如果线程花费的时间太长，如何停止线程

Question

I have a situation that i export data to a file and what i have been asked to do is to provide a cancel button which on click will stop the export if it takes too much time to export.

I started exporting to the file in a thread. And i try to abort the thread on the button click. But it do not work.

I searched on Google and i found that abort() is not recommended. But what else should I choose to achieve it?

My current code is:

private void ExportButtonClick(object param)
{
    IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
    DataTable dtData = ExportHelper.ToDataTable(data);
    thread = new Thread(new ThreadStart(()=>ExportHelper.DataTableToCsv(dtData, "ExportFile.csv")));
    thread.SetApartmentState(ApartmentState.STA);
    thread.IsBackground = true;
    thread.Name = "PDF";
    thread.Start();
}

private void StopButtonClick(object param)
{
    if (thread.Name == "PDF")
    {
        thread.Interrupt();
        thread.Abort();
    }
}

Answer 1

Aborting a thread is a bad idea, especially when dealing with files. You won't have a chance to clean up half-written files or clean-up inconsistent state.

It won't harm the .NET Runtime bat it can hurt your own application eg if the worker method leaves global state, files or database records in an inconsistent state.

It's always preferable to use cooperative cancellation - the thread periodically checks a coordination construct like a ManualResetEvent or CancellationToken . You can't use a simple variable like a Boolean flag, as this can lead to race conditions, eg if two or more threads try to set it at the same time.

You can read about cancellation in .NET in the Cancellation in Managed Threads section of MSDN.

The CancellationToken/CancellationTokenSource classes were added in .NET 4 to make cancellation easier that passing around events.

In your case, you should modify your DataTableToCsv to accept a CancellationToken . That token is generated by a CancellationTokenSource class.

When you call CancellationTokenSource.Cancel the token's IsCancellationRequested property becomes true. Your DataTableToCsv method should check this flag periodically. If it's set, it should exit any loops, delete any inconsistent files etc.

Timeouts are directly supported with CancelAfter . Essentially, CancelAfter starts a timer that will fire Cancel when it expires.

Your code could look like this:

CancellationTokenSource _exportCts = null;

private void ExportButtonClick(object param)
{
    IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
    DataTable dtData = ExportHelper.ToDataTable(data);

    _exportCts=new CancellationTokenSource();
    var token=_exportCts.Token;

    thread = new Thread(new ThreadStart(()=>
            ExportHelper.DataTableToCsv(dtData, "ExportFile.csv",token)));
    thread.SetApartmentState(ApartmentState.STA);
    thread.IsBackground = true;
    thread.Name = "PDF";

    _exportCts.CancelAfter(10000);
    thread.Start();

}


private void StopButtonClick(object param)
{
    if (_exportCts!=null)
    {
        _exportCts.Cancel();
    }
}

DataTableToCsv should contain code similar to this:

foreach(var row in myTable)
{
    if (token.IsCancellationRequested)
    {
        break;
    }
    //else continue with processing
    var line=String.Join(",", row.ItemArray);
    writer.WriteLine(line);

}

You can clean up your code quite a bit by using tasks instead of raw threads:

private async void ExportButtonClick(object param)
{
    IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
    DataTable dtData = ExportHelper.ToDataTable(data);

    _exportCts=new CancellationTokenSource();
    var token=_exportCts.Token;

    _exportCts.CancelAfter(10000);
    await Task.Run(()=> ExportHelper.DataTableToCsv(dtData, "ExportFile.csv",token)));
    MessageBox.Show("Finished");
}

You could also speed it up by using asynchronous operations, eg to read data from the database or write to text files without blocking or using threads . Windows IO (both file and network) is asynchronous at the driver level. Methods like File.WriteLineAsync don't use threads to write to a file.

Your Export button handler could become :

private void ExportButtonClick(object param)
{
    IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
    DataTable dtData = ExportHelper.ToDataTable(data);

    _exportCts=new CancellationTokenSource();
    var token=_exportCts.Token;

    _exportCts.CancelAfter(10000);
    await Task.Run(async ()=> ExportHelper.DataTableToCsv(dtData, "ExportFile.csv",token)));
    MessageBox.Show("Finished");
}

and DataTableToCsv :

public async Task DataTableToCsv(DataTable table, string file,CancellationToken token)
{
...
    foreach(var row in myTable)
    {
        if (token.IsCancellationRequested)
        {
            break;
        }
        //else continue with processing
        var line=String.Join(",", row.ItemArray);
        await writer.WriteLineAsync(line);
    }

Answer 2

You can use a boolean flag. Use a volatile boolean for that.

In the helper do something like:

 this.aborted = false;
 while(!finished && !aborted) {
      //process one row
 }

Whenever you want to cancel the operation, you call a method to set aborted to true:

 public void Abort() {
     this.aborted = true;
 }

Answer 3

Have a read here: https://msdn.microsoft.com/en-us/library/system.threading.threadabortexception(v=vs.110).aspx

When a call is made to the Abort method to destroy a thread, the common language runtime throws a ThreadAbortException. ThreadAbortException is a special exception that can be caught, but it will automatically be raised again at the end of the catch block. When this exception is raised, the runtime executes all the finally blocks before ending the thread. Because the thread can do an unbounded computation in the finally blocks or call Thread.ResetAbort to cancel the abort, there is no guarantee that the thread will ever end. If you want to wait until the aborted thread has ended, you can call the Thread.Join method. Join is a blocking call that does not return until the thread actually stops executing.

Since Thread.Abort() is executed by another thread, it can happen anytime and when it happens ThreadAbortException is thrown on target thread.

Inside ExportHelper.DataTableToCsv:

catch(ThreadAbortException e) {
    Thread.ResetAbort();
}

On StopButtonClick

if (thread.Name == "PDF")
{
    thread.Interrupt();
    thread.Join();
}

Answer 4

To Stop a thread you have one option of Thread.Abort .However because this method thrown ThreadAbortException on the target thread when it executed by another thead. Which is not recommended. The second option to stop a thread is by using shared variable that both your target and your calling thread can access. See the Example ::

public static class Program
{
    public static void ThreadMethod(object o)
    {
        for (int i = 0; i < (int)o; i++)
        {
            Console.WriteLine("ThreadProc: { 0}", i);
            Thread.Sleep(0);
        }
    }
    public static void Main()
    {
        bool stopped = false;
        Thread t = new Thread(new ThreadStart(() =>
        {
            while (!stopped)
            {
                Console.WriteLine("Running...");
                Thread.Sleep(1000);
            }
        }));
        t.Start();
        Console.WriteLine("Press any key to exit");
        Console.ReadKey();
        stopped = true;
        t.Join();
    }
}

//Source :: Book --> Programming in c#