Performance Tip: for vs. foreach in Microsoft .NET

Collections are one of the most commonly used types in programming. For any program that uses data, you will be dealing with collections. One of the most common things we do is to iterate over the collection to process the data. There are three main ways to iterate over a collection in .NET.

for

For is a common statement in many programming languages by using a counter variable to iterate over a collection.

for (int personCount = 0; personCount < _personCollection.Count - 1; 
         personCount++)
{
    var name = _personCollection[personCount].FirstName;
}

foreach

Foreach was the statement I usually used because it’s cleaner and easier to read.

foreach (var person in _personCollection)
{    
    var name = person.FirstName;
}

foreach with AsParallel()

AsParallel() is part of LINQ and could increase performance if there is a sugnificant load with processing the data.

foreach (var person in _personCollection.AsParallel())
{    
    var name = person.FirstName;
}

Performance

I’ve done a lot of benchmarking using for, foreach and foreach AsParallel() for my book on code performance. I ran the benchmark four times using a collection count of 100, 500, 2000 and 5000. Included in this test is the comparison between .NET Clr 4.7.2 and .NET Core 2.2. Below are the results.

Test	Runtime	Collection Count	Mean (ns)	CLR vs CORE Difference
for	Clr 4.7.2	100	108.8216
foreach	Clr 4.7.2	100	308.1016
foreach AsParallel()	Clr 4.7.2	100	853.7294
for	Core 2.2	100	108.3985	-0.4231
foreach	Core 2.2	100	334.339	26.2374
foreach AsParallel()	Core 2.2	100	821.127	-32.60
for	Clr 4.7.2	500	672.2033
foreach	Clr 4.7.2	500	1,619.09
foreach AsParallel()	Clr 4.7.2	500	3,881.51
for	Core 2.2	500	671.033	-1.17
foreach	Core 2.2	500	1,662.23	43.14
foreach AsParallel()	Core 2.2	500	3,626.62	-254.89
for	Clr 4.7.2	2000	3,400.98
foreach	Clr 4.7.2	2000	7,615.78
foreach AsParallel()	Clr 4.7.2	2000	18,726.19
for	Core 2.2	2000	3,422.70	21.72
foreach	Core 2.2	2000	7,444.41	-171.36
foreach AsParallel()	Core 2.2	2000	13,814.03	-4,912.16
for	Clr 4.7.2	5000	10,242.85
foreach	Clr 4.7.2	5000	21,524.22
foreach AsParallel()	Clr 4.7.2	5000	38,018.76
for	Core 2.2	5000	10,330.39	87.54
foreach	Core 2.2	5000	21,290.93	-233.29
foreach AsParallel()	Core 2.2	5000	35,289.98	-2,728.78

The test was done using a business object called Person to mimic the a real world object. As you can see, using for is around 2-3 times faster than foreach! Wow, I was surprised when I first saw this. The benchmark comparing the .NET Clr 4.7.2 to .NET Core 3 produced similar results.

In most tests, .NET Core is faster than the Clr. Moving to .NET Core should be on your team’s roadmap if you aren’t already moving to it.

Summary

My recommendation is to always use for and avoid foreach. I would also avoid using foreach with AsParallel(), unless there is load during the processing of the data. In all my open-source projects and any code that I work on in my contracts, I am always using for from now on.

Of course, you should always benchmark your code to see how for, foreach, do and while looping performs in your own projects and servers.

To learn more about this subject and a lot more, pick up a copy of my book Rock Your Code: Code & App Performance for Microsoft .NET. Do you have any performance tips when using collections? Please make a comment below.