c# - Are Timers and Loops in .Net accurate?

Question

While developing a program to calculate the frequency and pulse width of the pulse generated by a 555 timer IC, coming to the PC via PC Parallel port. I noticed that every time I run the code it shows different values, so I start testing the Loops and timers for there accuracy. I have run the following code and come to the point that they are inaccurate (I might be wrong, please correct me, if I am!):

For Timers:

    int sec = 0;
    private void button2_Click(object sender, EventArgs e)
    {
        sec = DateTime.Now.Second;
        i = 0;
        timer1.Enabled = true;
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        if (sec == DateTime.Now.Second)
        {
            i++;
        }
        else
        {
            timer1.Enabled = false;
            MessageBox.Show(i.ToString(),"Timer Output");
        }
    }

OUTPUT: Should be same, but:

For LOOP:

    private void button1_Click(object sender, EventArgs e)
    {
        i = 0;
        CheckForIllegalCrossThreadCalls = false;
        Thread t1 = new Thread(LoopTest);
        t1.Start();
    }

    void LoopTest()
    {
        System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
        sw.Start();
        this.Cursor = Cursors.WaitCursor;
        while (true)
        {
            if (sw.ElapsedMilliseconds != 1000)
            {
                i++;
            }
            else
            {
                break;
            }
        }
        sw.Stop();
        this.Cursor = Cursors.Default;
        MessageBox.Show(i.ToString(), "Loop Output");
    }

OUTPUT: Should be same, but:

What should I do to make loops and timer accurate, Is there any way to do it? Or I have to go to hard and complex C code and DOS?

I think it is the basic reason for getting me wrong values in this question: Count Parallel port input frequency - C#

Answer 1

1) Don't use DateTime.Now for performance measurements, use StopWatch.

2) "OUTPUT: Should be same, but .."

Why should they? You are running managed/JIT'ed code on a non RTOS (real time operating system). You're code can get bounced at any time if the OS feels like it. What lead you to believe that running the same code N times in this environment should always produce the same results to such a small degree?

3) Timers on Windows have a ~15ms resolution. Your best bet for very accurate timings is the HighPerformanceTimer API on systems (CPU's) which support it. You haven't even shown us the timer's interval.

You're failing to consider many variables here and you're basing your predictions upon faulty assumptions. How many times are you even measuring this code? Are you taking into account the time needed to compile it the first time through? Are you running in release mode? Through VS? Are there many tasks running in the background? I could go on.