Develop Reference

Thread.Suspend() is obsolete

I have a problem with thread, I want to create n thread and write a log (with method write, already implemented)
This is an unit test, when I run it, it works nice, but an exception appears :
System.AppDomainUnloadedException: Attempted to access an unloaded AppDomain. This can happen if the test(s) started a thread but did not stop it. Make sure that all the threads started by the test(s) are stopped before completion.
So, I tried to use ThreadC.Suspend() and error disappears, but mehod Suspend is obsolete..
How can I fix it?
public void TestMethod1()
{
try
{
LogTest logTest = new LogTest(new FileLog());
logTest.PerformanceTest();
logTest = new LogTest(new CLogApi());
logTest.PerformanceTest();
logTest = new LogTest(new EmptyLog());
logTest.PerformanceTest();
}
catch (Exception)
{
Assert.IsTrue(false);
}
}
public class LogTest
{
private readonly Log log;
private int numberOfIterations = 5;
public LogTest(Log log)
{
this.log = log;
}
public void PerformanceTest()
{
for (int i = 0; i < this.numberOfIterations; i++)
{
try
{
Thread threadC = Thread.CurrentThread;
threadC = new Thread(this.ThreadProc);
threadC.Name = i.ToString();
threadC.Start();
// threadC.IsBackground = true;
}
catch (Exception)
{
Assert.IsTrue(false);
}
}
}
private void ThreadProc()
{
try
{
this.log.Write(" Thread : " + Thread.CurrentThread.Name.ToString());
this.log.Write(" Thread : " + Thread.CurrentThread.Name.ToString());
this.log.Write(" Thread : " + Thread.CurrentThread.Name.ToString());
this.log.Write(" Thread : " + Thread.CurrentThread.Name.ToString());
}
catch (Exception)
{
Assert.IsTrue(false);
}
}
}

1: You should use "Assert.Fail()" instead Assert.IsTrue(false);
2: Read the Microsoft documentation if you use an obsolete method. They write what you can use instead."Thread.Suspend has been deprecated. Please use other classes in System.Threading, such as Monitor, Mutex, Event, and Semaphore, to synchronize Threads or protect resources."
3: If i understand you correctly you want to kill all running threads or wait for them. You can use "Thread.Join()" https://msdn.microsoft.com/de-de/library/95hbf2ta(v=vs.110).aspx
You can store all threads in an Array or list an join all threads at the end.
4: Instead using threads you can use the async pattern and wait for all Tasks with Task.WaitAll(tasks) https://msdn.microsoft.com/en-us/library/dd270695(v=vs.110).aspx

C# Enqueue Failure

I have a simple logging mechanism that should be thread safe. It works most of the time, but every now and then I get an exception on this line, "_logQ.Enqueue(s);" that the queue is not long enough. Looking in the debugger there are sometimes just hundreds of items, so I can't see it being resources. The queue is supposed to expand as needed. If I catch the exception as opposed to letting the debugger pause at the exception I see the same error. Is there something not thread safe here? I don't even know how to start debugging this.
static void ProcessLogQ(object state)
{
try
{
while (_logQ.Count > 0)
{
var s = _logQ.Dequeue();
string dir="";
Type t=Type.GetType("Mono.Runtime");
if (t!=null)
{
dir ="/var/log";
}else
{
dir = #"c:\log";
if (!Directory.Exists(dir))
Directory.CreateDirectory(dir);
}
if (Directory.Exists(dir))
{
File.AppendAllText(Path.Combine(dir, "admin.log"), DateTime.Now.ToString("hh:mm:ss ") + s + Environment.NewLine);
}
}
}
catch (Exception)
{
}
finally
{
_isProcessingLogQ = false;
}
}
public static void Log(string s) {
if (_logQ == null)
_logQ = new Queue<string> { };
lock (_logQ)
_logQ.Enqueue(s);
if (!_isProcessingLogQ) {
_isProcessingLogQ = true;
ThreadPool.QueueUserWorkItem(ProcessLogQ);
}
}
Note that the threads all call Log(string s). ProcessLogQ is private to the logger class.
* Edit *
I made a mistake in not mentioning that this is in a .NET 3.5 environment, therefore I can't use Task or ConcurrentQueue. I am working on fixes for the current example within .NET 3.5 constraints.
** Edit *
I believe I have a thread-safe version for .NET 3.5 listed below. I start the logger thread once from a single thread at program start, so there is only one thread running to log to the file (t is a static Thread):
static void ProcessLogQ()
{
while (true) {
try {
lock (_logQ);
while (_logQ.Count > 0) {
var s = _logQ.Dequeue ();
string dir = "../../log";
if (!Directory.Exists (dir))
Directory.CreateDirectory (dir);
if (Directory.Exists (dir)) {
File.AppendAllText (Path.Combine (dir, "s3ol.log"), DateTime.Now.ToString ("hh:mm:ss ") + s + Environment.NewLine);
}
}
} catch (Exception ex) {
Console.WriteLine (ex.Message);
} finally {
}
Thread.Sleep (1000);
}
}
public static void startLogger(){
lock (t) {
if (t.ThreadState != ThreadState.Running)
t.Start ();
}
}
private static void multiThreadLog(string msg){
lock (_logQ)
_logQ.Enqueue(msg);
}

Look at the TaskParallel Library. All the hard work is already done for you. If you're doing this to learn about multithreading read up on locking techniques and pros and cons of each.
Further, you're checking if _logQ is null outside your lock statement, from what I can deduce it's a static field that you're not initializing inside a static constructor. You can avoid doing this null check (which should be inside a lock, it's critical code!) you can ensure thread-safety by making it a static readonly and initializing it inside the static constructor.
Further, you're not properly handling queue states. Since there's no lock during the check of the queue count it could vary on every iteration. You're missing a lock as your dequeuing items.
Excellent resource:
http://www.yoda.arachsys.com/csharp/threads/

For a thread-safe queue, you should use the ConcurrentQueue instead:
https://msdn.microsoft.com/en-us/library/dd267265(v=vs.110).aspx

Multithreading design-pattern

While keeping in mind that:
I am using a blocking queue that waits for ever until something is added to it
I might get a FileSystemWatcher event twice
The updated code:
{
FileProcessingManager processingManager = new FileProcessingManager();
processingManager.RegisterProcessor(new ExcelFileProcessor());
processingManager.RegisterProcessor(new PdfFileProcessor());
processingManager.Completed += new ProcessingCompletedHandler(ProcessingCompletedHandler);
processingManager.Completed += new ProcessingCompletedHandler(LogFileStatus);
while (true)
{
try
{
var jobData = (JobData)fileMonitor.FileQueue.Dequeue();
if (jobData == null)
break;
_pool.WaitOne();
Application.Log(String.Format("{0}:{1}", DateTime.Now.ToString(CultureInfo.InvariantCulture), "Thread launched"));
Task.Factory.StartNew(() => processingManager.Process(jobData));
}
catch (Exception e)
{
Application.Log(String.Format("{0}:{1}", DateTime.Now.ToString(CultureInfo.InvariantCulture), e.Message));
}
}
}
What are are you suggestions on making the code multi-threaded while taking into consideration the possibility that two identical string paths may be added into the blocking queue? I have left the possibility that this might happen and in this case.. the file would be processed twice, the thing is that sometimes I get it twice, sometimes not, it is really awkward, if you have suggestions on this, please tell.
The null checking is for exiting the loop, I intentionally add a null from outside the threaded loop to determine it to stop.

For multi-threading this... I would probably add a "Completed" event to your FileProcessingManager and register for it. One argument of that event will be the "bool" return value you currently have. Then in that event handler, I would do the checking of the bool and re-queueing of the file. Note that you will have to keep a reference to the FileMonitorManager. So, I would have this ThreadProc method be in a class where you keep the FileMonitorManager and FileProcessingManager instances in a property.
To deduplicate, in ThreadProc, I would create a List outside of the while loop. Then inside the while loop, before you process a file, lock that list, check to see if the string is already in there, if not, add the string to the list and process the file, if it is, then skip processing.
Obviously, this is based on little information surrounding your method but my 2 cents anyway.
Rough code, from Notepad:
private static FileMonitorManager fileMon = null;
private static FileProcessingManager processingManager = new FileProcessingManager();
private static void ThreadProc(object param)
{
processingManager.RegisterProcessor(new ExcelFileProcessor());
processingManager.RegisterProcessor(new PdfFileProcessor());
processingManager.Completed += ProcessingCompletedHandler;
var procList = new List<string>();
while (true)
{
try
{
var path = (string)fileMon.FileQueue.Dequeue();
if (path == null)
break;
bool processThis = false;
lock(procList)
{
if(!procList.Contains(path))
{
processThis = true;
procList.Add(path);
}
}
if(processThis)
{
Thread t = new Thread (new ParameterizedThreadStart(processingManager.Process));
t.Start (path);
}
}
catch (System.Exception e)
{
Console.WriteLine(e.Message);
}
}
}
private static void ProcessingCompletedHandler(bool status, string path)
{
if (!status)
{
fileMon.FileQueue.Enqueue(path);
Console.WriteLine("\n\nError on file: " + path);
}
else
Console.WriteLine("\n\nSucces on file: " + path);
}

Serially process ConcurrentQueue and limit to one message processor. Correct pattern?

I'm building a multithreaded app in .net.
I have a thread that listens to a connection (abstract, serial, tcp...).
When it receives a new message, it adds it to via AddMessage. Which then call startSpool. startSpool checks to see if the spool is already running and if it is, returns, otherwise, starts it in a new thread. The reason for this is, the messages HAVE to be processed serially, FIFO.
So, my questions are...
Am I going about this the right way?
Are there better, faster, cheaper patterns out there?
My apologies if there is a typo in my code, I was having problems copying and pasting.
ConcurrentQueue<IMyMessage > messages = new ConcurrentQueue<IMyMessage>();
const int maxSpoolInstances = 1;
object lcurrentSpoolInstances;
int currentSpoolInstances = 0;
Thread spoolThread;
public void AddMessage(IMyMessage message)
{
this.messages.Add(message);
this.startSpool();
}
private void startSpool()
{
bool run = false;
lock (lcurrentSpoolInstances)
{
if (currentSpoolInstances <= maxSpoolInstances)
{
this.currentSpoolInstances++;
run = true;
}
else
{
return;
}
}
if (run)
{
this.spoolThread = new Thread(new ThreadStart(spool));
this.spoolThread.Start();
}
}
private void spool()
{
Message.ITimingMessage message;
while (this.messages.Count > 0)
{
// TODO: Is this below line necessary or does the TryDequeue cover this?
message = null;
this.messages.TryDequeue(out message);
if (message != null)
{
// My long running thing that does something with this message.
}
}
lock (lcurrentSpoolInstances)
{
this.currentSpoolInstances--;
}
}

This would be easier using BlockingCollection<T> instead of ConcurrentQueue<T>.
Something like this should work:
class MessageProcessor : IDisposable
{
BlockingCollection<IMyMessage> messages = new BlockingCollection<IMyMessage>();
public MessageProcessor()
{
// Move this to constructor to prevent race condition in existing code (you could start multiple threads...
Task.Factory.StartNew(this.spool, TaskCreationOptions.LongRunning);
}
public void AddMessage(IMyMessage message)
{
this.messages.Add(message);
}
private void Spool()
{
foreach(IMyMessage message in this.messages.GetConsumingEnumerable())
{
// long running thing that does something with this message.
}
}
public void FinishProcessing()
{
// This will tell the spooling you're done adding, so it shuts down
this.messages.CompleteAdding();
}
void IDisposable.Dispose()
{
this.FinishProcessing();
}
}
Edit: If you wanted to support multiple consumers, you could handle that via a separate constructor. I'd refactor this to:
public MessageProcessor(int numberOfConsumers = 1)
{
for (int i=0;i<numberOfConsumers;++i)
StartConsumer();
}
private void StartConsumer()
{
// Move this to constructor to prevent race condition in existing code (you could start multiple threads...
Task.Factory.StartNew(this.spool, TaskCreationOptions.LongRunning);
}
This would allow you to start any number of consumers. Note that this breaks the rule of having it be strictly FIFO - the processing will potentially process "numberOfConsumer" elements in blocks with this change.
Multiple producers are already supported. The above is thread safe, so any number of threads can call Add(message) in parallel, with no changes.

I think that Reed's answer is the best way to go, but for the sake of academics, here is an example using the concurrent queue -- you had some races in the code that you posted (depending upon how you handle incrementing currnetSpoolInstances)
The changes I made (below) were:
Switched to a Task instead of a Thread (uses thread pool instead of incurring the cost of creating a new thread)
added the code to increment/decrement your spool instance count
changed the "if currentSpoolInstances <= max ... to just < to avoid having one too many workers (probably just a typo)
changed the way that empty queues were handled to avoid a race: I think you had a race, where your while loop could have tested false, (you thread begins to exit), but at that moment, a new item is added (so your spool thread is exiting, but your spool count > 0, so your queue stalls).
private ConcurrentQueue<IMyMessage> messages = new ConcurrentQueue<IMyMessage>();
const int maxSpoolInstances = 1;
object lcurrentSpoolInstances = new object();
int currentSpoolInstances = 0;
public void AddMessage(IMyMessage message)
{
this.messages.Enqueue(message);
this.startSpool();
}
private void startSpool()
{
lock (lcurrentSpoolInstances)
{
if (currentSpoolInstances < maxSpoolInstances)
{
this.currentSpoolInstances++;
Task.Factory.StartNew(spool, TaskCreationOptions.LongRunning);
}
}
}
private void spool()
{
IMyMessage message;
while (true)
{
// you do not need to null message because it is an "out" parameter, had it been a "ref" parameter, you would want to null it.
if(this.messages.TryDequeue(out message))
{
// My long running thing that does something with this message.
}
else
{
lock (lcurrentSpoolInstances)
{
if (this.messages.IsEmpty)
{
this.currentSpoolInstances--;
return;
}
}
}
}
}

Check 'Pipelines pattern': http://msdn.microsoft.com/en-us/library/ff963548.aspx
Use BlockingCollection for the 'buffers'.
Each Processor (e.g. ReadStrings, CorrectCase, ..), should run in a Task.
HTH..

Exception handling in threads

Recently i have attended an interview . A code snippet is given to me.I know,the interviewer took it from albhari's threading sample.
public static void Main()
{
try
{
new Thread (Go).Start();
}
catch (Exception ex)
{
// We'll never get here!
Console.WriteLine ("Exception!");
}
}
static void Go() { throw null; }
The modification of the above code as
public static void Main()
{
new Thread (Go).Start();
}
static void Go()
{
try
{
...
throw null; // this exception will get caught below
...
}
catch (Exception ex)
{
Typically log the exception, and/or signal another thread
that we've come unstuck
...
}
}
would be the good candidate to handle the exception.
I have been asked, "Except the above trail what are the other alternatives would fit as good solution?. It was hard to find the alternative,so i raise it here to gather your suggestion.

Exception thrown in a thread normally couldn't be caught in another thread.
You'd better to catch it in function Go and pass it to main thread explicitly.
However, if you just want to log all unhandled messages from all threads, you may use AppDomain.UnhandledException event or equivalent events at Application class if you are developing WinForms or WPF app.

what are the other alternatives would fit as good solution?.
Solution to what? What problem are you trying to solve?
If you use BackgroundWorker, as opposed to Thread, it has an RunWorkerCompleted event, and within that you can check the RunWorkerCompletedEventArgs param for the Error property. This generally is used in WinForms or WPF apps, because there is good support for BackgroundWorker in the Visual Studio designer.
You could also define a delegate for Go(), and call BeginInvoke() on it. Of course you need the EndInvoke() too.
Also, it's generally not a good idea to start up random threads. ThreadPool.QueueUserWorkItem, BackgroundWorker, or asynch delegates all use the ThreadPool, and are recommended.

There are alternatives listed on Joe Albahari's website:
http://www.albahari.com/threading/#_Exception_Handling
"There are, however, some cases where you don’t need to handle exceptions on a worker thread, because the .NET Framework does it for you. These are covered in upcoming sections, and are:
-Asynchronous delegates
-BackgroundWorker
-The Task Parallel Library (conditions apply)"

You can use the AppDomain.UnhandledException event

I think this is the easiest way is:
BackgroundWorker bw = new BackgroundWorker();
bw.DoWork += new DoWorkEventHandler((object sender2, DoWorkEventArgs e2) =>
{
throw new Exception("something bad");
e2.Result = 1 + 1;
});
bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler((object sender2, RunWorkerCompletedEventArgs e2) =>
{
if (e2.Error != null)
{
Console.WriteLine("Error: " + e2.Error.Message);
}
});
bw.RunWorkerAsync();
but there is another way that some might prefer if you want to synchronize the thread (perhaps this is on a thread other than the GUI thread):
private class FileCopier
{
public bool failed = false;
public Exception ex = null;
public string localPath;
public string dstPath;
public FileCopier(string localPath, string dstPath)
{
this.localPath = localPath;
this.dstPath = dstPath;
}
public void Copy()
{
try{
throw new Exception("bad path");
}catch(Exception ex2)
{
ex = ex2;
failed = true;
}
}
}
public static void Main()
{
FileCopier fc = new FileCopier("some path", "some path");
Thread t = new Thread(fc.Copy);
t.Start();
t.Join();
if (fc.failed)
Console.WriteLine(fc.ex.Message);
}
Note that the second example would make more sense if you have several threads and you loop through them and join all...but I kept the example simple.
the 3rd pattern would be using Task Factory which is cleaner:
private static test(){
List<Task<float>> tasks = new List<Task<float>>();
for (float i = -3.0f; i <= 3.0f; i+=1.0f)
{
float num = i;
Console.WriteLine("sent " + i);
Task<float> task = Task.Factory.StartNew<float>(() => Div(5.0f, num));
tasks.Add(task);
}
foreach(Task<float> t in tasks)
{
try
{
t.Wait();
if (t.IsFaulted)
{
Console.WriteLine("Something went wrong: " + t.Exception.Message);
}
else
{
Console.WriteLine("result: " + t.Result);
}
}catch(Exception ex)
{
Console.WriteLine("Error: " + ex.Message);
}
}
}
private static float Div(float a, float b)
{
Console.WriteLine("got " + b);
if (b == 0) throw new Exception("Divide by zero");
return a / b;
}