Develop Reference

Catching an exception thrown in a Thread

I need to know if an exception that happens inside a method called by a Thread can be catch in the main application.
I'm doing a Windows forms application and one of the things I have to do is store some data in a database, but I need to inform the user if, for some reason, the operation was unsuccessful (e.g if the application couldn't connect to the database). The thing is that I have to call the method to insert the values in the DB from a new Thread, and, therefore, I use the try;catch blocks from inside that method. But if an error occur and the exception is thrown there is nobody able to catch it so the program crashes.
I have been doing some google search but all that I could find recommended to use the class Task instead of Thread, but, because this is an assignment from my university, I need to use Threads.
So, is there a way to "transfer" the exception from a Thread to the main thread of the application ? Here's my code so far:
//This is how I create the new Thread
public static Correo operator +(Correo c, Paquete p)
{
foreach (Paquete paq in c.Paquetes)
{
if (paq == p)
throw new TrackingIDRepetidoException("El paquete ya se encuentra cargado en la base de datos");
}
c.Paquetes.Add(p);
Thread hilo = new Thread(p.MockCicloDeVida);
hilo.Start();
c.mockPaquetes.Add(hilo);
return c;
}
public void MockCicloDeVida()
{
while (this.Estado != EEstado.Entregado)
{
Thread.Sleep(10000);
this.Estado += 1;
this.InformaEstado(this, new EventArgs());
}
try
{
// A simple method to insert an object in a DB. The try catch to check if the connection to the DB was succesfull or not is implemented here.
PaqueteDAO.Insertar(this);
}
catch (System.Data.SqlClient.SqlException e)
{
// I can't catch the exception here
}
}
Any help or tips is greatly appreciated. Thanks!

I would use this very useful class: TaskCompletionSource
var tcs = new TaskCompletionSource<object>();
var th = new Thread(() => MockCicloDeVida(tcs));
th.Start();
try
{
var returnedObj = tcs.Task.Result;
}
catch(AggregateException aex)
{
Console.WriteLine(aex.InnerExceptions.First().Message);
}
public void MockCicloDeVida(TaskCompletionSource<object> tcs )
{
Thread.Sleep(10000);
tcs.TrySetException(new Exception("something bad happened"));
//tcs.TrySetResult(new SomeObject());
}

Thread.Suspend() is obsolete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);
}

Infrequent hangs in a multi-threaded C# console application when using Console.Writeline() or Console.Write()

I have written a console application that makes use of console.write and console.writeline to provide some logging. The application is a server application that uses asynchronous beginacceptconnection() and beginread() ( Sockets ) for communication. Occasionally i get reports of it hanging and from the limited debug i can do i am able to see the problem being Console.Writeline() or Console.write().
Being multi-threaded I have been careful to have a lock around the logging class so only one thread can log a message at once.....when I've caught a hang all i get are threads blocking on the lock and VS reporting that the control has passed into Console.Write and it is waiting for it to come back....it never does.
A couple of days ago i got another report of a failure but this time during bootup....where no asynch connections have yet been kicked off( the main thread does spawn a thread to bootup though ) and I was sent a picture.....see below.( i added the begin and end critical section lines to prevent this and it did not )
// Logging Class
public class Logging
{
// Lock to make the logging class thread safe.
static readonly object _locker = new object();
public delegate void msgHandlerWriteLineDelegate(string msg, Color col);
public static event msgHandlerWriteLineDelegate themsgHandlerWriteLineDelegate;
public delegate void msgHandlerWriteDelegate(string msg, Color col);
public static event msgHandlerWriteDelegate themsgHandlerWriteDelegate;
public static void Write(string a, Color Col)
{
if (themsgHandlerWriteDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteDelegate(a, Col);
}
}
}
public static void Write(string a)
{
if (themsgHandlerWriteDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteDelegate(a, Color.Black);
}
}
}
public static void WriteLine(string a, Color Col)
{
if (themsgHandlerWriteLineDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteLineDelegate(a, Col);
}
}
}
public static void WriteLine(string a)
{
if (themsgHandlerWriteLineDelegate != null)
{
lock (_locker)
{
themsgHandlerWriteLineDelegate(a, Color.Black);
}
}
}
// Console Methods That implement the delegates in my logging class.
public static void ConsoleWriteLine(string message, Color Col)
{
try
{
if (Col == Color.Black)
{
Console.ForegroundColor = ConsoleColor.Gray;
}
else
{
Console.ForegroundColor = (ConsoleColor)Enum.Parse(typeof(ConsoleColor), Col.Name);
}
Thread.BeginCriticalRegion();
Console.WriteLine(message);
Thread.EndCriticalRegion();
Console.ForegroundColor = ConsoleColor.Gray;
}
catch (ThreadAbortException ex)
{
Console.WriteLine("ThreadAbortException : " + ex.Message);
}
catch (Exception ex)
{
Console.WriteLine("Exception : " + ex.Message);
}
}
public static void ConsoleWrite(string message, Color Col)
{
try
{
if (Col == Color.Black)
{
Console.ForegroundColor = ConsoleColor.Gray;
}
else
{
Console.ForegroundColor = (ConsoleColor)Enum.Parse(typeof(ConsoleColor), Col.Name);
}
Thread.BeginCriticalRegion();
Console.Write(message);//**THIS IS WHERE IS HANGS...IT NEVER RETURNS **
Thread.EndCriticalRegion();
Console.ForegroundColor = ConsoleColor.Gray;
}
catch (ThreadAbortException ex)
{
Console.WriteLine("ThreadAbortException : " + ex.Message);
}
catch (Exception ex)
{
Console.WriteLine("Exception : " + ex.Message);
}
}
public static void ConsoleUpdate(string message)
{
try
{
Thread.BeginCriticalRegion();
Console.WriteLine(message);//**THIS IS WHERE IS HANGS...IT NEVER RETURNS **
Thread.EndCriticalRegion();
}
catch (ThreadAbortException ex)
{
Console.WriteLine("ThreadAbortException : " + ex.Message);
}
catch (Exception ex)
{
Console.WriteLine("Exception : " + ex.Message);
}
}
// The main method...subscribes to delegates and spawns a thread to boot HW..main thread then exits.
public static void Main()
{
Logging.themsgHandlerWriteDelegate += new Logging.msgHandlerWriteDelegate(ConsoleWrite);
Logging.themsgHandlerWriteLineDelegate += new Logging.msgHandlerWriteLineDelegate(ConsoleWriteLine);
Logging.themsgHandlerUpdateDelegate += new Logging.msgHandlerUpdateDelegate(ConsoleUpdate);
}
}
public class ClassOnOtherThread
{
// In a different class running on a different thread the following line occasionly invokes the error:
private void BootHw(string Resource, string Resource2)
{
Logging.Write("\t\t[");
}
}
My reading of the MSDN suggests Console.WriteLine and Console.Write are threadsafe and therefore i don't actually need a lock around it....i also can't believe it Microsoft's code is wrong(;-) and so I am guessing it is some interaction my code is doing which creates the error.
Now my question : Should i be doing anything to prevent Console.WriteLine and Console.Write being interrupted?...it is my guess that something it interrupting it...but i don't really know that!!
Any help would me very much appreciated.
Regards,
Gordon.

I had the same problem.
I was using console.readkey() in main thread to prevent closing application in debug mode.
After I replaced it with an infinite loop my problem was solved.

You should solve your problem by removing the locks around the logging. The logging is done via Console.WriteLine which is synchronized (and thread safe). You are possibly causing a deadlock through your own locking mechanism (though I can't verify without seeing the code).

I guess your application is started by another process which does redirect stderr and stdout. If your "watcher" process uses ReadToEnd() for both streams on the same thread you can deadlock.
Another option to deadlock is to send the child process input via stdin which in turn starts another process which has a console which waits for input indefinitely. This happend once to me with wmic.exe which does block when stdin is redirected.
If you have played with your log class I suspect that you change the underlying Stream of Console.Out with your own one. Please post at least the callstack where your application is hanging so we have something to analyze. There are many ways to shoot yourself in the foot if you replace the console stream with your own one.
Yours,
Alois Kraus

This is a bit of a long shot, but I wonder if you are calling Console.WriteLine with objects whose ToString() method takes a lock. If so, you can get yourself in a deadlock situation with respect to the lock taken internally by Console.WriteLine.
I once posted this bug report to Microsoft Connect, though sadly they declined to fix it.