windows forms, async task exception catching - asynchronous

Question

I have a form, 2 buttons , 1 textbox. Button 1 processes the TaskException_click.
What I wanted to do is understanding the async task/void difference. But checking multiple examples I still do not understand or get it to work. Below my code.
When I click the taskexception button, the unobservedtaskexception is not executed (I expected that).
When I click it another time, the event is executed with the exception of the first click. However the UI is not updated (actually it hangs). Would like to know what I am doing wrong.
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace test
{
public partial class main : Form
{
public main()
{
InitializeComponent();
TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;
}
void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
{
//textBox1.Text = "Unobserved Exception caught ";
e.SetObserved();
if (this.InvokeRequired)
{
this.Invoke((MethodInvoker)delegate()
{
//codes to do whatever i wan to do with the GUI
//Examples of it would be disposing a flowlayout panel
//and re-adding it back and populating it again to
//show the refreshed values.
textBox1.Text = "Unobserved Exception caught " + e.Exception.Message;
});
}
else
{
textBox1.Text = "Unobserved Exception caught " + e.Exception.Message;
}
}
private int i = 0;
// Add async here! You can always add these to events
private async void TaskException_Click(object sender, EventArgs e)
{
textBox1.Text = "";
try
{
Task t = TaskThrowAnException();
textBox1.Text = "done";
t = null;
Thread.Sleep(100);
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
}
catch (Exception ex)
{
textBox1.Text = "Exception caught";
}
}
private async Task TaskThrowAnException()
{
//await Task.Delay(1000);
i++;
throw new Exception("Task" + i.ToString());
}
private async void VoidException_Click(object sender, EventArgs e)
{
textBox1.Text = "";
try
{
VoidThrowAnException();
textBox1.Text = "done";
}
catch (Exception ex )
{
textBox1.Text = "Exception caught";
}
}
private async void VoidThrowAnException()
{
//await Task.Delay(1000);
throw new Exception("Void");
}
}
}

Answer 1

For the TaskException case the exception is stored in the Task this is the expected behavior for async methods returning Task. If you want the exception to be thrown you need to observe the exception by awaiting the Task or calling Result or Wait() on the Task.
If the exception is unobserved it should get thrown when the Task is finalized, the only thing I can conclude is that somehow the task is not being finalized when you call
Thread.Sleep(100);
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
I am nor sure why the local variable is not GCed, if add another button and you put the code above in the button handler (e.g Clear_Click) everything works as expected. I thought the generated code must somehow have a link to the task variable but I couldn't find any link.
Here is the generated code from Reflector:
namespace WindowsFormsApplication1
{
using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Drawing;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
public class Form1 : Form
{
private IContainer components = null;
private int i = 0;
private Button TaskException;
private TextBox textBox1;
private Button VoidException;
public Form1()
{
this.InitializeComponent();
TaskScheduler.UnobservedTaskException += new EventHandler<UnobservedTaskExceptionEventArgs>(this.TaskScheduler_UnobservedTaskException);
}
protected override void Dispose(bool disposing)
{
if (disposing && (this.components != null))
{
this.components.Dispose();
}
base.Dispose(disposing);
}
private void InitializeComponent()
{
this.textBox1 = new TextBox();
this.TaskException = new Button();
this.VoidException = new Button();
base.SuspendLayout();
this.textBox1.Location = new Point(13, 13);
this.textBox1.Multiline = true;
this.textBox1.Name = "textBox1";
this.textBox1.Size = new Size(0x20d, 0x13f);
this.textBox1.TabIndex = 0;
this.TaskException.Location = new Point(13, 0x16d);
this.TaskException.Name = "TaskException";
this.TaskException.Size = new Size(0x9b, 0x17);
this.TaskException.TabIndex = 1;
this.TaskException.Text = "TaskException";
this.TaskException.UseVisualStyleBackColor = true;
this.TaskException.Click += new EventHandler(this.TaskException_Click);
this.VoidException.Location = new Point(0xda, 0x16d);
this.VoidException.Name = "VoidException";
this.VoidException.Size = new Size(0xab, 0x17);
this.VoidException.TabIndex = 2;
this.VoidException.Text = "VoidException";
this.VoidException.UseVisualStyleBackColor = true;
this.VoidException.Click += new EventHandler(this.VoidException_Click);
base.AutoScaleDimensions = new SizeF(6f, 13f);
base.AutoScaleMode = AutoScaleMode.Font;
base.ClientSize = new Size(550, 430);
base.Controls.Add(this.VoidException);
base.Controls.Add(this.TaskException);
base.Controls.Add(this.textBox1);
base.Name = "Form1";
this.Text = "Form1";
base.ResumeLayout(false);
base.PerformLayout();
}
[DebuggerStepThrough, AsyncStateMachine(typeof(<TaskException_Click>d__4))]
private void TaskException_Click(object sender, EventArgs e)
{
<TaskException_Click>d__4 d__;
d__.<>4__this = this;
d__.sender = sender;
d__.e = e;
d__.<>t__builder = AsyncVoidMethodBuilder.Create();
d__.<>1__state = -1;
d__.<>t__builder.Start<<TaskException_Click>d__4>(ref d__);
}
private void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
{
MethodInvoker method = null;
e.SetObserved();
if (base.InvokeRequired)
{
if (method == null)
{
method = (MethodInvoker) (() => (this.textBox1.Text = "Unobserved Exception caught " + e.Exception.Message));
}
base.BeginInvoke(method);
}
else
{
this.textBox1.Text = "Unobserved Exception caught " + e.Exception.Message;
}
}
[AsyncStateMachine(typeof(<TaskThrowAnException>d__6)), DebuggerStepThrough]
private Task TaskThrowAnException()
{
<TaskThrowAnException>d__6 d__;
d__.<>4__this = this;
d__.<>t__builder = AsyncTaskMethodBuilder.Create();
d__.<>1__state = -1;
d__.<>t__builder.Start<<TaskThrowAnException>d__6>(ref d__);
return d__.<>t__builder.Task;
}
[DebuggerStepThrough, AsyncStateMachine(typeof(<VoidException_Click>d__8))]
private void VoidException_Click(object sender, EventArgs e)
{
<VoidException_Click>d__8 d__;
d__.<>4__this = this;
d__.sender = sender;
d__.e = e;
d__.<>t__builder = AsyncVoidMethodBuilder.Create();
d__.<>1__state = -1;
d__.<>t__builder.Start<<VoidException_Click>d__8>(ref d__);
}
[AsyncStateMachine(typeof(<VoidThrowAnException>d__a)), DebuggerStepThrough]
private void VoidThrowAnException()
{
<VoidThrowAnException>d__a _a;
_a.<>4__this = this;
_a.<>t__builder = AsyncVoidMethodBuilder.Create();
_a.<>1__state = -1;
_a.<>t__builder.Start<<VoidThrowAnException>d__a>(ref _a);
}
[CompilerGenerated]
private struct <TaskException_Click>d__4 : IAsyncStateMachine
{
public int <>1__state;
public Form1 <>4__this;
public AsyncVoidMethodBuilder <>t__builder;
public EventArgs e;
public object sender;
private void MoveNext()
{
try
{
if (this.<>1__state != -3)
{
this.<>4__this.textBox1.Text = "";
try
{
Task task = this.<>4__this.TaskThrowAnException();
this.<>4__this.textBox1.Text = "done";
task = null;
}
catch (Exception)
{
this.<>4__this.textBox1.Text = "Exception caught";
}
Thread.Sleep(100);
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
}
}
catch (Exception exception2)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception2);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
[DebuggerHidden]
private void SetStateMachine(IAsyncStateMachine param0)
{
this.<>t__builder.SetStateMachine(param0);
}
}
[CompilerGenerated]
private struct <TaskThrowAnException>d__6 : IAsyncStateMachine
{
public int <>1__state;
public Form1 <>4__this;
public AsyncTaskMethodBuilder <>t__builder;
private void MoveNext()
{
try
{
if (this.<>1__state != -3)
{
this.<>4__this.i++;
throw new Exception("Task" + this.<>4__this.i.ToString());
}
}
catch (Exception exception)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
[DebuggerHidden]
private void SetStateMachine(IAsyncStateMachine param0)
{
this.<>t__builder.SetStateMachine(param0);
}
}
[CompilerGenerated]
private struct <VoidException_Click>d__8 : IAsyncStateMachine
{
public int <>1__state;
public Form1 <>4__this;
public AsyncVoidMethodBuilder <>t__builder;
public EventArgs e;
public object sender;
private void MoveNext()
{
try
{
if (this.<>1__state != -3)
{
this.<>4__this.textBox1.Text = "";
try
{
this.<>4__this.VoidThrowAnException();
this.<>4__this.textBox1.Text = "done";
}
catch (Exception)
{
this.<>4__this.textBox1.Text = "Exception caught";
}
}
}
catch (Exception exception2)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception2);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
[DebuggerHidden]
private void SetStateMachine(IAsyncStateMachine param0)
{
this.<>t__builder.SetStateMachine(param0);
}
}
[CompilerGenerated]
private struct <VoidThrowAnException>d__a : IAsyncStateMachine
{
public int <>1__state;
public Form1 <>4__this;
public AsyncVoidMethodBuilder <>t__builder;
private void MoveNext()
{
try
{
if (this.<>1__state != -3)
{
SynchronizationContext current = SynchronizationContext.Current;
throw new Exception("Void");
}
}
catch (Exception exception)
{
this.<>1__state = -2;
this.<>t__builder.SetException(exception);
return;
}
this.<>1__state = -2;
this.<>t__builder.SetResult();
}
[DebuggerHidden]
private void SetStateMachine(IAsyncStateMachine param0)
{
this.<>t__builder.SetStateMachine(param0);
}
}
}
}