Declaring a variable inside or outside an foreach loop: which is faster/better?

Question

Which one of these is the faster/better one?

This one:

List list = new List();
User u;

foreach (string s in l)
{
    u = new         


        

Answer 1

There should be no percievable difference in performance.

Answer 2

I went to verify this issue. Suprisingly finding out at my dirty tests that the 2nd option is even slightly faster all the time.

namespace Test
{
  class Foreach
  {
    string[] names = new[] { "ABC", "MNL", "XYZ" };

    void Method1()
    {
      List<User> list = new List<User>();
      User u;

      foreach (string s in names)
      {
        u = new User();
        u.Name = s;
        list.Add(u);
      }
    }

    void Method2()
    {

      List<User> list = new List<User>();

      foreach (string s in names)
      {
        User u = new User();
        u.Name = s;
        list.Add(u);
      }
    }
  }

  public class User { public string Name; }
}

I verified the CIL but it is not identical.

So I prepared something what I wanted to be much better test.

namespace Test
{
  class Loop
  { 

    public TimeSpan method1 = new TimeSpan();
    public TimeSpan method2 = new TimeSpan();

    Stopwatch sw = new Stopwatch();

    public void Method1()
    {
      sw.Restart();

      C c;
      C c1;
      C c2;
      C c3;
      C c4;

      int i = 1000;
      while (i-- > 0)
      {
        c = new C();
        c1 = new C();
        c2 = new C();
        c3 = new C();
        c4 = new C();        
      }

      sw.Stop();
      method1 = method1.Add(sw.Elapsed);
    }

    public void Method2()
    {
      sw.Restart();

      int i = 1000;
      while (i-- > 0)
      {
        var c = new C();
        var c1 = new C();
        var c2 = new C();
        var c3 = new C();
        var c4 = new C();
      }

      sw.Stop();
      method2 = method2.Add(sw.Elapsed);
    }
  }

  class C { }
}

Also in this case 2nd method was always winning but then I verified the CIL finding no difference.

I am not CIL-reading guru but I see no decleration issue. As was already pointed out declaration is not allocation so there is no performance penalty on it.

Test

namespace Test
{
  class Foreach
  {
    string[] names = new[] { "ABC", "MNL", "XYZ" };

    public TimeSpan method1 = new TimeSpan();
    public TimeSpan method2 = new TimeSpan();

    Stopwatch sw = new Stopwatch();

    void Method1()
    {
      sw.Restart();

      List<User> list = new List<User>();
      User u;

      foreach (string s in names)
      {
        u = new User();
        u.Name = s;
        list.Add(u);
      }

      sw.Stop();
      method1 = method1.Add(sw.Elapsed);
    }

    void Method2()
    {
      sw.Restart();

      List<User> list = new List<User>();

      foreach (string s in names)
      {
        User u = new User();
        u.Name = s;
        list.Add(u);
      }

      sw.Stop();
      method2 = method2.Add(sw.Elapsed);
    }
  }

  public class User { public string Name; }

Answer 3

Whenever you've a question about performance, the only thing to do is measure - run a loop around your test and time it.

To answer your question - without measuring :-) or looking at the generated ilasm - any difference wouldn't be noticeable in a meaningful number of iterations and the most expensive operation in your code there is likely to be the user allocation by a few orders of magnitude, so concentrate on code clarity (as you should in general) and go with 2.

Oh, its late and I guess I'm just trying to say don't worry about this sort of thing or get caught up in details like this.

K

Answer 4

Technically, the first example will save a few nanoseconds because the stack frame will not have to be moved to allocate a new variable, but this is such a tiny amount of CPU time you won't notice it, that's if the compiler doesn't optimize any difference away anyays.