asp.net global synclock object

问题

is there a way in asp.net to make sure that a certain threaded sub is not run twice concurrently, no matter what?

the code i have now is

Public Class CheckClass
ReadOnly Property CheckSessionsLock As Object
    Get
        If HttpRuntime.Cache("CheckSessionsLock") Is Nothing Then HttpRuntime.Cache("CheckSessionsLock") = New Object
        Return HttpRuntime.Cache("CheckSessionsLock")
    End Get
End Property
Sub TryThreads()
    Dim thread = New Thread(AddressOf TryLock)
    thread.Priority = ThreadPriority.Lowest
    thread.Start()
End Sub
Sub TryLock()
    SyncLock CheckSessionsLock
        DoTrace("entered locker")
        For x = 0 To 10000
        Next
        DoTrace("exiting locker")
    End SyncLock
    DoTrace("exited locker")
End Sub
End Class

if i run this code on every page then several times the code overlaps. the DoTrace function in the code simply writes the message to a table.

the messages in the table should appear in order (entered,exiting,exited) again and again, but in reality, they don't. i get like entered, exiting,entered,exited,exiting...

this means that the synclock is not complete. is that true?

if so, how can we implement a complete synclock on a block of code, across requests and across sessions?

EDIT: i need this lock, as the real code will be sending emails, according to a list of mailing types in a db. after each mailing type is sent, its marked, then it continues with the next mailing. i cant have in middle of processing, another thread should see this mailing as unprocessed.

please advise

回答1:

Rather than using the HttpRuntime Cache have you considered using a static variable?

Just as a note (it might be helpful to explain why you want this functionality) your website is not going to be very scalable if this can only be run once at a time.

回答2:

In C# (sorry, don't know VB syntax) I use this:

private static readonly object Padlock = new object();

It's a field, not a property,
It's static (in VB, that's "shared" if I'm not mistaken) so it's the same throughout the entire application
It's initialised once as soon as you use this class, not when you explicitly use the field.

With your property/cache version, you could have two threads trying to get the lock-object and each creating a different one:

Thread 1 checks the cache and doesn't find the object
Thread 1 is parked
Thread 2 checks the cache, doesn't find the object
Thread 2 creates the object and caches it, retrieves it again and returns from the property
Thread 1 resumes
Thread 1 creates a new object and caches it, retrieves it again and returns a different lock object than thread 2 uses
Any further threads will use the lock object of thread 1

来源：https://stackoverflow.com/questions/4920481/asp-net-global-synclock-object

标签

ASP.NET

multithreading

synclock