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Using SyncLock to control access to a hastable

Posted on 2012-03-14
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Last Modified: 2012-08-13
Hi, I have the following code

Dim htb as hashtable = htbTrades.Clone
htbTrades.Clear

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In this example htbTrades is a hashtable that is declared as a global variable and can be added to by other threads.  I want to make sure that no items can be added to the htbTrades hashtable between the Clone and Clear operations.  Is SyncLock what I should be using.  Will the following achieve my objective.

Dim htb as hashtable
SyncLock htbTrades.SyncRoot
        htb = htbTrades.Clone
        htbTrades.Clear()
End SyncLock

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Thanks
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Question by:nicksbell
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by:lojk
ID: 37720401
http://msdn.microsoft.com/en-us/library/system.collections.hashtable.clone.aspx

Well - yes and no. It will only shallow copy the hashtable so a lot depends on what you are containing in the hashtable. Heres the best way i can explain that...

Imagine you have 10 cups on a table and each cup is connected to a Keyring via a peice of string. Cloning the hashtable merely adds another keyring and another 10 peices of string connected to the original cups.  If you want 10 more cups, then i think you may need to CopyTo instead.

If you are only storing value types (and/or strings) it should work fine but if you are using it to hold instances of complex types/classes you might need to check what you are trying to acheive is actually working correctly.
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Mike Tomlinson earned 250 total points
ID: 37720565
You'd also have to place all ADDS to that HashTable within a similar SyncLock block.
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Expert Comment

by:lojk
ID: 37720642
I'm with idlemind - in fact i wouldn't even expose it is a global variable at all rather a private one with a readonly property and a few public methods to proxy access to it (AddToHash, CopyHash etc) with the synclocks in place in there.
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Author Comment

by:nicksbell
ID: 37720768
Thanks for your replies.

lokj, re my first comment, my issue is not with the Clone method.  I know that works for what I am trying to do as I already have that in production.  I simply want to make sure that no items can be added to the hashtable after Clone and before Clear, as then they would be lost altogether.  I am not worried about the underlying objects contained in the hashtable as they are static and are never updated, only new ones are added.

Idle_mind, ok, I didn't realise that but that makes sense.

lokj, just to clarify, are you suggesting I create a class that inherits the hashtable class and then expose a public AddToHash method (which would handle the synclock) and all other code would call AddToHash?  I guess that makes a lot of sense too as it stops the need for synclocks all over the place.
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Assisted Solution

by:lojk
lojk earned 250 total points
ID: 37721120
not explicity saying an inherited class here more just a couple of helper methods in the global module to manage the utilisation of Idleminds point but i was going to suggest it and avoided it for simplicity reasons.

I think i would just create a Shared Class (module - not getting into that discussion now) with an private instance of the hashtable  and Get,Add,Delete,Clone,Clear methods that all implement the sync methods - it would always be fairly thread safe then.

MySafeHashTable.AddNewItem("key",Item);
Item = MySafeHashTable.GetItem("key");
MySafeHashTable.FlushAll();

The knock-on benefit to this approach would be by removing that global hash , you'd also weed out all those instances in your code where it's used allowing a nice simple hit list of places to change to the new methods.
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by:nicksbell
ID: 37721344
Thanks, very useful
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