C# how to check exact time difference in if condition

Question

I have this if condition where I need to do some stuff if the difference is 15 min or so

if (DateTime.UtcNow - x.Timestamp == TimeSpan.FromMinutes(15))
{
    //Do something
}

The above condition never satisfies when I check with == but when I do >= it gets into the condition but that's not what I need, I want 15 min or so difference.

Can someone explain this behavior?

Sample Data:

x = 2/17/2020 8:45:17 PM

Answer 1

//I cannot comment on the above answer i dont have enough Rep

I think a better solution for a time trigger to do something would be like so:

static System.Timers.Timer _t = new System.Timers.Timer();

void Init()
{
    //Start a timer trigger every 300000 milliseconds = 5 minutes
     _t.Interval = 300000;
     _t.Elapsed += fiveMinutesElapsed;
     _t.Enabled = true;
}    
public void fiveMinutesElapsed()
{
    t.Enabled = false;
    //Do Something
    t.Enabled = true;
}

Answer 2

Can someone explain this behavior?

Yes: It is very unlikely that the elapsed time calculated from DateTime.Now is exactly equal to your specified TimeSpan.

From your comments, it seems that you want to periodically check if it's time to do something. One easy way to do this is to use a Stopwatch to keep track of the amount of time that has elapsed since the last time you did the thing:

using System;
using System.Diagnostics;
using System.Threading;

namespace ConsoleApp1
{
    class Program
    {
        static void Main()
        {
            throttleTimer.Start();

            // This loop simulates your periodic check to see if the action
            // needs to be performed:

            while (true)
            {
                doActionIfTimeElapsed(TimeSpan.FromSeconds(5));
                Thread.Sleep(1000);  // Simulate checking every 1 second or so.
            }
        }

        static Stopwatch throttleTimer = new Stopwatch();

        static void doActionIfTimeElapsed(TimeSpan period)
        {
            if (throttleTimer.Elapsed <= period)
                return;

            doAction();
            throttleTimer.Restart();
        }

        static void doAction()
        {
            Console.WriteLine("Doing the action.");
        }
    }
}

This sample program will perform doAction() whenever you call doActionIfTimeElapsed() and more than 5 seconds has elapsed since the last time it called doAction().

Alternatively, if you want to perform an action after every N seconds without having to call a method to periodically check if the action should be performed, then you can use a System.Threading.Timer instead.

Answer 3

Option 1:

If x.Timestamp is something you can reset (not a server-/client-side readonly value), then use >= (or > as = is less likely to ever happen) and reset x.Timestamp to the current time. Also, check this answer for determining relative time.

Option 2:

If you just want to raise a function in every 15 minutes, you have the answer here.

Note that 3 different timers exist, if you will need one that operate within seconds or milliseconds, you may want to avoid overlapping: which is (only?) possible with System.Timers.Timer via its Enabled property.

Option 3:

If you want to raise a function inside a while loop in every 15 minutes, I propose to use a StopWatch.

var watch = new StopWatch();
watch.Start();
while(...)
{
    if (watch.Elapsed > TimeSpan.FromMinutes(15))
    {
        // do your stuff
        watch.Restart();
    }
}