Exceptions in multithreaded application.

Question

I have heard from a very discerning person that an exception being thrown (and not caught) in a thread is being propagated to the parent thread. Is that true? I have tried some

Answer 1

Here is one way of catching it and handling it in a safe way:

BackgroundWorker bg = new BackgroundWorker();
object o;
bg.DoWork += (c1,c2) =>
{         
        Thread.Sleep(2000);        
        throw new IndexOutOfRangeException();
};

bg.RunWorkerCompleted += (c3,c4) => 
{

 if (((RunWorkerCompletedEventArgs)e).Error != null)
    {
        MessageBox.Show(((RunWorkerCompletedEventArgs)e).Error.Message);
    }
};

Answer 2

No, it won't. In order to catch thread exceptions you need to use Application.ThreadException.

Answer 3

The thread's exception will not propogate to the main thread's context. This really makes sense - by the time the exception is thrown, the main thread will typically be in a completely different scope than the one containing your exception handler.

You can catch these exceptions (typically to log them) by hooking into AppDomain.UnhandledException. See that page for details, including differences in Windows Forms applications, etc.

Answer 4

We can use CustomExceptionHandler for this. Below code might help you.

    using System;
    using System.Threading;
    using System.Windows.Forms;

    // Create a form with a button that, when clicked, throws an exception.
     public class ErrorForm : System.Windows`enter code here`.Forms.Form
     {
        internal Button button1;

        public ErrorForm() : base()
        {
           // Add the button to the form.
           this.button1 = new System.Windows.Forms.Button();
           this.SuspendLayout();
           this.button1.Location = new System.Drawing.Point(100, 43);
           this.button1.Size = new System.Drawing.Size(75, 23);
           this.button1.Text = "Click!";
           this.Controls.Add(this.button1);
           this.button1.Click += new EventHandler(this.button1_Click);

           this.Text = "ThreadException";
           this.ResumeLayout(false);
        }

        // Throw an exception when the button is clicked.
        private void button1_Click(object sender, System.EventArgs e)
        {
           throw new ArgumentException("The parameter was invalid");
        }

        public static void Main()
        {
           // Add the event handler.
           Application.ThreadException += new ThreadExceptionEventHandler(CustomExceptionHandler.OnThreadException);

           // Start the example.
           Application.Run(new ErrorForm());
        }
     }

     // Create a class to handle the exception event.
     internal class CustomExceptionHandler
     {
         // Handle the exception event
        public static void OnThreadException(object sender, ThreadExceptionEventArgs t)
        {
           DialogResult result = ShowThreadExceptionDialog(t.Exception);

           // Exit the program when the user clicks Abort.
           if (result == DialogResult.Abort) 
              Application.Exit();
        }

        // Create and display the error message.
        private static DialogResult ShowThreadExceptionDialog(Exception e)
        {
           string errorMsg = "An error occurred.  Please contact the adminstrator " +
                "with the following information:\n\n";
           errorMsg += String.Format("Exception Type: {0}\n\n", e.GetType().Name);
           errorMsg += "\n\nStack Trace:\n" + e.StackTrace;
           return MessageBox.Show(errorMsg, "Application Error", 
                MessageBoxButtons.AbortRetryIgnore, MessageBoxIcon.Stop);
        }
     }

2nd approach:-

using System;
using System.IO;
using System.Threading.Tasks;

class Example
{
   static async Task Main(string[] args)
   {
      // Get a folder path whose directories should throw an UnauthorizedAccessException.
      string path = Directory.GetParent(
                              Environment.GetFolderPath(
                              Environment.SpecialFolder.UserProfile)).FullName;

      // Use this line to throw UnauthorizedAccessException, which we handle.
      Task<string[]> task1 = Task<string[]>.Factory.StartNew(() => GetAllFiles(path));

      // Use this line to throw an exception that is not handled.
      // Task task1 = Task.Factory.StartNew(() => { throw new IndexOutOfRangeException(); } );
      try {
          await task1;
      }
      catch (AggregateException ae) {
          ae.Handle((x) =>
          {
              if (x is UnauthorizedAccessException) // This we know how to handle.
              {
                  Console.WriteLine("You do not have permission to access all folders in this path.");
                  Console.WriteLine("See your network administrator or try another path.");
                  return true;
              }
              return false; // Let anything else stop the application.
          });
      }

      Console.WriteLine("task1 Status: {0}{1}", task1.IsCompleted ? "Completed," : "", 
                                                task1.Status);
   }

   static string[] GetAllFiles(string str)
   {
      // Should throw an UnauthorizedAccessException exception.
      return System.IO.Directory.GetFiles(str, "*.txt", System.IO.SearchOption.AllDirectories);
   }
}