A static property in static class when used concurrently

Question

I have a static class 'Logger' with a public property called 'LogLevels' as in code below.

When the property is used concurrently in a multi-user or multi-threaded environment, could it cause problems?

Do I need to use thread synchronization for the code within the property 'LogLevels'?

 public class Logger
{
    private static List<LogLevel> _logLevels = null;


    public static List<LogLevel> LogLevels
    {
        get
        {
            if (_logLevels == null)
            {
                _logLevels = new List<LogLevel>();
                if (!string.IsNullOrWhiteSpace(System.Configuration.ConfigurationManager.AppSettings["LogLevels"]))
                {

                    string[] lls = System.Configuration.ConfigurationManager.AppSettings["LogLevels"].Split(",".ToCharArray());
                    foreach (string ll in lls)
                    {

                        _logLevels.Add((LogLevel)System.Enum.Parse(typeof(LogLevel), ll));
                    }
                }
            }

            if (_logLevels.Count == 0)
            {
                _logLevels.Add(LogLevel.Error);
            }
            return _logLevels;
        }
    }
}

UPDATE: I ended up using thread synchronization to solve concurrency problem in a static class, as in code below.

public class Logger
{
    private static readonly System.Object _object = new System.Object();

    private static List<LogLevel> _logLevels = null;


private static  List<LogLevel> LogLevels
    {
        get
        {
            //Make sure that in a multi-threaded or multi-user scenario, we do not run into concurrency issues with this code.
            lock (_object)
            {
                if (_logLevels == null)
                {
                    _logLevels = new List<LogLevel>();
                    if (!string.IsNullOrWhiteSpace(System.Configuration.ConfigurationManager.AppSettings["SimpleDBLogLevelsLogger"]))
                    {

                        string[] lls = System.Configuration.ConfigurationManager.AppSettings["SimpleDBLogLevelsLogger"].Split(",".ToCharArray());
                        foreach (string ll in lls)
                        {

                            _logLevels.Add((LogLevel)System.Enum.Parse(typeof(LogLevel), ll));
                        }
                    }
                }

                if (_logLevels.Count == 0)
                {
                    _logLevels.Add(LogLevel.Error);
                }
            }
            return _logLevels;
        }
    }
}

Answer 1

When the property is used concurrently in a multi-user or multi-threaded environment, could it cause problems?

Absolutely. List<T> is not designed for multiple threads, except for the case where there are just multiple readers (no writers).

Do I need to use thread synchronization for the code within the property 'LogLevels'?

Well that's one approach. Or just initialize it on type initialization, and then return a read-only wrapper around it. (You really don't want multiple threads modifying it.)

Note that in general, doing significant amounts of work in a static constructor is a bad idea. Are you happy enough that if this fails, every access to this property will fail, forever?

Answer 2

This code posses race conditions and cannot be safely executed from multiple threads. The primary problem is the List<T> type is not thread safe yet this code will freely write to. This mean the writes can occur in parallel and hence break the implicit contract of List<T>

Answer 3

The short answer is "yes" and "yes" you do need threads synchronization. The other question is, why re-invent the wheel? You can use something like log4net or .NET logging framework.