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Joining a worker thread in a windows forms app

My program works like this:
I press a radio button which opens the port.
Next i press a button "Read" which starts a thread that reads data continously from the Serial Port using port.ReadLine() and prints it in a textbox;
I have another radio which should first join the thread and after that close the port;the problem is the printing goes well until i close the port when the UI freezes.
public Form1()
{
mythread = new Thread(ReadFct);
myPort = new SerialPort("COM3", 9600);
myPort.ReadTimeout = 3500;
InitializeComponent();
foreach (var t in Constants.ComboParameters)
this.paramCombo.Items.Add(t);
radioClose.CheckedChanged += new EventHandler(radioButtonCheckedChanged);
radioOpen.CheckedChanged += new EventHandler(radioButtonCheckedChanged);
}
Below is the function attached to the thread
void ReadFct()
{
string aux = "";
while (readCondition)
{
if (myPort.IsOpen)
aux = myPort.ReadLine();
this.SetText(aux);
}
}
Below is the radio button event handler
public void radioButtonCheckedChanged(object sender,EventArgs e)
{
if (radioOpen.Checked && !myPort.IsOpen)
try
{
myPort.Open();
mythread.Start();
}
catch (Exception)
{
MessageBox.Show("Nu s-a putut deschide port-ul");
}
if (radioClose.Checked && myPort.IsOpen)
{
readCondition = false;
mythread.Join();
myPort.Close();
// myPort.DataReceived -= DataReceivedHandler;
}
}
The read button function:
private void readbtn_Click(object sender, EventArgs e)
{
if (!myPort.IsOpen)
MessageBox.Show("PORT NOT OPENED!");
else
{
// myPort.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);
readCondition = true;
if (!mythread.IsAlive)
{
mythread = new Thread(ReadFct);
mythread.Start();
}
}
I have used what MSDN suggest when changing control from another thread:
private void SetText(string text)
{
if (this.textBox1.InvokeRequired)
{
StringTb del = new StringTb(SetText);
this.Invoke(del, new object[] { text });
}
else
SetData = text;
}

It's hard to know exactly what you need, lacking a good Minimal, Complete, and Verifiable code example to illustrate the question. That said, the issue here is that the Thread.Join() method causes that thread to stop doing any other work, and the thread you use to call that method is the thread that handles all of the user interface. Worse, if your port never receives another newline, the thread you're waiting on will never terminate, because you're stuck waiting on the ReadLine() method. Even worse, even if you do get a newline, if that happens while you're stuck waiting on the Thread.Join(), the call to Invoke() will deadlock, because it needs the UI thread to do its work, and the Thread.Join() call is preventing it from getting the UI thread.
In other words, your code has multiple problems, any one of which could cause problems, but all of which together mean it just can't possibly work.
There are a variety of strategies to fix this, but IMHO the best is to use await. The first step in doing that is to change your I/O handling so that it's done asynchronously instead of dedicating a thread to it:
// Ideally, you should rename this method to "ReadFctAsync". I am leaving
// all names intact for the same of the example though.
async Task ReadFct()
{
string aux = "";
using (StreamReader reader = new StreamReader(myPort.BaseStream))
{
while (true)
{
aux = await reader.ReadLineAsync();
// This will automatically work, because the "await" will automatically
// resume the method execution in the UI thread where you need it.
this.SetText(aux);
}
}
}
Then, instead of creating a thread explicitly, just create a Task object by calling the above:
public Form1()
{
// In this approach, you can get rid of the "mythread" field altogether
myPort = new SerialPort("COM3", 9600);
myPort.ReadTimeout = 3500;
InitializeComponent();
foreach (var t in Constants.ComboParameters)
this.paramCombo.Items.Add(t);
radioClose.CheckedChanged += new EventHandler(radioButtonCheckedChanged);
radioOpen.CheckedChanged += new EventHandler(radioButtonCheckedChanged);
}
public async void radioButtonCheckedChanged(object sender,EventArgs e)
{
if (radioOpen.Checked && !myPort.IsOpen)
{
try
{
myPort.Open();
await ReadFct();
// Execution of this method will resume after the ReadFct() task
// has completed. Which it will do only on throwing an exception.
// This code doesn't have any continuation after the "await", except
// to handle that exception.
}
catch (Exception)
{
// This block will catch the exception thrown when the port is
// closed. NOTE: you should not catch "Exception". Figure out what
// *specific* exceptions you expect to happen and which you can
// handle gracefully. Any other exception can mean big trouble,
// and doing anything other than logging and terminating the process
// can lead to data corruption or other undesirable behavior from
// the program.
MessageBox.Show("Nu s-a putut deschide port-ul");
}
// Return here. We don't want the rest of the code executing after the
// continuation, because the radio button state might have changed
// by then, and we really only want this call to do work for the button
// that was selected when the method was first called. Note that it
// is probably even better if you just break this into two different
// event handlers, one for each button that might be checked.
return;
}
if (radioClose.Checked && myPort.IsOpen)
{
// Closing the port should cause `ReadLineAsync()` to throw an
// exception, which will terminate the read loop and the ReadFct()
// task
myPort.Close();
}
}
In the above, I have completely ignored the readbtn_Click() method. Lacking a good MCVE, it's not clear what role that button plays in the overall scheme. You seem to have a radio button group (of two buttons) that control whether the port is open or closed. It is not clear why then you have an additional regular button that is seemingly able to also open the port and start reading, independently of the radio group.
If you want that extra button, it seems to me that all it ought to do is change the radio group state, by checking the "open" radio button. Then let the radio group buttons handle the port state and reading. If you need more specific advice as to how to fully integrate my code example above with your entire UI, you will need to provide more detail, preferably in a new question. That new question must include a good MCVE.

Run .exe processes in parallel and update progress bar in .NET 4.5

I have a dataGridView where I run a Process for each entry
and then update a toolStripProgressBar based on the output from the Process.
I have looked at the following threads,
Run two async tasks in parallel and collect results in .NET 4.5
Progress bar in parallel loop invocation
how to update the progress bar from tasks running concurrently
but I am not sure how to change my current code to something along these lines.
The main difference from these threads, as I see it, is that my computations are done by an outside application,
which I then need to collect the output from.
I guess I have to define each Process as an async task and then somehow collect the output.
For simplicity the processes are equal weighted in the sample code.
private iNumProcesses;
private void RunApps()
{
iNumProcesses = dataGridView1.Rows.Count;
string sPath = .exe application path
for (int i = 0; i < iNumProcesses; i++)
{
string sArgs = dataGridView1.Rows[i]["Arguments"].ToString();
ExecuteProgram(sPath, sArgs);
}
}
private void ExecuteProgram(string sProcessName, string sArgs)
{
using (cmd = new Process())
{
cmd.StartInfo.FileName = sProcessName;
cmd.StartInfo.Arguments = sArgs;
cmd.StartInfo.UseShellExecute = false;
cmd.StartInfo.CreateNoWindow = true;
cmd.StartInfo.ErrorDialog = true;
cmd.StartInfo.RedirectStandardOutput = true;
cmd.StartInfo.RedirectStandardError = true;
cmd.OutputDataReceived += new DataReceivedEventHandler(SortOutputHandler);
cmd.ErrorDataReceived += new DataReceivedEventHandler(SortOutputHandler);
cmd.Start();
cmd.BeginOutputReadLine();
while (!cmd.HasExited) { Application.DoEvents(); }
}
}
private void SortOutputHandler(object sender, DataReceivedEventArgs e)
{
Trace.WriteLine(e.Data);
this.BeginInvoke(new MethodInvoker(() =>
{
if (e.Data == "Start") { do something... }
else if (e.Data == "Finish") { do something... }
else if (e.Data == "End") { do something... }
else
{
// .exe application output numbers 1 through 100
toolStripProgressBar1.Value += Math.Round(Convert.ToInt32(e.Data)/iNumProcesses,0);
}
}));
}
How can I run the processes in parallel and update the progress bar
based on the output numbers 1 through 100 I get from the .exe applications?
Any advice or suggestions will be greatly appreciated.

My answer to this question is not particularly associated with "C#" and therefore I'm not going to speak directly in those terms: this unexpectedly-thorny issue is actually universal.
The first thing that you must do is to arrange to periodically update the user display. To avoid nasty race-conditions and other problems, you should have "the main thread" perform this task, driven by a millisecond timer. The thread consults a shared array of progress-information and updates all of the progress-bars accordingly. (If you want to use an event to avoid outright "timed waiting," feel free to do so.)
Since each of the launched child processes will have their own input and output streams, and will be writing to those streams asynchronously, you will find it necessary to spawn a "mommy thread" within your application to supervise each child. This thread corresponds to a particular external process, and, in fact, is the one that launches it. The thread continues to exist until it determines that the process that it launched has died.
The "mommy thread" observes the output-stream(s) of its appointed ward to determine its "progress." It updates the progress-variables in the shared array accordingly. (A key element of this design is that each "mommy thread" is able to pay 100% of its attention to just its child. And, because all the "mommies" are threads, they can easily share information with the thread that's updating those progress bars.)
Do you actually have to use semaphores-and-such to coordinate access between the mommy-threads and the main? Depending of course upon the exact internal implementation of that data-structure, the answer just might be, "probably not." (Oh, but it's probably safest to do it anyway.)

C# BackgroundWorker

I have a button that on click event I get some information from the network.
When I get information I parse it and add items to ListBox. All is fine, but when I do a fast double-click on button, it seems that two background workers are running and after finishing all work, items in the list are dublicated.
I want to do so that if you click button and the proccess of getting information is in work, this thread is stopping and only after first work is completed the second one is beginning.
Yes, I know about AutoResetEvent, but when I used it it helped me only one time and never more. I can't implement this situation and hope that you will help me!
Now I even try to make easier but no success :( : I added a flag field(RefreshDialogs)(default false), when the user clicks on button, if flag is true(it means that work is doing), nothing is doing, but when flag field is set to false, all is fine and we start a new proccess.
When Backgroundwork completes, I change field flag to false(it means that user can run a new proccess).
private void Message_Refresh_Click(object sender, EventArgs e)
{
if (!RefreshDialogs)
{
RefreshDialogs = true;
if (threadBackgroundDialogs.WorkerSupportsCancellation)
{
threadBackgroundDialogs.CancelAsync();
}
if (!threadBackgroundDialogs.IsBusy)
{
downloadedDialogs = 0;
threadBackgroundDialogs = new BackgroundWorker();
threadBackgroundDialogs.WorkerSupportsCancellation = true;
threadBackgroundDialogs.DoWork += LoadDialogs;
threadBackgroundDialogs.RunWorkerCompleted += ProcessCompleted;
threadBackgroundDialogs.RunWorkerAsync();
}
}
}
void ProcessCompleted(object sender, RunWorkerCompletedEventArgs e)
{
RefreshDialogs = false;
}

So you want to keep the second process running while the first works, but they shouldn't disturb each other? And after the first one finishes the second one continues?
Crude way: While loop:
if (!RefreshDialogs)
{
RefreshDialogs = true;
this becomes:
while(RefreshDialogs)
{
}
RefreshDialogs = true;
After you set it false the second process wwill jump out of the while. (Note this is extremly inefficent since both processes will be running all the time, i'm pretty sure the second one will block the first one, but with multitasking now it shouldn't, if it block use a Dispatcher.Thread)
Elegant way: Use A Semaphore
http://msdn.microsoft.com/de-de/library/system.threading.semaphore%28v=vs.80%29.aspx
If you find it impossible to have both processes running at the same time, or want another way:
Add an Array/List/int and when the second process notices there is the first process running, like with your bool, increase your Added variable, and at the end of the process, restart the new process and decrese the variable:
int number;
if (!RefreshDialogs)
{
RefreshDialogs = true;
your code;
if(number > 0)
{
number--;
restart process
}
}
else
{
number++;
}
I have to admit, i like my last proposal the most, since its highly efficent.

Make your thread blocking. That is easy;
lock(someSharedGlobalObject)
{
Do Work, Exit early if cancelled
}
This way other threads will wait until the first thread releases the lock. They will never execute simultaneously and silently wait until they can continue.
As for other options; why not disable the button when clicked and re-enable it when the backgroundworker completes. Only problem is this does not allow for cancelling the current thread. The user has to wait for it to finish. It does make any concurrency go away very easily.

How about this approach?
Create a request queue or counter which will be incremented on every button click. Every time that count is > 0. Start the background worker. When the information comes, decrement the count and check for 0. If its still > 0 restart the worker. In that your request handler becomes sequential.
In this approach you may face the problem of continuous reference of the count by two threads, for that you may use a lock unlock condition.
I hav followed this approach for my app and it works well, hope it does the same for you.

I'm not an Windows Phone expert, but as I see it has support for TPL, so following code would read nicely:
private object syncRoot =new object();
private Task latestTask;
public void EnqueueAction(System.Action action)
{
lock (syncRoot)
{
if (latestTask == null)
latestTask = Task.Factory.StartNew(action);
else
latestTask = latestTask.ContinueWith(tsk => action());
}
}

Use can use semaphores
class TheClass
{
static SemaphoreSlim _sem = new SemaphoreSlim (3);
static void Main()
{
for (int i = 1; i <= 5; i++)
new Thread (Enter).Start (i);
}
static void Enter (object name)
{
Console.WriteLine (name + " wants to enter");
_sem.Wait();
Console.WriteLine (name + " has entered!");
Thread.Sleep (1000 * (int) name );
Console.WriteLine (name + " is leaving");
_sem.Release(); }
}
}

I found the solution and thanks to #Giedrius. Flag RefreshingDialogs is set to true only when proccess is at the end, when I added items to Listbox. The reason why I'am using this flag is that state of process changes to complete when the asynchronous operation of getting content from network(HttpWebRequest, method BeginGetRequestStream) begins, but after network operaion is complete I need to make UI operations and not only them(parse content and add it to Listbox)My solution is:
private object syncRoot = new object();
private Task latestTask;
public void EnqueueAction(System.Action action)
{
lock (syncRoot)
{
if (latestTask == null)
{
downloadedDialogs = 0;
latestTask = Task.Factory.StartNew(action);
}
else if(latestTask.IsCompleted && !RefreshingDialogs)
{
RefreshingDialogs = true;
downloadedDialogs = 0;
latestTask = Task.Factory.StartNew(action);
}
}
}
private void Message_Refresh_Click(object sender, EventArgs e)
{
Action ac = new Action(LoadDialogs2);
EnqueueAction(ac);
}

c# - Pass information to BackgroundWorker From UI during execution

I have a c# application that uses a background worker thread, and quite successfully updates the UI from the running thread. The application involves shortest path routing on a network, and I display the network and the shortest path, on the UI, as the background worker proceeds. I would like to allow the user to slow down the display through use of a slider, while the application is running.
I found this as a suggestion, but it is in vb.net, I am not clear on how to get it to work in c#.
How can the BackgroundWorker get values from the UI thread while it is running?
I can pass the value of the slider to the backgroundworker as follows:
// Start the asynchronous operation.
delay = this.trackBar1.Value;
backgroundWorker1.RunWorkerAsync(delay);
and use it within the backgroundworker thread, but it only uses the initially-sent value. I am not clear on how to pick up the value from inside the backgroundworker when I move the slider on the UI.
I have previously used multiple threads and delegates, but if it is possible to utilize the background worker, I would prefer it for its simplicity.
5/10/2012
Thanks to all for your responses. I am still having problems, most likely because of how I have structured things. The heavy duty calculations for network routing are done in the TransportationDelayModel class. BackgroundWorker_DoWork creates an instance of this class, and then kicks it off. The delay is handled in TransportationDelayModel.
The skeleton of code is as follows:
In UI:
private void runToolStripMenuItem1_Click(object sender, EventArgs e)
{
if (sqliteFileName.Equals("Not Set"))
{
MessageBox.Show("Database Name Not Set");
this.chooseDatabaseToolStripMenuItem_Click(sender, e);
}
if (backgroundWorker1.IsBusy != true)
{
// Start the asynchronous operation.
delay = this.trackBar1.Value;
// pass the initial value of delay
backgroundWorker1.RunWorkerAsync(delay);
// preclude multiple runs
runToolStripMenuItem1.Enabled = false;
toolStripButton2.Enabled = false;
}
}
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker worker = sender as BackgroundWorker;
if (!backgroundWorkerLaunched)
{
// instantiate the object that does all the heavy work
TransportationDelayModel TDM = new TransportationDelayModel(worker, e);
// kick it off
TDM.Run(sqliteFileName, worker, e);
backgroundWorkerLaunched = true;
}
}
The TransportationDelayModel constructor is:
public TransportationDelayModel(BackgroundWorker worker, DoWorkEventArgs e)
{
listCentroids = new List<RoadNode>();
listCentroidIDs = new List<int>();
listNodes = new List<RoadNode>();
listNodeIDs = new List<int>();
listRoadLink = new List<RoadLink>();
roadGraph = new AdjacencyGraph<int, RoadLink>(true); // note parallel edges allowed
tdmWorker = worker;
tdmEvent = e;
networkForm = new NetworkForm();
}
so I have the tdmWorker, which allows me to pass information back to the UI.
In the internal calculations in TransportationDelayModel, I sleep for the delay period
if (delay2 > 0)
{
tdmWorker.ReportProgress(-12, zzz);
System.Threading.Thread.Sleep(delay2);
}
so the problem seems to be how to pass an updated slider value from the UI back to the object that is executing in the background worker. I have tried a number of combinations, sort of thrashing around, to no avail, either nothing happens or I get a message about not being allowed to access what is happening on the other thread. I realize that if I were doing all the work in the DoWork event handler, then I should be able to do things as you suggest, but there is too much complexity for that to happen.
Again, thank you for your suggestions and help.
6/2/2012
I have resolved this problem by two methods, but I have some questions. Per my comment to R. Harvey, I have built a simple application. It consists of a form with a run button, a slider, and a rich text box. The run button launches a background worker thread that instantiates an object of class "Model" that does all the work (a simplified surrogate for my TransportationModel). The Model class simply writes 100 lines to the text box, incrementing the number of dots in each line by 1, with a delay between each line based on the setting of the slider, and the slider value at the end of the line, something like this:
....................58
.....................58
......................58
.......................51
........................44
.........................44
The objective of this exercise is to be able to move the slider on the form while the "Model" is running, and get the delay to change (as in above).
My first solution involves the creation of a Globals class, to hold the value of the slider:
class Globals
{
public static int globalDelay;
}
then, in the form, I update this value whenever the trackbar is scrolled:
private void trackBar1_Scroll(object sender, EventArgs e)
{
Globals.globalDelay = this.trackBar1.Value;
}
and in the Model, I just pick up the value of the global:
public void Run(BackgroundWorker worker, DoWorkEventArgs e)
{
for (int i = 1; i < 100; i++)
{
delay = Globals.globalDelay; // revise delay based on static global set on UI
System.Threading.Thread.Sleep(delay);
worker.ReportProgress(i);
string reportString = ".";
for (int k = 0; k < i; k++)
{
reportString += ".";
}
reportString += delay.ToString();
worker.ReportProgress(-1, reportString);
}
}
}
This works just fine.
My question: are there any drawbacks to this approach, which seems very simple to implement and quite general.
The second approach, based on suggestions by R. Harvey, makes use of delegates and invoke.
I create a class for delegates:
public class MyDelegates
{
public delegate int DelegateCheckTrackBarValue(); // create the delegate here
}
in the form, I create:
public int CheckTrackBarValue()
{
return this.trackBar1.Value;
}
and the Model class now has a member m_CheckTrackBarValue
public class Model
{
#region Members
Form1 passedForm;
public static MyDelegates.DelegateCheckTrackBarValue m_CheckTrackBarValue=null;
#endregion Members
#region Constructor
public Model(BackgroundWorker worker, DoWorkEventArgs e, Form1 form)
{
passedForm = form;
}
When the background thread is launched by the run button, the calling form is passed
private void button1_Click(object sender, EventArgs e)
{
if (backgroundWorker1.IsBusy != true)
{
backgroundWorker1.RunWorkerAsync();
}
}
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker worker = sender as BackgroundWorker;
if (!backgroundWorkerLaunched)
{
// instantiate the object that does all the heavy work
Model myModel= new Model(worker, e, this);
Model.m_CheckTrackBarValue = new MyDelegates.DelegateCheckTrackBarValue(this.CheckTrackBarValue);
// kick it off
myModel.Run(worker, e);
backgroundWorkerLaunched = true;
}
}
Finally, in the Model, the Invoke method is called on the passed form to get the value of the trackbar.
public void Run(BackgroundWorker worker, DoWorkEventArgs e)
{
for (int i = 1; i < 100; i++)
{
int delay = (int)passedForm.Invoke(m_CheckTrackBarValue,null); // invoke the method, note need the cast here
System.Threading.Thread.Sleep(delay);
worker.ReportProgress(i);
string reportString = ".";
for (int k = 0; k < i; k++)
{
reportString += ".";
}
reportString += delay.ToString();
worker.ReportProgress(-1, reportString);
}
}
This works as well. I kept getting an error until I made the member variable static, e.g.
public static MyDelegates.DelegateCheckTrackBarValue m_CheckTrackBarValue=null;
My questions on this solution: Are there advantages to this solution as regards to the previous version? Am I making things too complicated in the way I have implemented this? Why does m_CheckTrackBarValue need to be static.
I apologize for the length of this edit, but I thought that the problem and solutions might be of interest to others.

You have to pass the TrackBar object to the BackgroundWorker, not delay. delay doesn't change once you set it.
To simplify the needed Invoke(), you can use a helper method, such as this one:
Async.UI(delegate { textBox1.Text = "This is way easier!"; }, textBox1, true);

I will assume that you are already familiarized with cross-thread invocation to update the UI. So, the solution is very simple: in your worker thread, after each iteration, invoke the UI to get the slider thumb position.

To use a backgroundworker, you add a method to the DoWork property, like this:
this.backgroundWorker1.WorkerSupportsCancellation = true;
this.backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
In the DoWork method, you need to check the variable where the updated delay is set.
This could be an integer field that is available on the containing Form or UI control, or it could be the TrackBar itself.

Running a BackgroundWorker within another BackgroundWorker

I'm attempting a very DB intensive task in a project. Here is a walk-through:
We need to search our DB of workers, we called Locums, and find one for a specific job. This procedure starts when we decide to process x number of jobs. So, on the click of a button, we process using the ProcessJobBatch() method. However, this method only process against a very limited number of Locums. So it takes less then 10 seconds to fill up a scheduler control. Now, once the limited number of Locums are served, we need to run a background task to check the rest of the Locums. There are around 1250 of them!
So, once ProcessJobBatch() finishes, a BackgroundWorker, BackgroundWorkerMoreLocums, goes off. Now, this worker basically does a simple loop: For each job, go through the whole 1250 employees. This takes way too long. I need to plan this out using an alternate strategy that I can't of ATM or I need to show a secondary progress bar for the inner for-each loop.
More Explanation: We import a batch of Jobs (10 to 70) numerous times on daily bases. Once a batch is imported, the application instructs the logged-in user to "Preference Find" those newly created jobs. The user already has a list of his favorite locums (1 to 20). He wants to distribute the jobs among his favorites first. That is done through ProcessJobBatch(). But, there are two scenarios that prevent the flow there and then:
What if certain jobs didn't fall to
any favorite locum?
What if there is a locum in the whole
DB who can do almost all the jobs but
since he isn't favorite?
So, I end up with a scenario of matching a job with each Locum.
Question:
Can second BackgroundWorker run within a BackgroundWorker's DoWork?
Am I doing the second scan wrong?
Environment: Windows 7 Pro 64-bit, Visual Studio 2010, C#, .NET 4.0, and Windows Forms
private void ButtonPreferenceFind_Click(object sender, EventArgs e) {
if (LookUpBatches.EditValue != null) {
JobBatch JobBatchSelected = DbContext.JobBatches.FirstOrDefault(job_batch=> job_batch.OID == LookUpBatches.EditValue.ToString());
if (JobBatchSelected != null && JobBatchSelected.Jobs.Count(condition => condition.JobStatusID == 1) > 0) {
if (XtraMessageBox.Show(String.Format("Are you sure to process {0} job(s)?", JobBatchSelected.Jobs.Count(condition => condition.JobStatusID == 1)), Text, MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes) {
ProcessJobBatch(JobBatchSelected);
IEnumerable<Job> SpecificJobs = from req_jobs in JobBatchSelected.Jobs
where req_jobs.JobStatusID == 1
select req_jobs;
ProgressBarControlPreferenceFinder.EditValue = 0;
ProgressBarControlPreferenceFinder.Properties.Minimum = 0;
ProgressBarControlPreferenceFinder.Properties.Maximum = SpecificJobs.Count() - 1;
BackgroundWorkerMoreLocums.RunWorkerAsync(SpecificJobs);
} else {
LookUpBatches.Focus();
}
} else {
XtraMessageBox.Show("Unable to retrieve the selected batch or the batch has no processable jobs.", Text, MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
LookUpBatches.Focus();
}
} else {
XtraMessageBox.Show("Select a batch first.", Text, MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
LookUpBatches.Focus();
}
}
#region Background Searching
private void BackgroundWorkerMoreLocums_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e) {
try {
e.Result = GetTableData(e.Argument);
}
catch (Exception ex) {
XtraMessageBox.Show("Background Error: " + ex.Message, "Excite Engine 2", MessageBoxButtons.OK, MessageBoxIcon.Error);
e.Result = ex;
}
}
private void BackgroundWorkerMoreLocums_ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e) {
// only display progress, do not assign it to grid
ProgressBarControlPreferenceFinder.Increment(e.ProgressPercentage);
}
private void BackgroundWorkerMoreLocums_RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e) {
if (e.Result is DataTable) {
//dataGridView1.DataSource = e.Result as DataTable;
}
else if (e.Result is Exception) {
}
}
private DataTable GetTableData(Object JobList) {
DataTable ResultDataTable = new DataTable();
ResultDataTable.Columns.Add();
IEnumerable<Job> JobBatchSelected = (IEnumerable<Job>)JobList;
IEnumerable<Locum> LeftOverLocums = from lefties in DbContext.Locums
//where SchedulerMatrixStorage.Resources.Items.Select(res => (long)res.Id).ToList().Contains(lefties.OID) == false
select lefties;
int NumOfJobsProcessed = 0;
List<KeyValuePair<long, TemporaryPreferenceFindLocum>> AlreadyPrefferedLocums = new List<KeyValuePair<long, TemporaryPreferenceFindLocum>>();
foreach (Job oneJob in JobBatchSelected) {
foreach (Locum oneLocum in LeftOverLocums) {
if (DbContext.Availabilities.Any(check => check.LocumID == oneLocum.OID && check.AvailableDate == oneJob.JobDate && check.AvailabilityStatusID == 1)) {
//This Locum can do this job
//Now check if he/she has been just alloted
if (AlreadyPrefferedLocums.Any(search => search.Key == oneLocum.OID && search.Value.JobDate == oneJob.JobDate) == false) {
//No? Cool!
//Add to the list to prevent double allocation
AlreadyPrefferedLocums.Add(new KeyValuePair<long, TemporaryPreferenceFindLocum>(oneJob.OID, new TemporaryPreferenceFindLocum(oneJob.JobDate, oneJob.OID, oneLocum.OID, oneLocum.FirstName + " " + oneLocum.LastName)));
}
else {
continue;
}
}
else {
//Not marked as Avaliable on the required job date...
continue;
}
}
NumOfJobsProcessed++;
BackgroundWorkerMoreLocums.ReportProgress((int)(NumOfJobsProcessed * 100F / (JobBatchSelected.Count() - 1)));
}
return ResultDataTable;
}
#endregion

A BackgroundWorker can be started from within the DoWork handler of another BackgroundWorker, but you need to be aware of the consequences of using such a scheme. When you start a background worker from your main UI thread the DoWork handler is executed on a thread pool thread while the ProgressChanged and RunWorkerCompleted are executed back on the main UI thread making it safe for you to interact with windows forms controls.
This scenario is guaranteed when you start the worker from the main UI thread because it picks up the SynchronizationContext available on that thread and which is initialized by the windows forms infra-structure.
However, when you start a background worker from the DoWork handler of another worker, you'll be starting it from a thread pool thread that lacks the synchronization context causing the ProgressChanged and RunWorkerCompleted handlers to also be executed on thread pool threads and not in your main UI thread making it unsafe for you to interact with windows forms controls from within those handlers.

It is quite common to have one background thread spawn new background threads. I don't think it is a problem if you scan the list on a background thread and process each list item on another thread.
In such cases there is no background worker within another. There is just a background worker starting other threads.
Things you should consider -
Be aware of what you do in the completed event handlers in case you handle that event.
Consider the performance implications of running so many threads for small tasks. You should consider using PLINQ or parallel tasks so that .Net can handle the partitioning of input and merging of results giving you optimum performance.