Is the condition in a for loop evaluated each iteration?

Question

When you do stuff like:

for (int i = 0; i < collection.Count; ++i ) 

is collection.Count called on every iteration?

Would the result change if the Count property dynamically gets the count on call?

Answer 1

Yes Count will be evaluated on every single pass. The reason why is that it's possible for the collection to be modified during the execution of a loop. Given the loop structure the variable i should represent a valid index into the collection during an iteration. If the check was not done on every loop then this is not provably true. Example case

for ( int i = 0; i < collection.Count; i++ ) {   collection.Clear(); } 

The one exception to this rule is looping over an array where the constraint is the Length.

for ( int i = 0; i < someArray.Length; i++ ) {   // Code } 

The CLR JIT will special case this type of loop, in certain circumstances, since the length of an array can't change. In those cases, bounds checking will only occur once.

Reference: http://blogs.msdn.com/brada/archive/2005/04/23/411321.aspx

Answer 2

Count would be evaluated on every pass. If you continued to add to the collection and the iterator never caught up, you would have an endless loop.

class Program     {         static void Main(string[] args)         {             List<int> intCollection = new List<int>();             for(int i=-1;i < intCollection.Count;i++)             {                 intCollection.Add(i + 1);             }         }     } 

This eventually will get an out of memory exception.