Efficiencies of loops

Question

I had a long debate last night about the effeciences of for loops, for each loops, and the instantiation of variables. Take a look at the following loops:

//Loop #1
MyObject obj = null;
for (int i = 0; i < myObjectCollection.Count; i++)
{ 
   obj = myObjectCollection.Count[i];
   //Do something with obj...
}

OR

//Loop #2 for (int i = 0; i < myObjectCollection.Count; i++) { MyObject obj = myObjectCollection.Count[i]; //Do something with obj... }

Essentially both of these loops do the same thing. However I have a few questions:

1) In loop #2 it appears that "obj" gets instantiated each time through the loop therefore allocating a new location in the stack. Is this correct or does the compiler optimize this to allow the variable to remain in the same location each time just passing it a new reference to the object in the collection?

2) In either loop is it necessary/more effecient to set "obj = null" at any point to speed up garbage collection?

3) In general which of these is more effecient and why?

Thanks,

Micah Martin

Alan · Answer

Yes if you don't need to refer to the 'obj variable outside of the loop then I agree that it's better to declare it within (and restrict its scope to) the loop.

Micah Martin · Answer

Alan - Thanks for the response. If there is no difference from an efficiency standpoint would it then make more sense to use loop #2 since the obj variable will immediately fall out of scope when the loop terminates as opposed to having the variable declared outside of the loop?

Alan · Answer

Hi Micah 1. When the JIT compiler compiles a method it allocates a slot on the stack to hold each local variable used in that method whether the variable is declared within a loop or not. So in both loops the same memory slot is always used for the 'obj' variable and it doesn't therefore make any difference whether you declare it inside or outside the loop. 2. All objects referenced by local variables will become eligible for garbage collection when the method ends provided there are no other external references to those objects. However if you no longer need those objects it may speed up GC if you set the variables to null before the method ends depending on the state of the heap at the time. If the objects are holding onto any unmanaged resources it may also be sensible to call the Dispose() method first. 3. As the code stands the second loop is slightly more efficient because the 'obj' variable is being set to null before the first loop starts. However this is unnecessary - the code will still compile if you leave it unassigned - in which case there is nothing to choose between the two from an efficiency viewpoint.