Parallel.ForEach IndexOutOfRangeException

Question

I'm trying to go through my event logs with a Parallel.ForEach. But at random it stops with the error IndexOutOfRangeException on the foreach.

this one:

foreach (EventLogEntry message in evlog.Entries)

Sometimes it stops with the first time it goes through the loop and sometimes after already gone through the loop 44 times.

Also I sometimes get the error that the given ID already exists.

Here's my full code:

string ID = null;
Logs.Add("System", 0);
Logs.Add("Application", 1000);
Logs.Add("Setup", 2000);
Logs.Add("Forwarded Events", 3000);  

EventLog evlog = new EventLog();
evlog.MachineName = ".";  

Parallel.ForEach(Logs.Keys, logname =>
{
    System.Threading.Thread.Sleep(1000);
    evlog.Source = logname;
    string lognameWithoutSpaces = logname.Replace(" ", "");

    foreach (EventLogEntry message in evlog.Entries)
    {
        string type = message.EntryType.ToString();
        if (type == "0" | type == "Warning")
        {
            ID = lognameWithoutSpaces + "_" + Logs[logname].ToString();
            Console.WriteLine("ID: " + ID);
            dictLogs.Add(ID, new List<string>());
            dictLogs[ID].Add(evlog.Log);
            dictLogs[ID].Add(message.Source);
            dictLogs[ID].Add(message.InstanceId.ToString());
            dictLogs[ID].Add(type);
            dictLogs[ID].Add(message.UserName);
            dictLogs[ID].Add(message.TimeGenerated.ToString());
            dictLogs[ID].Add(message.Category);
            dictLogs[ID].Add(message.MachineName);
            dictLogs[ID].Add(message.Message);
            Logs[logname]++;
        }
    }
});
dictLogs.Clear();

This is what I see in de console:

ID: ForwardedEvents_3000
ID: System_0
ID: Setup_2000
ID: Application_1000
'DashboardBackEnd.vshost.exe' (CLR v4.0.30319: DashboardBackEnd.vshost.exe):    Loaded 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.resources\v4.0_4.0.0.0_nl_b77a5c561934e089\System.resources.dll'. Module was built without symbols.
A first chance exception of type 'System.Collections.Generic.KeyNotFoundException' occurred in mscorlib.dll
A first chance exception of type 'System.IndexOutOfRangeException' occurred in System.dll

Answer 1

dictLogs are being added to at the same time by multiple threads. If this is a normal dictionary this will give you issues.

Ask yourself what happens when

dictLogs.Add(ID, new List<string>());

is called at the exact same time.

You have the same kind of issue with

Logs[logname]++;

where the same entry will be updated by multiple threads. However this should not cause an IndexOutOfRangeException but will cause incorrect counts.

Your ID already exists exception I think is because of this

ID = lognameWithoutSpaces + "_" + Logs[logname].ToString();

Logs[logname] could have the same count at the same time. Hence this will cause duplicate keys to be entered

Also

evlog.Source = logname;

might not cause any exceptions but it will give you incorrect results because it might be changed by another thread before it reaches the loop:

foreach (EventLogEntry message in evlog.Entries)

In all honesty I would say take out the Parallel loop before you blow up the universe.

Answer 2

I expect the collection is not thread safe, what makes you think it is?

Also it is very possible that your actions when processing a message changes the collection in some unexpected way.

What are you trying to achieve by writing complex multi threaded code when 1 thread would do the job?

Answer 3

Each instance of the parallel loop is writing to the same evlog. So one looping is setting the Source while another loop is already running.