September 20, 2010

CSharp/.NET - Use Stopwatch() Instead of DateTime() for Accurate High Precision Timing

(originally posted at coreygoldberg.blogspot.com)

In C# (.NET), the DateTime() class is not accurate for high precision timing. Instead, use the Stopwatch() class if you need a timer. Most hardware and operating systems support a high-resolution performance counter that can be accessed through System.Diagnostics.Stopwatch.

Don't use DateTime() like this if you need accuracy:

 
using System;

class Program
{
    public static void Main()
    {
        DateTime start = DateTime.Now;

            // do timed work here

        DateTime stop = DateTime.Now;

        // don't do this. you won't get accurate timing
        Console.WriteLine("{0} ms", (stop - start).TotalMilliseconds);

        // definitely don't do this. you won't get accurate timing or full timer resolution
        Console.WriteLine("{0} ms", (stop - start).Milliseconds);
    }
}

Stopwatch() uses operating system's high-resolution performance counter:

 
using System;
using System.Diagnostics;

class Program
{
    public static void Main()
    {
        Stopwatch stopWatch = Stopwatch.StartNew();

            // do timed work here

        stopWatch.Stop();

        // don't do this. you won't get full timer resolution
        Console.WriteLine("{0} ms", stopWatch.ElapsedMilliseconds);

        // do this to get accurate high precision timing
        Console.WriteLine("{0} ms", stopWatch.Elapsed.TotalMilliseconds);
    }
}

The Stopwatch class is in the System.Diagnostics namespace:

 
using System.Diagnostics;

Stopwatch measures elapsed time by counting timer ticks in the underlying timer mechanism. If the installed hardware and operating system support a high-resolution performance counter, then the Stopwatch class uses that counter to measure elapsed time. Otherwise, the Stopwatch class uses the system timer (DateTime class) to measure elapsed time.

To see if your system supports a high-resolution performance counter, check the Stopwatch.IsHighResolution property:

 
if (Stopwatch.IsHighResolution)
    Console.WriteLine("Using the system's high-resolution performance counter.");
else 
    Console.WriteLine("Using the DateTime class.");

To check the timing accuracy, use the Stopwatch.Frequency property:

 
long frequency = Stopwatch.Frequency;

Console.WriteLine("Timer frequency in ticks per second: {0}", frequency);

long nanosecPerTick = (1000L*1000L*1000L) / frequency;

Console.WriteLine("Timer is accurate within {0} nanoseconds", nanosecPerTick);
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