如何停止在C＃类中运行的计时器

Question

I have a timer running inside a class. I have two push buttons. Push button 1 is to start the timer(which is inside a class) Push button 2 is to stop the timer forcefully. Timer runs smoothly when click on push button 1 and perform calculations. But when I press Push button 2 the timer does not stopped forcefully and throws an exception

An unhandled exception of type 'System.NullReferenceException' occurred ".

Note:if I use public static System.Timers.Timer theTimer1; then timer could be stopped with push button 2.But I dont want to use static and when if condition will be true then it stopped automatically but stop method is not working with push button. How can I stop this timer with push button. Thanks.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();             
    } 

    Test acc = new Test();

    private void button1_Click(object sender, EventArgs e) 
    {
        acc.SerialPortEvent()
    }

    private void button2_Click(object sender, EventArgs e) 
    {
        acc.stop();
    }
}

class Test
{
    public System.Timers.Timer theTimer1;

    public void SerialPortEvent()
    {
        theTimer1 = new System.Timers.Timer(10000);
        theTimer1.Elapsed += timer_Tick;
        theTimer1.Start();
    }

    void timer_Tick(object sender, EventArgs e)
    {
        //Perform some calculations here.
        if (something like counter)
        {
            stop();
        }
    }

    public void stop()
    {
        theTimer1.Stop();
        theTimer1.Dispose();
    }
}

Answer 1

Initialize the timer in the constructor of Test :

class Test
{
   public Test()
   {
       theTimer1 = new System.Timers.Timer(10000);
   }

Your code initializes the timer only when SerialPortEvent() is called.

This is not called until you press the Start button, hence you get a null reference exception when you press Stop without pressing Start first.

Furthermore your stop() function disposes the timer. This might be an issue when you try to stop the timer after it was disposed.

Answer 2

It is most likely that you're trying to stop your Timer from a different thread

So let's assume that you are in thread 1 and you create a reference to your timer that is instantiated in thread 2. After some time thread 1 has ended and you are in thread 3 where you're trying to reach that reference from thread 1. That's why you got null reference exception.

You should either use static, volatile or Task and Task Cancellation Token.

public volatile System.Timers.Timer theTimer1;