C# TimeSpan gives 0 as TotalMilliseconds [duplicate]

Question

I was trying to "benchmark" two nearly identical loops. When I inspect them one by one the subtraction is without any hiccups, and the TimeSpan gives back the correct numbers as the TotalMilliseconds. When I benchmark the two loops together however the first subtraction gives back a number between 3 - 4 (which should be right), and the second one always gives back 0. What am I doing wrong? Thanks!

class Program { static void Main(string[] args) { Console.WriteLine(":: Simple for ::"); var a = 0; var start = DateTime.Now; for (int i = 0; i < 10; i++) { a += i; } Console.WriteLine("Elapsed time: {0} ms", (DateTime.Now - start).TotalMilliseconds); // 3 - 4 ms Console.WriteLine(":: Fancy for ::"); a = 0; start = DateTime.Now; foreach (var i in Enumerable.Range(0, 9)) { a += i; } Console.WriteLine("Elapsed time: {0} ms", (DateTime.Now - start).TotalMilliseconds); // 0 ms } }

Its wierd that you are getting 4 ms. Because counting to 10 should be done in few micro seconds not even 1 ms.... Unless you are running on wooden pc but still.... — M.kazem Akhgary
– M.kazem Akhgary, Commented Jan 8, 2017 at 22:32
See also How to measure code performance in .NET? and In .NET, which loop runs faster, 'for' or 'foreach'?. — CodeCaster
– CodeCaster, Commented Jan 8, 2017 at 22:37

Community · Accepted Answer · 2020-06-20 09:12:55Z

From the documentation:

The resolution of this property depends on the system timer, which is approximately 15 milliseconds on Windows systems.As a result, repeated calls to the Now property in a short time interval, such as in a loop, may return the same value.

The Now property is frequently used to measure performance. However, because of its low resolution, it is not suitable for use as a benchmarking tool. A better alternative is to use the Stopwatch class.

Dmitrii Bychenko · Accepted Answer · 2017-01-08 22:42:12Z

Try using Stopwatch class which is specially designed for benchmarking:

 Stopwatch timer = new Stopwatch(); timer.Start(); for (int i = 0; i < 10; i++) { a += i; } timer.Stop(); Console.WriteLine($"Elapsed time: {timer.Elapsed}");

Outcome (.Net 4.6 IA-64, Core i7 3.2 GHz)

 00:00:00.0000003

please, notice that the time is about 300 nano seconds

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