How To: Measure execution time in C#

public class Timer
{
    public static readonly bool IsHighPerformance;

    [DllImport("Kernel32.dll")]
    private static extern bool QueryPerformanceCounter(out long lpPerformanceCount);
    //Retrieves the current value of the high-resolution performance counter.

    [DllImport("Kernel32.dll")]
    private static extern bool QueryPerformanceFrequency(out long lpFrequency);
    //Retrieves the frequency of the high-resolution performance counter, if one exists.
    //The frequency cannot change while the system is running.

    private long m_startTime;
    private long m_stopTime;
    private static long m_freq;

    static Timer()
    {
        try
        {
            IsHighPerformance = QueryPerformanceFrequency(out m_freq);
        }
        catch (Exception)
        {
            IsHighPerformance = false;
        }
    }

    public Timer()
    {
        m_startTime = 0;
        m_stopTime = 0;
    }

    ///
    /// Start the timer
    ///
    public void Start()
    {
        // let the waiting threads do their work
        Thread.Sleep(0);

        if (IsHighPerformance)
        {
            QueryPerformanceCounter(out m_startTime);
        }
        else
        {
            m_startTime = DateTime.Now.Ticks;
        }
    }

    ///
    /// Stop the timer
    ///
    public void Stop()
    {
        if (IsHighPerformance)
        {
            QueryPerformanceCounter(out m_stopTime);
        }
        else
        {
            m_stopTime = DateTime.Now.Ticks;
        }
    }

    ///
    /// Returns the duration of the timer (in fraction of seconds)
    ///
    public double DurationSeconds
    {
        get
        {
            if (IsHighPerformance)
            {
                return (double)(m_stopTime - m_startTime) / (double)m_freq;
            }
            else
            {
                TimeSpan span = (new DateTime(m_stopTime)) - (new DateTime(m_startTime));
                return span.TotalSeconds;
            }
        }
    }

    public double DurationMilliseconds
    {
        get
        {
            if (IsHighPerformance)
            {
                return (double)(m_stopTime - m_startTime) / (double)m_freq;
            }
            else
            {
                TimeSpan span = (new DateTime(m_stopTime)) - (new DateTime(m_startTime));
                return span.TotalMilliseconds;
            }
        }
    }
}


//////demo
//static void Main() {
//Timer timer = new Timer();
//timer.Start();
// do something here
//timer.Stop();
//Console.WriteLine(timer.DurationSeconds);
//}


//////////the second class time of without using the system function
class Timing2
{
    TimeSpan startingTime;
    TimeSpan duration;
    
    public Timing2()
    {
        startingTime = new TimeSpan(0);
        duration = new TimeSpan(0);
    }

    public void StopTime()
    {
        duration = Process.GetCurrentProcess().Threads[0].UserProcessorTime.Subtract(startingTime);
    }

    public void startTime()
    {
        GC.Collect();
        GC.WaitForPendingFinalizers();
        startingTime = Process.GetCurrentProcess().Threads[0].UserProcessorTime;
    }

    public TimeSpan Result()
    {
        return duration;
    }
}