Unexpected release of named mutex

I have a mutex that releases while I am still holding a reference to
it! Maybe this is "garbage collection 101" and this old C++ programmer
just needs to get used to it...

Here's a boiled-down snippet of VB I used in order to implement a
single-instance app. (I can't use the application framework's single-
instance feature because it chokes on my form.)

Sub Main
Dim goodToGo As Boolean = False
Dim singleInstance As Threading.Mutex = Nothing
Try
singleInstance = New Threading.Mutex(True,
Application.ProductName & ".SingleInstance", goodToGo)
If Not goodToGo Then
' (find the existing instance and activate it)
End
End If
MainForm.ShowDialog()
Catch ex As Exception
' (log errors)
Finally
' doesn't work if you remove the next line
If goodToGo Then singleInstance.ReleaseMutex()
End Try
End Sub

This works, but if I take out the call to ReleaseMutex in the Finally
block I can them start as many instances as I want. The mutex releases
all by itself shortly after being acquired. This was highly
unexpected.

Silly me, thinking that I could keep a reference to an object by, uh,
keeping a reference to an object.

It appears that the garbage collector is outsmarting me by realizing
that the mutex object is never again being referenced and killing it.

I couldn't demonstrate this with a small test program. This is a
moderate sized app. A couple dozen forms, maybe two hundred source
files total.

(And yes, I know it's bad practice to not explicitly release a mutex
but one normally assumes that program exit cleans stuff like that up.)

Any other interpretations of what's going on here?

Thank you and be warned.
-- Carl
.

Relevant Pages

  • Re: [PATCH RFD] alternative kobject release wait mechanism
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    (Linux-Kernel)
  • Re: WaitForSingleObject() will not deadlock
    ... You keep talking about needing to know the reference count. ... algorithm that uses a recursive mutex that cares in the slightest what the reference count ... attempts to lock appears to be isomorphic to a recursive lock, ... cycle detection until you reach the end of the list, ...
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  • Re: [PATCH RFD] alternative kobject release wait mechanism
    ... Tejun Heo wrote: ... Here's an example where immediate-detach cannot be implemented. ... There is no reason to hold reference to it. ... I probably have over simplified it but using both mutex and reference ...
    (Linux-Kernel)
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