I need to write this recursive function in an iterative way: 

return root;

if (t != null)

iuliax, why do you need this? Recursive approach is quite the best here. Or gimme contr arguments.

In other case you do need to know the deep of controls hierarchy to make it iterative

To make sure have a look at sources (by using .Net Reflector) of Control.FindControl method. It does it recursively.

I can write up some psudo code fast:

findcontrol(Control root, string id){
 ArrayList stack = new ArrayList();
stack.AddAll( root.Controls );
while(!stack.empty){
   Control cur = stack.pop();
   stack.AddAll(cur.Controls);
   if (cur.id == id)
      return cur;
   }

}

Recursive is good when the detph is not high. But in my case if I look with ANTS Profiler

i have performance improvements in the case of iterative function.

Yes, Control.FindControl is recursive, but only on the same level (seabling)

I agree with you. Iterative is always more faster. But, if the performance is not the issue recursive algorithms are mostly more understandable to read than the iterative ones.

And in many cases the performance bottlenecks are usually in different places, such as: connection to database, bad sql execution plan, redundant hits to database, etc. You'll gain more benefits when optimise these stuff.

Just my 2 cents.

You must take into account that not all the application are based on the Database servers.

What about client server applications, or COM app or many others?!!?

Best iterative way:

OBS: if replacing foreach with a classice for is faster, if replace the classic for with a IEnumerable Next, current is faster more:

Recursive MAY be better if the hierarchy is not high, but after deep 2-3 in the control tree recursion blows us in exponential black hole

private static Control FindControlIterative(Control root, string id)
        {
            Control ctl = root;
            LinkedList ctls = new LinkedList();

            while (ctl != null)
            {
                if (ctl.ID == id)
                    return ctl;
                foreach (Control child in ctl.Controls)
                {
                    if (child.ID == id)
                        return child;
                    if (child.HasControls())
                        ctls.AddLast(child);
                }
                ctl = ctls.First.Value;
                ctls.Remove(ctl);
            }
            return null;
        }