Execute specified function every X seconds

I have a Windows Forms application written in C#. The following function checks whenever printer is online or not:

public void isonline()
{
    PrinterSettings settings = new PrinterSettings();
    if (CheckPrinter(settings.PrinterName) == "offline")
    {
        pictureBox1.Image = pictureBox1.ErrorImage;
    }
}

and updates the image if the printer is offline. Now, how can I execute this function isonline() every 2 seconds so when I unplug the printer, the image displayed on the form (pictureBox1) turns into another one without relaunching the application or doing a manual check? (eg. by pressing "Refresh" button which runs the isonline() function)

Use System.Windows.Forms.Timer.

private Timer timer1; 
public void InitTimer()
{
    timer1 = new Timer();
    timer1.Tick += new EventHandler(timer1_Tick);
    timer1.Interval = 2000; // in miliseconds
    timer1.Start();
}

private void timer1_Tick(object sender, EventArgs e)
{
    isonline();
}

You can call InitTimer() in Form1_Load().

The most beginner-friendly solution is:

Drag a Timer from the Toolbox, give it a Name, set your desired Interval, and set "Enabled" to True. Then double-click the Timer and Visual Studio (or whatever you are using) will write the following code for you:

private void wait_Tick(object sender, EventArgs e)
{
    refreshText(); // Add the method you want to call here.
}

No need to worry about pasting it into the wrong code block or something like that.

Threaded:

    /// <summary>
    /// Usage: var timer = SetIntervalThread(DoThis, 1000);
    /// UI Usage: BeginInvoke((Action)(() =>{ SetIntervalThread(DoThis, 1000); }));
    /// </summary>
    /// <returns>Returns a timer object which can be disposed.</returns>
    public static System.Threading.Timer SetIntervalThread(Action Act, int Interval)
    {
        TimerStateManager state = new TimerStateManager();
        System.Threading.Timer tmr = new System.Threading.Timer(new TimerCallback(_ => Act()), state, Interval, Interval);
        state.TimerObject = tmr;
        return tmr;
    }

Regular

    /// <summary>
    /// Usage: var timer = SetInterval(DoThis, 1000);
    /// UI Usage: BeginInvoke((Action)(() =>{ SetInterval(DoThis, 1000); }));
    /// </summary>
    /// <returns>Returns a timer object which can be stopped and disposed.</returns>
    public static System.Timers.Timer SetInterval(Action Act, int Interval)
    {
        System.Timers.Timer tmr = new System.Timers.Timer();
        tmr.Elapsed += (sender, args) => Act();
        tmr.AutoReset = true;
        tmr.Interval = Interval;
        tmr.Start();

        return tmr;
    }

.NET 6 has added the PeriodicTimer class.

var periodicTimer= new PeriodicTimer(TimeSpan.FromSeconds(1));
while (await periodicTimer.WaitForNextTickAsync())
{
    // Place function in here..
    Console.WriteLine("Printing");
}

You can run it in the background with this:

async Task RunInBackground(TimeSpan timeSpan, Action action)
{
    var periodicTimer = new PeriodicTimer(timeSpan);
    while (await periodicTimer.WaitForNextTickAsync())
    {
        action();
    }
}

RunInBackground(TimeSpan.FromSeconds(1), () => Console.WriteLine("Printing"));

Things have changed a lot with time. You can use the solution below:

static void Main(string[] args)
{
    var timer = new Timer(Callback, null, 0, 2000);

    //Dispose the timer
    timer.Dispose();
}
static void Callback(object? state)
{
    //Your code here.
}

You can do this easily by adding a Timer to your form (from the designer) and setting it's Tick-function to run your isonline-function.

using System;
using System.Timers;
namespace SnirElgabsi
{
  class Program
  {
     private static Timer timer1;
     static void Main(string[] args)
     {
         timer1 = new Timer(); //new Timer(1000);
         timer1.Elpased += (sender,e) =>
         {
            MyFoo();
         }
         timer1.Interval = 1000;//miliseconds
         timer1.Start();
       
         Console.WriteLine("press any key to stop");
         Console.ReadKey();
     }

     private static void MyFoo()
     {
         Console.WriteLine(string.Format("{0}", DateTime.Now));
     }
  }
}