Logger()
endif
return mLogger
end get
end property

private sub New()
end sub
end class

[=?Utf-8?B?RmFtaWx5IFRyZWUgTWlrZQ==?=]

RE: Class Scoping Confusion

public class Logger

private mLogger as Logger = Nothing
>
private property Instance as Logger
get
if (mLogger is Nothing) then
mLogger = new Logger()
endif
return mLogger
end get
end property
>
private sub New()
end sub
end class
>

Sorry, temporary insanity, and typing off the IDE. The Instance property
should be public, not private...

[=?Utf-8?B?RmFtaWx5IFRyZWUgTWlrZQ==?=]

Re: Class Scoping Confusion



"Phill W." wrote:

Grok wrote:
>

In writing a class library, one of the classes should only ever be
instantiated once, and its object should be accessible to every other
class in the library. How can I do that?

>

To provide some actual names for discussion, I'm trying to do this
with a custom log-writing class, named LogWriter.vb. So I would like
to declare ..

Class library project:
LogWriter.vb
Drives.vb
Folders.vb
Files.vb

Friend Logger As LogWriter = new LogWriter(LogFi lename)

and have Drivers, Folders, and Files classes all be able to use the
sole instantiation "Logger" in their code.

Hints on how to organize this properly?

>
Properly? Don't know, but here's how I'd throw it together:
>
Public Class LogWriter
Public ReadOnly Property Drives() as Drives
Get
Return New Drives( Me )
End Get
End Property
End Class
>
Public Class Drives
Private Sub New()
End Sub
>
Friend Sub New( ByVal parent as LogWriter )
m_parent = parent
End Sub
>
Private m_parent as LogWriter = Nothing
>
Private ReadOnly Property Parent() as LogWriter
Get
return m_parent
End Get
End Property
>
End Class
>
Note the accessors on the Classes and Constructors.
>
Anyone "out there" can create an instance of LogWriter - it's
constructor is Public. They can't, however, create an instance of
Drives /directly/ - it's constructor is only accessible /inside/ your
library [assembly].
>
To get one, they have to use the Drives property on the LogWriter class,
which means that your code gets a look in and can deal with linking the
two objects together.
Of course, the Drives property doesn't /have/ to be Public...
>
HTH,
Phill W.
>

Sorry Phill. I missed this when I posted... I didn't copy your paper!

[Grok]

Re: Class Scoping Confusion

Thanks for the replies. Yes, I described the question poorly. It is
solved now. Here is the completed, working solution and test code.

Family Tree Mike, I was not grasping how to use the Instance property
as you intended, so was getting two LogWriter objects instead of one.

All your solution needed was the Shared keyword.

===== Implementation =====
'file: LogWriter.vb
Imports System.IO
Namespace GrokQuestion
'file: LogWriter.vb
'singleton to write logfile; used by public classes
Friend Class LogWriter
Shared mLogWriter As LogWriter = Nothing
Shared _filename As String
Friend Sub New()
End Sub
Shared ReadOnly Property Instance() As LogWriter
Get
Console.WriteLi ne("Get Instance() called.")
If (mLogWriter Is Nothing) Then
mLogWriter = New LogWriter()
End If
Console.WriteLi ne("Old Filename = """ + _filename +
"""")
Return mLogWriter
End Get
End Property
Public Property Filename() As String
Get
Return _filename
End Get
Set(ByVal value As String)
_filename = value
End Set
End Property
Public Sub WriteLine(ByVal Message As String)
'open file, write Message, close file
Console.WriteLi ne(" Message = """ + Message + """")
Dim sw As System.IO.Strea mWriter = New
System.IO.Strea mWriter(_filena me)
sw.WriteLine(Me ssage)
sw.Close()
End Sub
End Class
End Namespace

'file: Folders.vb
Namespace GrokQuestion
'file: Folders.vb
Public Class Folders
Private Logger As LogWriter
Public Sub New(ByVal LogFilename As String)
Logger = LogWriter.Insta nce()
Logger.Filename = LogFilename
Logger.WriteLin e("Folders class online!")
End Sub
End Class
End Namespace

'file: Files.vb
Namespace GrokQuestion
'file: Files.vb
Public Class Files
Private Logger As LogWriter
Public Sub New(ByVal LogFilename As String)
Logger = LogWriter.Insta nce()
Logger.Filename = LogFilename
Logger.WriteLin e("Files class online!")
End Sub
End Class
End Namespace

===== Test =====
Imports GrokQuestion

Module Module1

Sub Main()
TestFiles()
TestFolders()
End Sub

Private Sub TestFiles()
Console.WriteLi ne("TestFiles( ) .. expecting old filename =
""""" + vbCrLf)
Dim LogFilename As String = "C:\TEMP\First. txt"
Dim pFiles As GrokQuestion.Gr okQuestion.File s = New
GrokQuestion.Gr okQuestion.File s(LogFilename)
Console.WriteLi ne()
End Sub

Private Sub TestFolders()
Console.WriteLi ne("TestFolders () .. expecting old filename =
""C:\Test\First .txt""" + vbCrLf)
Dim LogFilename As String = "C:\TEMP\Second .txt"
Dim pFolders As GrokQuestion.Gr okQuestion.Fold ers = New
GrokQuestion.Gr okQuestion.Fold ers(LogFilename )
Console.WriteLi ne()
End Sub

End Module

===== Test Output =====
TestFiles() .. expecting old filename = ""

Get Instance() called.
Old Filename = ""
Message = "Files class online!"

TestFolders() .. expecting old filename = "C:\Test\First. txt"

Get Instance() called.
Old Filename = "C:\TEMP\First. txt"
Message = "Folders class online!"

--end