Why concatenating two null string results an empty string?

Question

I just saw a weird result while I tried to concatenate two null strings: it returns an empty one! I can not imagine if it have some utility or why this occurs.

Example:

string sns = null;
sns = sns + sns;
// It results in a String.Empty

string snss = null;
snss = String.Concat(snss, snss);
// It results in a String.Empty too!

Can someone tell me why it returns a String.Empty instead of null?

The `+` operator is a shorthand for the [`String.Concat`](https://msdn.microsoft.com/en-us/library/a6d350wd(v=vs.110).aspx) method. If the arguments passed were `null` then it turns them into an empty string. — Raktim Biswas, Aug 23 '16 at 22:10
I have been programming in C#, C++, and C for many years and only just now discovered this today! In my head I was thinking string-concatenation would always yield null if all arguments were null. How have I never noticed this behavior before? I feel like such a newb now. — MikeTeeVee, May 10 '19 at 18:12

score 4 · Accepted Answer · edited Aug 23 '16 at 22:25

4

Here is a fragment from C# Language Specification, the “7.8.4 Addition operator” section:

String concatenation:
string operator +(string x, string y);
string operator +(string x, object y);
string operator +(object x, string y);
These overloads of the binary + operator perform string concatenation. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.

edited Aug 23 '16 at 22:25

poke

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answered Aug 23 '16 at 22:03

AndreyAkinshin

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Good find, I could find it D=. – TheNoob Aug 23 '16 at 22:04
See more at [Google Books](https://books.google.com.br/books?id=s-IH_x6ytuQC&lpg=PT432&ots=lsc19Y7M6V&dq=%E2%80%9C7.8.4%20Addition%20operator%E2%80%9D&hl=pt-BR&pg=PT433#v=onepage&q=%E2%80%9C7.8.4%20Addition%20operator%E2%80%9D&f=false) – Akira Yamamoto Aug 23 '16 at 22:08

score 2 · Answer 2 · answered Aug 23 '16 at 21:59

2

http://referencesource.microsoft.com/#mscorlib/system/string.cs

If the string is null, it returns String.Empty.

As for the operator +, I am not too sure.

answered Aug 23 '16 at 21:59

TheNoob

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For the downvote, look at ln 3002. =) – TheNoob Aug 23 '16 at 22:00
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`+` just uses the `Concat` method. http://stackoverflow.com/questions/10341188/string-concatenation-using-operator – crthompson Aug 23 '16 at 22:04
@paqogomez nice thank you, ill check out that SO question. – TheNoob Aug 23 '16 at 22:05
Thank you for clarify. Unfortunately I only can chose one answer, but I upvoted both. Living and learning! – Striter Alfa Aug 23 '16 at 22:17

Why concatenating two null string results an empty string?

2 Answers2