Run once a day in C#

I achieved this by doing the following...

1. Set up a timer that fires every 20 minutes (although the actual timing is up to you - I needed to run on several occasions throughout the day).
2. on each Tick event, check the system time. Compare the time to the scheduled run time for your method.
3. If the current time is less than the scheduled time, check a in some persistent storage to get the datetime value of the last time the method ran.
4. If the method last ran more than 24 hours ago, run the method, and stash the datetime of this run back to your data store
5. If the method last ran within the last 24 hours, ignore it.

HTH

*edit - code sample in C# :: Note : untested...

using System;
using System.Collections.Generic;
using System.Text;
using System.Timers;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Timer t1 = new Timer();
            t1.Interval = (1000 * 60 * 20); // 20 minutes...
            t1.Elapsed += new ElapsedEventHandler(t1_Elapsed);
            t1.AutoReset = true;
            t1.Start();

            Console.ReadLine();
        }

        static void t1_Elapsed(object sender, ElapsedEventArgs e)
        {
            DateTime scheduledRun = DateTime.Today.AddHours(3);  // runs today at 3am.
            System.IO.FileInfo lastTime = new System.IO.FileInfo(@"C:\lastRunTime.txt");
            DateTime lastRan = lastTime.LastWriteTime;
            if (DateTime.Now > scheduledRun)
            {
                TimeSpan sinceLastRun = DateTime.Now - lastRan;
                if (sinceLastRun.Hours > 23)
                {
                    doStuff();
                    // Don't forget to update the file modification date here!!!
                }
            }
        }

        static void doStuff()
        {
            Console.WriteLine("Running the method!");
        }
    }
}

Take a look at quartz.net. It is a scheduling library for .net.

More specifically take a look here.

If the time when it is run is not relevant and can be reset each time the program starts you can just set a timer, which is the easiest thing to do. If that's not acceptable it starts getting more complex, like the solution presented here and which still doesn't solve the persistence problem, you need to tackle that separately if you truly wish to do what Scheduled Tasks would. I'd really consider again if it's worth going through all the trouble to replicate a perfectly good existing functionality.

Here's a related question (Example taken from there).

using System;
using System.Timers;

public class Timer1
{
    private static Timer aTimer = new System.Timers.Timer(24*60*60*1000);

    public static void Main()
    {
        aTimer.Elapsed += new ElapsedEventHandler(ExecuteEveryDayMethod);
        aTimer.Enabled = true;

        Console.WriteLine("Press the Enter key to exit the program.");
        Console.ReadLine();
    }

    // Specify what you want to happen when the Elapsed event is 
    // raised.
    private static void ExecuteEveryDayMethod(object source, ElapsedEventArgs e)
    {
        Console.WriteLine("The Elapsed event was raised at {0}", e.SignalTime);
    }
}