Timer Tick after FormClosing?

Question

Is there any chance that timer_Tick could be called after myForm_FormClosing in the code below.

If there is a chance: Is it sufficient to call timer.Stop() within myForm_FormClosing in order to avoid that timer_Tick gets called after myForm_FormClosing?

using System;
using System.Windows.Forms;
using System.ComponentModel;

namespace Test
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MyForm());
        }
    }

    class MyForm : Form
    {
        private IContainer components;
        private Timer timer;

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        public MyForm()
        {
            components = new Container();
            timer = new Timer(components);

            timer.Interval = 50;
            timer.Tick += timer_Tick;
            timer.Enabled = true;

            FormClosing += myForm_FormClosing;
        }

        private void timer_Tick(object sender, EventArgs e)
        {
        }

        private void myForm_FormClosing(object sender, FormClosingEventArgs e)
        {
        }
    }
}

Update: After receving a few hints (thanks for helping) I basically have chosen the following code to achive what I want. Please not that timer1_Tick could still be called after myForm_FormClosing was called! This solution just introduces a flag (i called it doWork) which stops the code within timer1_Tick to be executed after myForm_FormClosing was called.

using System;
using System.Windows.Forms;
using System.ComponentModel;

namespace Test
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MyForm());
        }
    }

    class MyForm : Form
    {
        private IContainer components;
        private Timer timer;
        private bool  doWork = true;

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        public MyForm()
        {
            components = new Container();
            timer = new Timer(components);

            timer.Interval = 50;
            timer.Tick += timer_Tick;
            timer.Enabled = true;

            FormClosing += myForm_FormClosing;
        }

        private void timer_Tick(object sender, EventArgs e)
        {
            if (doWork)
            {
                //do the work
            }
        }

        private void myForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            doWork = false;
        }
    }
}

Answer 1

Yes it is possible. According to the docs ...

Occurs before the form is closed.

When a form is closed, it is disposed, releasing all resources associated with the form.

The timer will not be disposed until after the FormClosing event. Here is a very contrived example of how it can happen. You will see that you hit the debugger breakpoint on the Timer tick after FormClosing has been called.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Diagnostics;
using System.Threading.Tasks;
using System.Threading;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private System.Windows.Forms.Timer _time = new System.Windows.Forms.Timer();
        private Task _t1 = null;
        private Task _t2 = null;
        private bool _closingFlag = false;
        public Form1()
        {
            InitializeComponent();

            _time.Interval = 50;
            _time.Tick += (s, e) => { 
                textBox1.Text = DateTime.Now.ToString();
                if (_closingFlag) Debugger.Break();
            };

            _time.Start();
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            _closingFlag = true;

            _t1 = Task.Factory.StartNew(() => { Thread.Sleep(1000); });
            _t2 = Task.Factory.StartNew(() => { Thread.Sleep(1000); });

            Task.WaitAll(_t1, _t2);
        }
    }
}

Answer 2

If you use a the timer System.Windows.Forms.Timer the UI thread must be idle to process a tick from the timer, so from my understanding that's is not possible.

Could happen if you use a timer with its own thread, as a System.Timers.Timer. In this case we could avoid what you mention by implementing something like this:

class MyForm : Form
{
    private System.Timers.Timer timer;

    private Object justForLocking = new Object();
    private Boolean safeToProceed = true;

    [...]

    public MyForm()
    {
        components = new Container();
        timer = new System.Timers.Timer();

        timer.Interval = 50;
        timer.Elapsed += new System.Timers.ElapsedEventHandler(timer_Elapsed);
        timer.Start();

        FormClosing += myForm_FormClosing;
    }

    void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
    {
         // In case that this code is being executed after
         // the method myForm_FormClosing has adquired the 
         // exclusive lock over our dummy resource,
         // the timer thread will wait until the resource if released.
         // Once is released, our control flag will be set to false
         // and the timer should just return and never execute again.
         lock(justForLocking)
         {
             if (safeToProceed)
             {
                 // Do whatever you want to do at this point
             }
         }
    }

    private void myForm_FormClosing(object sender, FormClosingEventArgs e)
    {
         lock(justForLocking)
         {
             safeToProceed = false;
         }
         timer.Stop();

         // Do something else
    }

    [...]
}

This other SO question has relevant information.

EDIT: the code above is only valid if a System.Timers.Timer is used instead of System.Windows.Forms.Timer