Question

I have an array list which is continuously updated every second. I have to use the same array list in two other threads and make local copies of it. I have done all of this but i get weird exceptions of index out of bound , What i have found out so far is that i have to ensure some synchronization mechanism for the array list to be used across multiple threads.

this is how i am making it synchronized:

for (int i = 0; i < Globls.iterationCount; i++)
            {
                if (bw_Obj.CancellationPending)
                {
                    eve.Cancel = true;
                    break;
                }

                byte[] rawData4 = DMM4.IO.Read(4 * numReadings);
                TempDisplayData_DMM4.Add(rawData4);
                Globls.Display_DataDMM4 = ArrayList.Synchronized(TempDisplayData_DMM4);
                Globls.Write_DataDMM4 = ArrayList.Synchronized(TempDisplayData_DMM4);

            }

in other thread i do the following to make local copies:

ArrayList Local_Write_DMM4 = new ArrayList();
Local_Write_DMM4 = new ArrayList(Globls.Write_DataDMM4); 

Am i synchronizing the arraylist in right way?, also do i need to lock while copying array-list as well:

 lock (Globls.Display_DataDMM4.SyncRoot){Local_Temp_Display1 = new ArrayList(Globls.Display_DataDMM4);}

or for single operations its safe?. I haven’t actually ran this code i need to run it over the weekend and i don’t want to see another exception :(. please help me on this!

Answer 1

as @Trickery stated assignment needs to be locked since the source array Globls.Write_DataDMM4 can be modified by another thread during enumeration.
It is essential therefore to lock both when populating the original array and when making your copy

for (int i = 0; i < Globls.iterationCount; i++)
{
    if (bw_Obj.CancellationPending)
    {
        eve.Cancel = true;
        break;
    }

    byte[] rawData4 = DMM4.IO.Read(4 * numReadings);
    TempDisplayData_DMM4.Add(rawData4);

    lock (Globls.Display_DataDMM4.SyncRoot)
    {
    Globls.Write_DataDMM4 = ArrayList.Synchronized(TempDisplayData_DMM4);
    }

}

and

lock (Globls.Display_DataDMM4.SyncRoot)
{
     Local_Temp_Display1 = new ArrayList(Globls.Display_DataDMM4);
}