mcainc
asked on
VB.NET - Thread safe collections (use SyncLock or Synchronize?)
Ok this is becoming a huge pain, I can't figure out how to do this. I tried SyncLock but I must have done it wrong because I was still getting out of range exceptions.
I have a timer that fires every 1 second & loops through this collection to check some conditions... also have events that can be raised from other threads that can use / modify these collections the same way.
Here is an example of my code, how can I make this thread safe?
Dim OfficeCollection As New ArrayList
Public Structure OfficeData
Dim firstname As String
Dim lastname As String
End Structure
*** Timer Start
Dim ODC As OfficeData
ODC.firstname = "whatever"
ODC.lastname = "whatever"
For x = OfficeCollection.Count -1 to 0 Step -1
... some condition 1: OfficeCollection.Add(ODC)
... some condition 2: OfficeCollection.Remove(OD
Next
Timer End ***
*** Event 1 Start
Dim ODC As OfficeData
ODC.firstname = "whatever"
ODC.lastname = "whatever"
For x = OfficeCollection.Count -1 to 0 Step -1
... conditions
OfficeCollection.Add(ODC)
Next
Event 1 End ***
*** Event 2 Start
Dim ODC As OfficeData
ODC.firstname = "whatever"
ODC.lastname = "whatever"
For x = OfficeCollection.Count -1 to 0 Step -1
... conditions
OfficeCollection.Remove(OD
Next
Event 2 End ***
I'm pretty frustrated at this point... I'm not 100% sure where to use SyncLock (around the for loop? only around .Add / .Remove calls? Both?)
What about .Synchronize?
Thanks a ton
I have a timer that fires every 1 second & loops through this collection to check some conditions... also have events that can be raised from other threads that can use / modify these collections the same way.
Here is an example of my code, how can I make this thread safe?
Dim OfficeCollection As New ArrayList
Public Structure OfficeData
Dim firstname As String
Dim lastname As String
End Structure
*** Timer Start
Dim ODC As OfficeData
ODC.firstname = "whatever"
ODC.lastname = "whatever"
For x = OfficeCollection.Count -1 to 0 Step -1
... some condition 1: OfficeCollection.Add(ODC)
... some condition 2: OfficeCollection.Remove(OD
Next
Timer End ***
*** Event 1 Start
Dim ODC As OfficeData
ODC.firstname = "whatever"
ODC.lastname = "whatever"
For x = OfficeCollection.Count -1 to 0 Step -1
... conditions
OfficeCollection.Add(ODC)
Next
Event 1 End ***
*** Event 2 Start
Dim ODC As OfficeData
ODC.firstname = "whatever"
ODC.lastname = "whatever"
For x = OfficeCollection.Count -1 to 0 Step -1
... conditions
OfficeCollection.Remove(OD
Next
Event 2 End ***
I'm pretty frustrated at this point... I'm not 100% sure where to use SyncLock (around the for loop? only around .Add / .Remove calls? Both?)
What about .Synchronize?
Thanks a ton
ASKER CERTIFIED SOLUTION
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ASKER
** reading the list is only thread safe if it isn't modified.
http://msdn.microsoft.com/
http://msdn.microsoft.com/
The big problem seems to be when you're enumerating while something else is modifying the collection.
Do you want a picture of the collection at the time you modified it, and don't care what changes take place after you take the intial picture? Then lock the list, copy it to a temporary working copy of the list, then release your lock. Modification threads can then go to work.
Do you want your list to always reflect changes to it? Then use do-while loops and forget enumerators. Each time you fetch an item, lock the list while you fetch the item, then release the lock as soon as the fetch is complete. When you add an item or remove an item, include a try-catch block that handles the exceptions which occur when the item is already in the list or when it has already been removed. Don't use for loops, as sometimes the compiler will cache the end counter in a register at the start of the loop; always use a standalone if-then to check whether your current index is greater than the size of the list.
Try playing with some of these thoughts, after you take a break.
http://msdn.microsoft.com/
The big problem seems to be when you're enumerating while something else is modifying the collection.
Do you want a picture of the collection at the time you modified it, and don't care what changes take place after you take the intial picture? Then lock the list, copy it to a temporary working copy of the list, then release your lock. Modification threads can then go to work.
Do you want your list to always reflect changes to it? Then use do-while loops and forget enumerators. Each time you fetch an item, lock the list while you fetch the item, then release the lock as soon as the fetch is complete. When you add an item or remove an item, include a try-catch block that handles the exceptions which occur when the item is already in the list or when it has already been removed. Don't use for loops, as sometimes the compiler will cache the end counter in a register at the start of the loop; always use a standalone if-then to check whether your current index is greater than the size of the list.
Try playing with some of these thoughts, after you take a break.
ASKER
my list will change frequently during loops (the list is looped through every second by a timer)... so if at all possible. I would like to always be working with a list that reflects changes to it
I haven't thought to try a do while loop
i'm still having trouble wrapping my head around the way to structure that do while loop (as far as where to lock)
Dim x As Integer = 0
Do While x <> MyList.Count - 1
SyncLock MySync.SyncObject
ODC = MyList(x)
End SyncLock
' perform task & remove item from MyList if condition met?
x = x + 1
Loop
or
Dim x As Integer = 0
SyncLock MySync.SyncObject
Do While x <> MyList.Count - 1
ODC = MyList(x)
' perform task & remove item from MyList if condition met?
x = x + 1
Loop
End SyncLock
or am I still way off? (sorry still green when it comes to threading)
I haven't thought to try a do while loop
i'm still having trouble wrapping my head around the way to structure that do while loop (as far as where to lock)
Dim x As Integer = 0
Do While x <> MyList.Count - 1
SyncLock MySync.SyncObject
ODC = MyList(x)
End SyncLock
' perform task & remove item from MyList if condition met?
x = x + 1
Loop
or
Dim x As Integer = 0
SyncLock MySync.SyncObject
Do While x <> MyList.Count - 1
ODC = MyList(x)
' perform task & remove item from MyList if condition met?
x = x + 1
Loop
End SyncLock
or am I still way off? (sorry still green when it comes to threading)
SOLUTION
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ASKER
ok so just to confirm, my code would be something like this:
Private Sub TimerFired() - happens every 1 second
Dim x As Integer = 0
SyncLock MySync.SyncObject
Do While x <> MyList.Count - 1
ODC = MyList(x)
... condition 1: AddToMyList(ODC)
... condition 2: RemoveFromMyList(x)
x = x + 1
Loop
End SyncLock
End Sub
Private Sub AddToMyList(var)
SyncLock MySync.SyncObject
MyList.Add(var)
End SyncLock
End Sub
Private Sub RemoveFromMyList(index)
SyncLock MySync.SyncObject
MyList.RemoveAt(index)
End SyncLock
End Sub
should this do it? thanks guys!
Private Sub TimerFired() - happens every 1 second
Dim x As Integer = 0
SyncLock MySync.SyncObject
Do While x <> MyList.Count - 1
ODC = MyList(x)
... condition 1: AddToMyList(ODC)
... condition 2: RemoveFromMyList(x)
x = x + 1
Loop
End SyncLock
End Sub
Private Sub AddToMyList(var)
SyncLock MySync.SyncObject
MyList.Add(var)
End SyncLock
End Sub
Private Sub RemoveFromMyList(index)
SyncLock MySync.SyncObject
MyList.RemoveAt(index)
End SyncLock
End Sub
should this do it? thanks guys!
ASKER
one more thing... this probably sounds silly but, with the synclocks in the add/remove subs... i should be able to call those subs freely from other methods without issues right?
That should do it.
You can still call the add/remove subs from anywhere but remember that now that you have "synchronized" the threads you have the potential for deadlocks. Code carefully... =)
http://msdn.microsoft.com/en-us/library/fxy8dte8(VS.71).aspx
You can still call the add/remove subs from anywhere but remember that now that you have "synchronized" the threads you have the potential for deadlocks. Code carefully... =)
http://msdn.microsoft.com/en-us/library/fxy8dte8(VS.71).aspx
ASKER
oh yay, this is going to be fun
thanks guys
thanks guys
You're very welcome :)
ASKER
I guess I'm just not following exactly where to place these locks.. I appreciate your help, my head is mush right now :(