Memory Mapped Files -- does the file survive as long as ANY process using it is active?

Robert Gustafson 606

WHAT I HAVE:
Visual Basic 2019, WinForms, .NET 4.6+

MY ISSUE:
I'm trying to use memory-mapped files to share data between different instances (of undetermined number) of my app. What I need to know is this: Does the file die when the current instance/process terminates or disposes of it, or does it survive until all instances/processes using it have terminated or disposed of it? The point is, I can't count on my instances finishing in any given order relative to the order in which they were started, and I need to access the same MMF data so long as any instance is running.

Please give me an answer ASAP, and, as for any code samples, keep it simple and in VB.NET.

David Lowndes 4,711 Reputation points

2021-11-16T14:41:42.667+00:00

Generally, as long as any process keeps a handle on an object, the object will be retained.
Robert Gustafson 606 Reputation points

2021-11-16T15:34:16.96+00:00
Does that mean that if one process (or instance thereof) disposes of an MMF (with a given name) while another is using the MMF (with the same name), the MMF remains alive until no processes are using it?

I.e.,

Process 1:

Public mmf As MemoryMappedFile Sub Main() mmf = MemoryMappedFile.CreateOrOpen("This File", size) ' do some stuff mmf = Nothing End Sub

Process 2 (may be another instance of the same app):

Public mmf As MemoryMappedFile Sub Main() mmf = MemoryMappedFile.CreateOrOpen("This File", size) ' do some stuff mmf = Nothing End Sub

Right or wrong: It doesn't matter what order the processes close with the respect to the order in which they open--so long as any process/instance-thereof is using "This File", "This File" and its contents survive; only when all processes dispose of "This File" will the MMF file be lost.
David Lowndes 4,711 Reputation points

2021-11-16T16:05:48.53+00:00

I would expect so. Bear in mind that the MemoryMappedFile is IDisposable, so you ought to wrap its use up in a Using statement to ensure it's released in a determinable manner.
RLWA32 40,021 Reputation points

2021-11-16T17:36:28.307+00:00

You'll also want to make sure that any MemoryMappedViewAccessor objects are also released in a determinable manner.
Robert Gustafson 606 Reputation points

2021-11-18T01:40:45.027+00:00

One last thing: I need to ensure that when a given instance of my app is accessing the MMF, other processes--include other instances of my program--aren't accessing at the same time. Which is a better lock-out object: Mutex or Semaphore? Mind you that this is a WinForms app.
David Lowndes 4,711 Reputation points

2021-11-18T08:56:44.93+00:00

In your case, you'd use a mutex. Search for an explanation, such as this - see "Difference between Mutex and Semaphore".