Iterator in C#

Posted by in C# category on for Intermediate level | Points: 250 | Views : 2000 red flag

We are going to look into details of different kinds of iterator supported by C# and find out subtle differences between them.
Read How to implement Remoting in ASP.NET before this article.


There are following two different types of iterator supported by C#.

1)      External iterator

2)      Internal iterator

Whatever differences exist between them have been marked in Bold letter so, look at them very carefully.

1) External iterator:

A statement in which we need to specify each single step to complete the task. Consider below code snippet

List<string> cities = new List<string> { "Vadodara""Surat""Anand" };

for (int i = 0; i < cities.Count; i++)             {                 Console.WriteLine(cities[i]);             }

This is very simple code. we already have been using this kind of codebase in our day to day application. We are just iterating over elements (string type) of list collection. It might be others like dictionary, array, hash table or custom collection. But it contains so many moving parts. Some developers might argue that this very old way to iterate over collection. They could be writing same codebase in the following ways

foreach (var city in cities)
Despite the above and prior one code produces the same output, but above one is much better than the prior one because former one contains more moving parts. But even more important thing is that there are semantical differences between for and foreach construct, for which we must be very thoughtful. That difference is mutability.
List<string> cities = new List<string> { "Vadodara""Surat""Anand" };

for (int i = 0; i < cities.Count; i++)             {                 Console.WriteLine(cities[i]);                 i = 44;             }

Above codebase only print first value from the collection because variable i is mutable (reinitialize its value) and it affects the state of iterator. This one is the very big design flow in the loop construct itself because i should not be mutable if it was iterating the index. On the contrast look at the below code

foreach (var city in cities)
                city = "Nadiad";
Here variable(city) is immutable, means we cannot change value of city variable. If we try, then compiler become unhappy and gives above compile-time error message.

In above both (for and foreach) code, we need to specify what to do and how to do part such kind of programming style is known as imperative style of programming.

2)Internal iterator: 

An expression is more concise and expressive. Let’s look at the below example

List<string> cities = new List<string> { "Vadodara""Surat""Anand" };
            cities.ForEach(city => Console.Write(city));

Here foreach function is a higher-order function, means if a function takes one or more functions as arguments it is called high-order function (functional form/ functor). In the above code snippet ForEach function contains anonymous function(city=>Console.Write(city).  It produces the same result as prior external iterator (for and foreach). But this code snippet is declarative in nature. Declarative means we only need to specify what we want to do with each element, rather than how it does.

In contrast to the external iterator where we must specify every minute details like starting value, exit condition and so on, here we are giving up control of certain part of our code to underlying library (Base class library). So, we better focus on other important part of our business logic rather than the thing we don’t need to care about. It also relives us from many duties as programmer.

One more important point about internal iterator is it supports polymorphism (Dynamic binding). The dot(.) before ForEach function is a polymorphic means it says just go ahead and call me and I will tell you what I do but does not reveal its how I do it. At runtime, it will work out how to do it based on the context of Object it is working with. So, we get little bit flexibility through internal iterator in our code compare to external iterator. Both for/foreach construct are example of static binding. So, such kind of programming style is called declarative style of programming.


To summarize, whenever you need to do a lot of manual loops through the data structure, wait for a minute and start thinking about the internal iterator. Because C# language has the power but the biggest problem is our mindset. As C# developer, our mind is wired with for/foreach loop construct since the long-time only because we are familiar with it, not because it provides better way of programming. We just need to re-tuning our mindset. Then our code will express our ideas in nicer way. Only need to frame set of functions together like

List<string> cities = new List<string> { "Vadodara""Surat""Anand" };
            var query = cities.Where(n => n.Contains("V"))
                        .OrderBy(n => n.Length)
                        .Select(n => n.ToUpper());

Page copy protected against web site content infringement by Copyscape

About the Author

Full Name: Rajesh Patel
Member Level: Gold
Member Status: Member,MVP
Member Since: 2/14/2012 4:06:42 AM
Country: India
Rajesh Patel R.P.A Developer | Developer Trainer | Clean Code Evangelist
I am Rajesh Patel ,Microsoft certified trainer since 2013, developer, mentor and senior software engineer. He has trained and mentored many developers of fortune IT organizations (Allscripts, Cognizant, L&T info-tech, Automation anywhere, Odysseus, PmcRetail, Advance-india, Webmyne, to name but a few). He is also involved in organizing other, free educational events in the Vadodara, Anand, Ahmedabad and Nadiad city. He also enjoys running Seminars,workshops and hands on labs with anyone willing to learn.

Login to vote for this post.

Comments or Responses

Login to post response

Comment using Facebook(Author doesn't get notification)