Missing index in arraylist

[ramanaths]

Hi

My app has multiple threads reading data from pipe and storing it in an arraylist. When parsing the arraylist i got an error... seemed like a null value that's crept in. The thread processing continued and i could see data being stored in the arraylist.

Looking into it further threw up a weird problem. An index position in the arraylist was missing!! Well I thought maybe I could expect null value to be stored at some positions but a missing index position seems strange.

The arraylist is named DataHolder.
What I see is DataHolder[1281]='abc' and then DataHolder[1283]='xyz'. There is no DataHolder[1282].

Any ideas why this happens? Is this a common occurance?

 

[jmcilhinney]

So what happens if you execute this line:

MessageBox.Show(DataHolder(1282).ToString())

Also, have you synchronised access to your ArrayList? The help documentation specifically says that instance members are NOT thread-safe. Firstly you should be getting a thread-safe version of your ArrayList via the Synchronized method, then you should be wrapping an multi-line operations you perform on it in SyncLock blocks. Multi-threading is error-prone if not done properly.

 

[jmcilhinney]

For example:

        Dim list As New ArrayList
        Dim threadSafeList As ArrayList = ArrayList.Synchronized(list)

        SyncLock threadSafeList.SyncRoot
            For Each item As Object In threadSafeList
                MessageBox.Show(item.ToString())
            Next item
        End SyncLock