for vs. foreach
There is a common confusion that those two constructs are very similar and that both are interchangeable like this:
foreach (var c in collection)
{
DoSomething(c);
}
and:
for (var i = 0; i < collection.Count; i++)
{
DoSomething(collection[i]);
}
The fact that both keywords start by the same three letters doesn't mean that semantically, they are similar. This confusion is extremely error-prone, especially for beginners. Iterating through a collection and doing something with the elements is done with foreach; for doesn't have to and shouldn't be used for this purpose, unless you really know what you're doing.
Let's see what's wrong with it with an example. At the end, you'll find the full code of a demo application used to gather the results.
In the example, we are loading some data from the database, more precisely the cities from Adventure Works, ordered by name, before encountering "Boston". The following SQL query is used:
select distinct [City] from [Person].[Address] order by [City]
The data is loaded by ListCities() method which returns an IEnumerable<string>. Here is what foreach looks like:
foreach (var city in Program.ListCities())
{
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
Let's rewrite it with a for, assuming that both are interchangeable:
var cities = Program.ListCities();
for (var i = 0; i < cities.Count(); i++)
{
var city = cities.ElementAt(i);
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
Both return the same cities, but there is a huge difference.
- When using
foreach, ListCities() is called one time and yields 47 items.
- When using
for, ListCities() is called 94 times and yields 28153 items overall.
What happened?
IEnumerable is lazy. It means that it will do the work only at the moment when the result is needed. Lazy evaluation is a very useful concept, but has some caveats, including the fact that it's easy to miss the moment(s) where the result will be needed, especially in the cases where the result is used multiple times.
In a case of a foreach, the result is requested only once. In a case of a for as implemented in the incorrectly written code above, the result is requested 94 times, i.e. 47 × 2:
Querying a database 94 times instead of one is terrible, but not the worse thing which may happen. Imagine, for example, what would happen if the select query would be preceded by a query which also inserts a row in the table. Right, we would have for which will call the database 2,147,483,647 times, unless it hopefully crashes before.
Of course, my code is biased. I deliberately used the laziness of IEnumerable and wrote it in a way to repeatedly call ListCities(). One can note that a beginner will never do that, because:
The IEnumerable<T> doesn't have the property Count, but only the method Count(). Calling a method is scary, and one can expect its result to not be cached, and not suitable in a for (; ...; ) block.
The indexing is unavailable for IEnumerable<T> and it's not obvious to find the ElementAt LINQ extension method.
Probably most beginners would just convert the result of ListCities() to something they are familiar with, like a List<T>.
var cities = Program.ListCities();
var flushedCities = cities.ToList();
for (var i = 0; i < flushedCities.Count; i++)
{
var city = flushedCities[i];
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
Still, this code is very different from the foreach alternative. Again, it gives the same results, and this time the ListCities() method is called only once, but yields 575 items, while with foreach, it yielded only 47 items.
The difference comes from the fact that ToList() causes all data to be loaded from the database. While foreach requested only the cities before "Boston", the new for requires all cities to be retrieved and stored in memory. With 575 short strings, it probably doesn't make much difference, but what if we were retrieving only few rows from a table containing billions of records?
So what is foreach, really?
foreach is closer to a while loop. The code I previously used:
foreach (var city in Program.ListCities())
{
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
can be simply replaced by:
using (var enumerator = Program.ListCities().GetEnumerator())
{
while (enumerator.MoveNext())
{
var city = enumerator.Current;
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
}
Both produce the same IL. Both have the same result. Both have the same side effects. Of course, this while can be rewritten in a similar infinite for, but it would be even longer and error-prone. You're free to choose the one you find more readable.
Want to test it yourself? Here's the full code:
using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Diagnostics;
using System.Linq;
public class Program
{
private static int countCalls;
private static int countYieldReturns;
public static void Main()
{
Program.DisplayStatistics("for", Program.UseFor);
Program.DisplayStatistics("for with list", Program.UseForWithList);
Program.DisplayStatistics("while", Program.UseWhile);
Program.DisplayStatistics("foreach", Program.UseForEach);
Console.WriteLine("Press any key to continue...");
Console.ReadKey(true);
}
private static void DisplayStatistics(string name, Action action)
{
Console.WriteLine("--- " + name + " ---");
Program.countCalls = 0;
Program.countYieldReturns = 0;
var measureTime = Stopwatch.StartNew();
action();
measureTime.Stop();
Console.WriteLine();
Console.WriteLine();
Console.WriteLine("The data was called {0} time(s) and yielded {1} item(s) in {2} ms.", Program.countCalls, Program.countYieldReturns, measureTime.ElapsedMilliseconds);
Console.WriteLine();
}
private static void UseFor()
{
var cities = Program.ListCities();
for (var i = 0; i < cities.Count(); i++)
{
var city = cities.ElementAt(i);
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
}
private static void UseForWithList()
{
var cities = Program.ListCities();
var flushedCities = cities.ToList();
for (var i = 0; i < flushedCities.Count; i++)
{
var city = flushedCities[i];
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
}
private static void UseForEach()
{
foreach (var city in Program.ListCities())
{
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
}
private static void UseWhile()
{
using (var enumerator = Program.ListCities().GetEnumerator())
{
while (enumerator.MoveNext())
{
var city = enumerator.Current;
Console.Write(city + " ");
if (city == "Boston")
{
break;
}
}
}
}
private static IEnumerable<string> ListCities()
{
Program.countCalls++;
using (var connection = new SqlConnection("Data Source=mframe;Initial Catalog=AdventureWorks;Integrated Security=True"))
{
connection.Open();
using (var command = new SqlCommand("select distinct [City] from [Person].[Address] order by [City]", connection))
{
using (var reader = command.ExecuteReader(CommandBehavior.SingleResult))
{
while (reader.Read())
{
Program.countYieldReturns++;
yield return reader["City"].ToString();
}
}
}
}
}
}
And the results:
--- for ---
Abingdon Albany Alexandria Alhambra [...] Bonn Bordeaux Boston
The data was called 94 time(s) and yielded 28153 item(s).
--- for with list ---
Abingdon Albany Alexandria Alhambra [...] Bonn Bordeaux Boston
The data was called 1 time(s) and yielded 575 item(s).
--- while ---
Abingdon Albany Alexandria Alhambra [...] Bonn Bordeaux Boston
The data was called 1 time(s) and yielded 47 item(s).
--- foreach ---
Abingdon Albany Alexandria Alhambra [...] Bonn Bordeaux Boston
The data was called 1 time(s) and yielded 47 item(s).
LINQ vs. traditional way
As for LINQ, you may want to learn functional programming (FP) - not C# FP stuff, but real FP language like Haskell. Functional languages have a specific way to express and present the code. In some situations, it is superior to non-functional paradigms.
FP is known being much superior when it comes to manipulating lists (list as a generic term, unrelated to List<T>). Given this fact, the ability to express C# code in a more functional way when it comes to lists is rather a good thing.
If you're not convinced, compare the readability of code written in both functional and non-functional ways in my previous answer on the subject.
foreachis removing items from a collection while enumerating it, where usually aforloop is needed to start from the last element.