How to chain tasks within each thread - c#-4.0

Question

I have a requirement for copying several hundred tables using bulkcopy.
So far I got the following code.
// from UI main thread
private void test1()
{
LimitedTaskScheduler scheduler = new LimitedTaskScheduler(4);
TaskFactory factory = new TaskFactory(scheduler);
foreach (DataRow row in tabList.Rows) // Loop over the rows.
{
string tabName = row[tabList.Columns["TableName"]].ToString();
factory.StartNew<string>( () =>
{
Console.WriteLine("{0} on thread {1}", tabName, Thread.CurrentThread.ManagedThreadId);
TableCopyer tc1 = new TableCopyer();
// pass progressbar and label into tc1 instance to update the progressbar and label
tc1.Bar1 = bar1;
tc1.L1 = l1;
tc1.EntityCopy(AppInfo.SrcDBMSPath, AppInfo.DestDBMSPath, tabName, "");
return tabName;
});
}
}
// inside TableCopier class
private void OnSqlRowsCopied(object sender, SqlRowsCopiedEventArgs e)
{
try
{
int rowCopied = int.Parse(e.RowsCopied.ToString());
double result = (double.Parse(rowCopied.ToString()) / double.Parse(TotalRows.ToString()));
int prsent = int.Parse(Math.Round((result * 100), 0).ToString());
SetProgressbar(prsent);
}
catch { throw; }
}
LimitedTaskScheduler is coming from
http://msdn.microsoft.com/en-us/library/ee789351.aspx
My questions are
How do I chain the tasks so that each will start only upon the finish of the prior task within each thread? I limited the max parallel thread to 4. I want to chain the tasks within the threads because the tasks being performed within a thread will reuse the same progressbar in a way one finishes and another starts.
I have 4 progress bar painted on UI, bar1, bar2, bar3, bar4. How do I associate each progressbar with a particular thread so that the user can see 4 running progress at the same time?

Answer 1

I think it doesn't make much sense to tie yourself into thinking about specific threads. Instead, what you want is that your tasks run concurrently at most 4 at a time and you want to use your 4 progress bars from those tasks.
What you could do is to put those progress bars into a collection, and then take a progress bar from there at the start of each task and return it back there at the end of the task.
You have to be careful about thread-safety here, so you should either use locks when working with the collection of progress bars, or use a thread-safe collection like ConcurrentQueue. Also, updating the progress bar usually has to be done from the UI thread, so you will have to take care of that too.

Answer 2

How do I chain the tasks so that each will start only upon the finish
of the prior task within each thread? I limited the max parallel
thread to 4. I want to chain the tasks within the threads because the
tasks being performed within a thread will reuse the same progressbar
in a way one finishes and another starts.
You want to chain your tasks together. So essentially there is no parallel processing occurring, except that you wish to process this on a thread separate to the UI. So, why then do you want to spawn a new task for each iteration of your loop if you wan't your loop processing to run sequentially? You could just wrap a normal for loop in a task:
private void test1()
{
Task.Factory.StartNew(() =>
{
foreach (DataRow row in tabList.Rows) // Loop over the rows.
{
string tabName = row[tabList.Columns["TableName"]].ToString();
Console.WriteLine("{0} on thread {1}", tabName, Thread.CurrentThread.ManagedThreadId);
TableCopyer tc1 = new TableCopyer();
// pass progressbar and label into tc1 instance to update the progressbar and label
tc1.Bar1 = bar1;
tc1.L1 = l1;
tc1.EntityCopy(AppInfo.SrcDBMSPath, AppInfo.DestDBMSPath, tabName, "");
}
});
}
I have 4 progress bar painted on UI, bar1, bar2, bar3, bar4. How do I
associate each progressbar with a particular thread so that the user
can see 4 running progress at the same time?
There are many ways to do this and it really depends on your requirements as to which is the best. You could reference the progress bars in a Queue<T>, but only if you are convinced that you will have as many progress bars as you have table rows.
private void test1()
{
Task.Factory.StartNew(() =>
{
var progressBars = new Queue<ProgressBar>(new List<ProgressBar> { bar1, bar2, bar3, bar4 });
foreach (DataRow row in tabList.Rows) // Loop over the rows.
{
// ...
if(progressBars.Count() == 0}
throw new Exception("Not enough progress bars setup!");
tc1.Bar1 = progressBars.Dequeue();
// ...
}
});
}
If tabList.Rows is an IEnumerable, then you can ensure that the queue won't break by using Take():
foreach (DataRow row in tabList.Rows.Take(progressBars.Count()))
{ ... }