BigInteger logical operation takes more time in subsequent loops C#

Question

I have the below console application to check the time taken for doing a specific operation with BigInteger in C#.

using System.Diagnostics;
using System.Numerics;
 
Stopwatch timer = new Stopwatch();
for (int j=0; j<10; j++)
{
    Console.WriteLine("Loop Started {0}",j);
    timer.Start();
    BigInteger bitFields = 0;
    byte value = 0;
    for (int i = 0; i < 65400; ++i)
    {
        var bb = new BigInteger(value);
        var result = (bb << (i * 8));
        bitFields = bitFields ^ result;
    }
    Console.WriteLine("Elapsed duration: {0}", timer.Elapsed.ToString());
    timer.Stop();
    Console.WriteLine("Loop Completed");
    Thread.Sleep(2000);
}
Console.ReadLine();

Even though I expect approximately similar time taken for each loop, the result shows differently. Time to execute the loop increases gradually

Is there any reason for this behaviour.

Answer 1

This happens because you start/resume the same timer with Stopwatch.Start:

Starts, or resumes, measuring elapsed time for an interval.

You can switch to Stopwatch.Restart:

Stops time interval measurement, resets the elapsed time to zero, and starts measuring elapsed time.

for (int j=0; j<10; j++)
{
    Console.WriteLine("Loop Started {0}",j);
    timer.Restart();
    // ..
}

Also note that for benchmarking it is better to use specialized tools like BenchmarkDotNet.