vba project reference and shared state (for logging)

Question

say I have 2 add-ins A and B - both use VB functions from a shared add-in C (i.e. A and B have a reference to the VB Project C).

C will log some messages to a file (e.g. C.Log) when errors are detected. I want that C logs to A.log when the functions are called from A and to B.log when the functions are called from B.

the naive approach to set a global variable in C with the log-file name will not work, because the variable is a Singleton, so the last project to set the variable (A, or B) will win.

How can this easily be done in VBA?

Create an ILog interface with the file name or log method, have an implementation in a class in A/B and pass that to C.Log(ILog, "" ...) — Alex K., Jan 31 '14 at 15:42
[This answer](http://stackoverflow.com/a/4918805/445425) may be of some help — chris neilsen, Jan 31 '14 at 19:17

score 0 · Accepted Answer · answered Feb 10 '14 at 11:04

okay - so here's the solution I came up with (quite some code, but seems to work fine):

I have a logging project with a logging interface ILog (Instancing PublicNotCreatable) like this:

Public Sub Error(sLogMessage As String)
end sub
Public Sub Warning(sLogMessage As String)
end sub
' etc.

and of course an implementing class CLog that just writes the messages to a log-file.
and a public function so that the callers can get a new instance of the logger:

Public Function New_Log(ByVal logFilePathAndName) As ILog
    Set New_CLog = New CLog
    New_CLog.Initialise logFilePathAndName
End Function

then the project C must have all it's code in a class/or classes that get an ILog instance in their initialisation. For example CSharedClass could look like so:

Private Log as ILog ' note: the var is intentionally NOT called moLog

Public Sub Initialise(oLogger as ILog)
    Set Log = oLogger
End Sub

' other functions in this class that want to log just use this:
Public Sub SomeProcedure()
    Log.Error("error text")
End Sub

also this project must have one public function to let the callers create an instance, like this:

Public Sub New_SharedClass(oLogger as ILog) as CSharedClass ' NOTE: you may want to use an interface instead
    Set New_SharedClass = New CSharedClass
    New_SharedClass.Initialise oLogger
End Sub

Then projects A and B could use the logging like this:

Private moLog as ILog

Public Function Log() as ILog
    ' log-file name is of course different for Proejct B
    If moLog is Nothing then Set moLog = New_Log("ProjectA.log")
    Set Log = moLog
End Function

all other procedures in project A/B can now write log-messages like this (same as in the shared project)

Log.Error("Error text")

when project A/B wants to use the shared project, it can do it like so

Private moSharedClass as CSharedClass 

Public Function SharedClass() as CSharedClass    
    If moSharedClass is Nothing then Set moSharedClass = New_SharedClass(Log)
    Set SharedClass = moSharedClass
End Function

Now any code in project A/B that wants to call a proc on the shared class can simply do this:

SharedClass.SomeProcedure()

and the log messages written in SomeProcedure will end up in the correct log-file

Note: I decided to break some of my own naming rules, so that all projects can use the same code for logging (which has the benefit that I can just move procedures from projects A/B to the shared project or vice versa)

vba project reference and shared state (for logging)

1 Answers1