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c# Object synchronisation error is thrown when calling Monitor.Pulse()

I'm trying to sync between two threads in my GUI system.
The main task of the system initialising two synchronisation objects and running two threads:
private FetcherAPI fetcherAPI { get; set; }
private FileReader fileReader { get; set; }
private object readerLocker;
private object writerLocker;
public Form1() {
InitializeComponent();
this.fetcherAPI = new FetcherAPI();
this.fileReader = new FileReader();
readerLocker = new object();
writerLocker = new object();
new Thread(() => {
this.fileReader.run(readerLocker);
});
new Thread(() => {
this.fixerAPI.run(writerLocker, readerLocker);
});
}
private void button1_Click(object sender, EventArgs e) {
Monitor.Pulse(writerLocker);
}
Reader Thread:
public void run(object readerLocker) {
while(true) {
Monitor.Wait(readerLocker);
readDataFromFileAndPresent();
}
}
Writer Thread:
public void run(object writerLocker, object readerLocker) {
while(true) {
Monitor.Wait(writerLocker);
fetchCurrency();
Monitor.Pulse(readerLocker);
Monitor.Wait(readerLocker);
}
}
When pressing the button I'm getting the following error:
object synchronization method was called from an unsynchronized block of code
What am I doing wrong?

Both Wait and Pulse can only be called if you already have the lock, i.e. you're inside a lock statement, or you've successfully used Monitor.Enter to acquire the lock. You haven't done either of those things, so : indeed, it won't work. The simplest fix would be to add a lock(writerLocker).

Why this code which runs on seperate AppDomain Crashes my process?

I'm trying to figure out how to use AppDomains.
The Need:
I have a single process web application which dynamically loads dlls and invokes it using reflection.
I want to ensure that a crash in a loaded dll does not crash the process, in addition to creating a seperation between "external" code and my base code.
So I have this "isloated" class:
public sealed class Isolated<T> : IDisposable where T : MarshalByRefObject
{
private AppDomain _domain;
private T _value;
public Isolated()
{
_domain = AppDomain.CreateDomain("Isolated:" + Guid.NewGuid(),
null, AppDomain.CurrentDomain.SetupInformation);
Type type = typeof(T);
_value = (T)_domain.CreateInstanceAndUnwrap(type.Assembly.FullName, type.FullName);
}
public T Value
{
get
{
return _value;
}
}
public void Dispose()
{
if (_domain != null)
{
AppDomain.Unload(_domain);
_domain = null;
}
}
}
and I wrote this code below, my expectation is it would not crush the process, but it does.
public class Work : MarshalByRefObject
{
public void DoSomething()
{
Thread thread = new Thread(new ThreadStart(() =>
{
throw new Exception();
}));
thread.IsBackground = true;
thread.Start();
while (true)
{
System.Diagnostics.Trace.WriteLine("Hello from main thread");
}
}
}
protected void Page_Load(object sender, EventArgs e)
{
using (Isolated<Work> isolated = new Isolated<Work>())
{
isolated.Value.DoSomething();
}
}
Can you please help me understand what I'm doing wrong?

There is no way to prevent the process termination.
As suggested by #Jehof,
More info here: http://www.codeproject.com/Articles/635497/AppDomains-Wont-Protect-Host-From-a-Failing-Plugin

Monitor doesn't seem to lock the object

I'm trying to implement a basic Future class (yeah, I know about Task but this is for educational purposes) and ran into strange behavior of Monitor class. The class is implemented so that it enters the lock in constructor, queues an action which exits the lock to a thread pool. Result getter checks an instance variable to see if the action is completed and if it isn't, enters lock and then returns the result. Problem is that in fact result getter doesn't wait for the queued action to finish and proceeds anyway leading to incorrect results. Here's the code.
// The class itself
public class Future<T>
{
private readonly Func<T> _f;
private volatile bool _complete = false;
private T _result;
private Exception _error = new Exception("WTF");
private volatile bool _success = false;
private readonly ConcurrentStack<Action<T>> _callbacks = new ConcurrentStack<Action<T>>();
private readonly ConcurrentStack<Action<Exception>> _errbacks = new ConcurrentStack<Action<Exception>>();
private readonly object _lock = new object();
public Future(Func<T> f)
{
_f = f;
Monitor.Enter(_lock);
ThreadPool.QueueUserWorkItem(Run);
}
public void OnSuccess(Action<T> a)
{
_callbacks.Push(a);
if (_complete && _success)
a(_result);
}
public void OnError(Action<Exception> a)
{
_errbacks.Push(a);
if (_complete && !_success)
a(_error);
}
private void Run(object state)
{
try {
_result = _f();
_success = true;
_complete = true;
foreach (var cb in _callbacks) {
cb(_result);
}
} catch (Exception e) {
_error = e;
_complete = true;
foreach (var cb in _errbacks) {
cb(e);
}
} finally {
Monitor.Exit(_lock);
}
}
public T Result {
get {
if (!_complete) {
Monitor.Enter(_lock);
}
if (_success) {
return _result;
} else {
Console.WriteLine("Throwing error complete={0} success={1}", _complete, _success);
throw _error;
}
}
}
// Failing test
public void TestResultSuccess() {
var f = new Future<int>(() => 1);
var x = f.Result;
Assert.AreEqual (1, x);
}
I'm using Mono 3.2.3 on Mac OS X 10.9.

Only the thread that took the lock can exit the lock. You can't Enter it in the constructor on the calling thread then Exit from the thread-pool when it completes - the thread-pool worker does not have the lock.
And conversely: presumably it is the same thread that created the future that is accessing the getter: that is allowed to Enter again: it is re-entrant. Also, you need to Exit the same number of times that you Enter, otherwise it isn't actually released.
Basically, I don't think Monitor is the right approach here.

How to implement monitor and system.threading for the smokers situation

The problem that I am working on deals with out to use a functional Lock, or monitor structure, to provided exclusive access to only one member on separate threads. Below, is my class definition of the monitor (note that it is different than the actual monitor class given by c# library). What I am trying to do is make pictureboxes appear or disappear on my form.
I have attempted to add in an instance of the form so I can access the individual pictureboxes, however, my program seems to freeze.
namespace SmokersProblem
{
class monitor
{
Form1 form = new Form1();
Random num = new Random();
object obj = new object();
public monitor()
{
}
public void agent(){
form.pictureBox4.Visible = false;
int r = num.Next(1, 4);
if (r == 1)
{
// put lighter and paper on table
smoker1();
}
else if (r == 2)
{
// put lighter and tobacco on table
smoker2();
}
else
{
// put paper and tobacco on table
smoker3();
}
}
public void smoker1()
{
//lock (obj)
//{
form.pictureBox9.Visible = true;
form.pictureBox1.Visible = false;
System.Threading.Thread.Sleep(5000);
//agent();
// }
}
public void smoker2()
{
//lock (obj)
//{
form.pictureBox10.Visible = true;
form.pictureBox3.Visible = false;
System.Threading.Thread.Sleep(5000);
//agent();
//}
}
public void smoker3()
{
//lock (obj)
//{
form.pictureBox11.Visible = true;
form.pictureBox2.Visible = false;
System.Threading.Thread.Sleep(5000);
//agent();
// }
}
}
}
Below is my form code, as you can see here, i try to create three seperate threads, one for each smoker.
namespace SmokersProblem
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
Random rnd = new Random();
int num = rnd.Next(1, 4);
Object obj = new Object();
}
private void Form1_Load(object sender, EventArgs e)
{
}
private void button1_Click(object sender, EventArgs e)
{
pictureBox1.Visible = true;
pictureBox2.Visible = true;
pictureBox3.Visible = true;
pictureBox8.Visible = false;
pictureBox7.Visible = false;
pictureBox6.Visible = false;
monitor one = new monitor();
one.agent();
Thread vone = new Thread(one.smoker1);
Thread two = new Thread(one.smoker2);
Thread three = new Thread(one.smoker3);
vone.Start();
two.Start();
three.Start();
}
}
}

After implementing this, I went looking for the Smoker Thread Problem that it looks like OP is trying to implement. This code should be easily adaptable to that problem.
The reason your UI is freezing is that you're calling one.agent() without putting it in a new thread. one.agent() sleeps, which keeps your UI from processing events.
OK, I've implemented some code to do the smoker problem with labels. Obviously it could be improved, for instance by not coupling the form to the threads.
I put two different locking mechanisms in, and left one commented out.
Essentially, there are three labels that can either be "Smoking" or "Not Smoking". The main UI thread creates three threads:
Smoker1
Smoker2
Smoker3
Each of the threads continually tries to take the lock in a while loop. When they take the lock, they set their label to "Smoking", wait a few seconds, and then set their label to "Not Smoking". This uses thread safe code from this answer.
public partial class Form1 : Form
{
private bool running = false;
public Label OneLabel { get; set; }
public Label TwoLabel { get; set; }
public Label ThreeLabel { get; set; }
private MyMonitor one;
private Thread vone;
private Thread two;
private Thread three;
public Form1()
{
InitializeComponent();
OneLabel = new Label();
OneLabel.Text = "Not Smoking";
OneLabel.Location = new Point(10, 50);
OneLabel.AutoSize = true;
this.Controls.Add(OneLabel);
TwoLabel = new Label();
TwoLabel.Text = "Not Smoking";
TwoLabel.Location = new Point(150, 50);
this.Controls.Add(TwoLabel);
ThreeLabel = new Label();
ThreeLabel.Text = "Not Smoking";
ThreeLabel.Location = new Point(300, 50);
this.Controls.Add(ThreeLabel);
}
private void MainButton_Click(object sender, EventArgs e)
{
if (!running)
{
vone.Start();
two.Start();
three.Start();
MainButton.Text = "Stop";
running = true;
}
else
{
one.RequestStop();
MainButton.Text = "Run";
running = false;
}
}
private void Form1_Load(object sender, EventArgs e)
{
one = new MyMonitor(this);
vone = new Thread(one.Smoker1);
two = new Thread(one.Smoker2);
three = new Thread(one.Smoker3);
}
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (running)
{
one.RequestStop();
running = false;
}
}
}
class MyMonitor
{
private int x = 1;
private Object obj = new Object();
private Form1 _form;
bool _finished = false;
public MyMonitor(Form1 form)
{
_form = form;
}
public void Smoker1()
{
while (!_finished)
{
//lock (obj)
//{
// _form.OneLabel.SetPropertyThreadSafe(() => _form.OneLabel.Text, "Smoking");
// System.Threading.Thread.Sleep(2000);
// _form.OneLabel.SetPropertyThreadSafe(() => _form.OneLabel.Text, "Not Smoking");
//}
try
{
Monitor.Enter(obj);
try
{
_form.OneLabel.SetPropertyThreadSafe(() => _form.OneLabel.Text, "Smoking");
System.Threading.Thread.Sleep(2000);
_form.OneLabel.SetPropertyThreadSafe(() => _form.OneLabel.Text, "Not Smoking");
}
finally
{
Monitor.Exit(obj);
}
}
catch (SynchronizationLockException SyncEx)
{
Console.WriteLine(SyncEx.Message);
}
}
}
public void Smoker2()
{
while (!_finished)
{
//lock (obj)
//{
// _form.TwoLabel.SetPropertyThreadSafe(() => _form.TwoLabel.Text, "Smoking");
// System.Threading.Thread.Sleep(2000);
// _form.TwoLabel.SetPropertyThreadSafe(() => _form.TwoLabel.Text, "Not Smoking");
//}
try
{
Monitor.Enter(obj);
try
{
_form.TwoLabel.SetPropertyThreadSafe(() => _form.TwoLabel.Text, "Smoking");
System.Threading.Thread.Sleep(2000);
_form.TwoLabel.SetPropertyThreadSafe(() => _form.TwoLabel.Text, "Not Smoking");
}
finally
{
Monitor.Exit(obj);
}
}
catch (SynchronizationLockException SyncEx)
{
Console.WriteLine(SyncEx.Message);
}
}
}
public void Smoker3()
{
while (!_finished)
{
//lock (obj)
//{
// _form.ThreeLabel.SetPropertyThreadSafe(() => _form.ThreeLabel.Text, "Smoking");
// System.Threading.Thread.Sleep(2000);
// _form.ThreeLabel.SetPropertyThreadSafe(() => _form.ThreeLabel.Text, "Not Smoking");
//}
try
{
Monitor.Enter(obj);
try
{
_form.ThreeLabel.SetPropertyThreadSafe(() => _form.ThreeLabel.Text, "Smoking");
System.Threading.Thread.Sleep(2000);
_form.ThreeLabel.SetPropertyThreadSafe(() => _form.ThreeLabel.Text, "Not Smoking");
}
finally
{
Monitor.Exit(obj);
}
}
catch (SynchronizationLockException SyncEx)
{
Console.WriteLine(SyncEx.Message);
}
}
}
public void RequestStop()
{
_finished = true;
}
}
//Thread Safe Extension Method
public static class Extensions
{
private delegate void SetPropertyThreadSafeDelegate<TResult>(Control #this, Expression<Func<TResult>> property, TResult value);
public static void SetPropertyThreadSafe<TResult>(this Control #this, Expression<Func<TResult>> property, TResult value)
{
var propertyInfo = (property.Body as MemberExpression).Member as PropertyInfo;
if (propertyInfo == null ||
!#this.GetType().IsSubclassOf(propertyInfo.ReflectedType) ||
#this.GetType().GetProperty(propertyInfo.Name, propertyInfo.PropertyType) == null)
{
throw new ArgumentException("The lambda expression 'property' must reference a valid property on this Control.");
}
if (#this.InvokeRequired)
{
#this.Invoke(new SetPropertyThreadSafeDelegate<TResult>(SetPropertyThreadSafe), new object[] { #this, property, value });
}
else
{
#this.GetType().InvokeMember(propertyInfo.Name, BindingFlags.SetProperty, null, #this, new object[] { value });
}
}
}

my program seems to freeze
My one.agent() is the part of the code that allows one of the smokers
to be called on, so they can smoke. Why wouldnt I want it in the main
code?
Because you shouldn't be using Sleep() from the main UI thread, which is what happens when you call one.agent() from the Button Click event. When Sleep(5000) is hit, you're telling the forms main UI thread to not do ANYTHING for five seconds, thus the freezing you're seeing.
To fix this, you'd need agent() to execute smoker1(), smoker2(), or smoker3() in a separate thread like you're doing down below.
There are several other problems with the code, however, that must also be addressed before you can "fix" your code...
The next problem lies in you creating a new instance of Form1() inside your monitor() class. This instance of Form1() is not the same one that is visible on your screen. Acting upon it is modifying an invisible form that has never even been shown. To act upon the form that is actually visible on your screen would require you to either (a) pass a reference to it into your monitor() class when you create it, or (b) have your monitor() class raise custom events that Form1() subscribes to, again when it creates monitor().
The last problem lies in you attempting to change UI controls from within a thread other than the main UI thread. This will result in a cross-thread exception (unless you've turn this off, which you shouldn't). There are various ways to overcome this problem, the most basic of which involves using delegates and the Invoke() method of the Form/Control to which you are trying to update.

Asynchronously add value to Bindinglist/Cross-threading & Locking issue

I have a BindingList databound to a datgridview. I'm using it to keep track of some real-time prices. The method 'update(Quote quote)' is called multiple times a second by various threads. If the datagridview doesn't contain the Quote, it is added. If it does, the values of the quote are updated. I don't want the same quote to appear in the BindingList (or on the GUI) twice, so I tried to put a lock around the operation that checks whether the value is in the list or not. It doesn't work! What am I doing wrong? I've tried two different ways of locking, and am locking on a String object rather than just an object. The problem is definitely in the BeginInvoke(new MethodInvoker(delegate() { activeQuotes.Insert(0, quote); })); call (which is probably taking some time), but if I make that synchronous, the 'add' method throws a 'cross-threading' error. . . What can I do to avoid the cross-threading error, but ensure that the lock works also??
public BindingList<Quote> activeQuotes = new BindingList<Quote>();
object lockObject = "lockObject";
dataGridViewActive.DataSource = activeQuotes;
public void update(Quote quote)
{
//lock (lockObject)
if(Monitor.TryEnter(lockObject))
{
try
{
if (!activeQuotes.Contains(quote))
{
try
{
activeQuotes.Add(quote);
AddQuote(quote);
}
catch (Exception ex)
{
Console.WriteLine("Datagridview!!!!!!");
}
}
else
{
int index = activeQuotes.IndexOf(quote);
activeQuotes[index].Bid = quote.Bid;
activeQuotes[index].Ask = quote.Ask;
activeQuotes[index].Mid = quote.Mid;
activeQuotes[index].Spread = quote.Spread;
activeQuotes[index].Timestamp = quote.Timestamp;
}
finally
{
Monitor.Exit(lockObject);
}
}
private void AddQuote(Quote quote)
{
if (this.InvokeRequired)
{
BeginInvoke(new MethodInvoker(delegate() { activeQuotes.Insert(0, quote); }));
BeginInvoke(new MethodInvoker(delegate() { dataGridViewActive.Refresh(); }));
BeginInvoke(new MethodInvoker(delegate() { dataGridViewActive.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells); }));
}
else
{
activeQuotes.Add(quote);
dataGridViewActive.Refresh();
dataGridViewActive.AutoResizeColumns (DataGridViewAutoSizeColumnsMode.AllCells);
}
}
I'd appreciate any help at all on this.
Thanks.

this code I wrote before will do
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
BindingListInvoked<Name> names;
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
names = new BindingListInvoked<Name>(dataGridView1);
dataGridView1.DataSource = names;
new Thread(() => names.Add(new Name() { FirstName = "Larry", LastName = "Lan" })).Start();
new Thread(() => names.Add(new Name() { FirstName = "Jessie", LastName = "Feng" })).Start();
}
}
public class BindingListInvoked<T> : BindingList<T>
{
public BindingListInvoked() { }
private ISynchronizeInvoke _invoke;
public BindingListInvoked(ISynchronizeInvoke invoke) { _invoke = invoke; }
public BindingListInvoked(IList<T> items) { this.DataSource = items; }
delegate void ListChangedDelegate(ListChangedEventArgs e);
protected override void OnListChanged(ListChangedEventArgs e)
{
if ((_invoke != null) && (_invoke.InvokeRequired))
{
IAsyncResult ar = _invoke.BeginInvoke(new ListChangedDelegate(base.OnListChanged), new object[] { e });
}
else
{
base.OnListChanged(e);
}
}
public IList<T> DataSource
{
get
{
return this;
}
set
{
if (value != null)
{
this.ClearItems();
RaiseListChangedEvents = false;
foreach (T item in value)
{
this.Add(item);
}
RaiseListChangedEvents = true;
OnListChanged(new ListChangedEventArgs(ListChangedType.Reset, -1));
}
}
}
}
public class Name
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
}

I think you should change your BeginInvoke to just Invoke. You need to get it on the UI thread, not begin an async operation. Otherwise your lock could get released before the BeginInvoke target gets invoked because control is returned immediately upon calling BeginInvoke. Calling Invoke will block that thread on that call until the Invoke target completes then return control back to your thread, which will ensure the lock is kept.
Also, have you considered using a lock block instead of Monitor method calls? It's basically the same thing but prevents you from needing the try/finally. I don't see that you're using any retry or benefit from the TryEnter, but perhaps the code sample doesn't demonstrate that.