dispose of shared object instance

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How do I dispose of a shared object instance only once--after all
consumers of it have been disposed of?

More detail:
I have a VB.NET trace class that I invoke as a shared instance in the
business code I'm interested in logging. The trace class needs to close
out of the file when I'm done using the business objects--but since many
objects may be using it, I don't want to close the trace file before
everyone has had a chance to use it.

for example:

'----------.dll/server-side component

Public Class business_class
Implements IDisposable

Shared log_trace as New Trace_class

Public Sub SampleMethod()
Trace.WriteLine("starting SampleMethod")
'...
End Sub

Public Sub Dispose() Implements IDisposable.Dispose
log_trace.Dispose
End Sub
End Class

'----------client-side component

Public Sub Main()
Dim my_business as Business_class
Dim your_business as Business_class

my_business.SampleMethod
your_business.SampleMethod

my_business.Dispose() 'the problem here is that it shouldn't yet call
log_trace.Dispose
my_business = Nothing

your_business.Dispose()
your_business = Nothing

End Sub