Whenever possible, use for instead of foreach.

This is what I was told shortly after learning about loops in C#, and I blindly believed it. I had the idea, like many others, that lower level code means faster code.

But is it true? I decided to find out. To ensure an accurate and fair comparison of the two loops, the order in which they are tested is alternated. They are each tested 100 times and the very accurate Stopwatch class of the .NET framework is used for timing.

The following is the test code.

long forTime = 0;
long foreachTime = 0;
int dataSize = 100000000;
int[] data = new int[dataSize];
int a;

Stopwatch sw = new Stopwatch();
var random = new Random();

for (int i = 0; i < dataSize; i++)
{
  data[i] = random.Next(int.MaxValue);
}

for (int r = 0; r < 50; r++)
{
  sw.Start();

  for (int i = 0; i < data.Length; i++)
  {
    // access variable here
    a = data[i];
  }

  sw.Stop();
  forTime += sw.ElapsedTicks;
  sw.Reset();
  sw.Start();

  foreach (var num in data)
  {
    // access variable here
    a = num;
  }

  sw.Stop();
  foreachTime += sw.ElapsedTicks;
}

sw.Reset();

for (int r = 0; r < 50; r++)
{
  sw.Start();

  foreach (var num in data)
  {
    // access variable here
    a = num;
  }

  sw.Stop();
  foreachTime += sw.ElapsedTicks;

  sw.Reset();
  sw.Start();

  for (int i = 0; i < data.Length; i++)
  {
    // access variable here
    a = data[i];
  }
 
  sw.Stop();
  forTime += sw.ElapsedTicks;
}

The line below the “access variable here” comment is repeated for the number of times we want to access the variable in each iteration. I changed the number of times it was read for different tests, but was always consistent within each run of the code. The “datasize” variable is the size of the array that we iterate through. The time is in milliseconds and is rounded off to 4 decimal places as any more precision than that is insignificant. The time is the average run time of the loop.

Here are the results: