asp.net multithreading with synclock

i have some test code which i run at every load of a page in my asp.net website

this is the code

Sub TryThreads()
    Dim t1 = New Thread(AddressOf TryLock)
    t1.Priority = ThreadPriority.Lowest
    t1.Start()
    Dim t2 = New Thread(AddressOf TryLock)
    t2.Priority = ThreadPriority.Lowest
    t2.Start()
End Sub
Sub TryLock()
    Dim LockObject = New Object
    SyncLock LockObject
        DoTrace("entered locker")
      开发者_如何转开发  For x = 0 To 10000
        Next
        DoTrace("exiting locker")
    End SyncLock
    DoTrace("exited locker")
End Sub

the "dotrace" simply add a record to a log table in the db

now the right result would be that i should have the entries in the db in order "entered","exiting","exited"

but actually when i look in the db i see first 2 "entered" then 2 "exiting" etc. meaning that the multithreading is working ok, but not the synclock is that correct?

and how can this be fixed? the real code will be adding records to the db and might be called from several pages of different sessions, but the same code must not run twice concurrently

i do appreciate anybodys help

thank you very much!!!

EDIT:

in response to Sashas wonderful post i changed my code to a class (it was in a module) and now it looks like this:

Public Class CheckClass
Property LockObject As Object
    Get
        If HttpRuntime.Cache("CheckSessionsLock") Is Nothing Then HttpRuntime.Cache("CheckSessionsLock") = New Object
        Return HttpRuntime.Cache("CheckSessionsLock")
    End Get
    Set(ByVal value As Object)
        If value Is Nothing Then
            HttpRuntime.Cache.Remove("CheckSessionsLock")
        Else
            HttpRuntime.Cache("CheckSessionsLock") = value
        End If
    End Set
End Property
Sub TryThreads()
    Dim t1 = New Thread(AddressOf TryLock)
    t1.Priority = ThreadPriority.Lowest
    t1.Start()
    Dim t2 = New Thread(AddressOf TryLock)
    t2.Priority = ThreadPriority.Lowest
    t2.Start()
End Sub
Sub TryLock()
    SyncLock LockObject
        DoTrace("entered locker")
        For x = 0 To 10000
        Next
        DoTrace("exiting locker")
    End SyncLock
    DoTrace("exited locker")
End Sub
End Class

now it works 80-90% of the time. on page load i have:

Dim cc = New CheckClass
    cc.TryThreads()

if i open multiple pages at once, they still clash some times. but if i'm correct, the issue is now not with the synclock as much as with the httpruntime.cache, because when using a standard property, on one page, the code works 100%.

so how can i make sure that 2 threads, even from totally different sessions never run the trylock simultaneously?

thank you all for helping out

You are creating a new object instance when the TryLock method is called, and use that for locking. If you want mutual exclusion between the two threads, you need to use a common object instance for locking, e.g. a static member of your class or a parameter that you pass to both threads.