String concatenation : += operator versus string array joined

Question

The concatenation in many language using the += operator create a new string instance. It is preffered to use a string[] that we join at the end.

In Javascript :

var myString = new Array("Hello");
myString.push(" ");
myString.push("world !");
console.log(myString.join(''));

is more efficient that :

var myString = "Hello";
myString += " ";
myString += "world !";
console.log(myString);

In C#, does the += operator create a new string ? Is the StringBuilder more efficient that using a string[] ?

This is the kind of question that is best answered with real-world examples and benchmarks rather than pure theoretical examination. Can you provide examples that you would like to see benchmarked? — Louis
– Louis, Commented Sep 26, 2016 at 15:54
Note that a += b or a = a + b gets translated to a = string.Concat(a, b) by the C# compiler — Matias Cicero
– Matias Cicero, Commented Sep 26, 2016 at 15:56
They serve two different purposes. StringBuilder is used to directly concatenate an arbitrary number of strings, whether in a collection or not - string.Join is used to concatenate strings (from a collection) with a separator. If you want to use Join to concatenate strings with an empty separator there's nothing stopping you, but it's not the intended purpose. — D Stanley
– D Stanley, Commented Sep 26, 2016 at 15:59

Dominik · Accepted Answer · 2016-09-26 16:00:52Z

2

In C#, does the += operator create a new string ?

String is immutable in C# and Java. That means you can not modify it. Every method that modifys a string (+= executes a method too) returns a new instance of a string.

Is the StringBuilder more efficient that using .Join() on a string[] ?

StringBuilder is more performant (that are some nanosecs per call) than using .Join on an string[]. So it does make "sence" when you do that really often in a loop or something.

edited Sep 26, 2016 at 16:00

answered Sep 26, 2016 at 15:56

Dominik

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6 Comments

D Stanley Over a year ago

The second question is using StringBuilder versus calling Join on a string array.

Dominik Over a year ago

Thanks for that explaination. Editing my answer

D Stanley Over a year ago

"StringBuilder is more performant (that are some nanosecs per call) than using .Join on an string[]" how do you know this?

stuartd Over a year ago

String.Join vs. StringBuilder: which is faster?

Dominik Over a year ago

@stuartd The question is joining multiple string[]. Not using string.Join. At least i think so

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