Why concatenating two null string results an empty string? [duplicate]

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 method. If the arguments passed were null then it turns them into an empty string. — Raktim Biswas
– Raktim Biswas, Commented Aug 23, 2016 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
– MikeTeeVee, Commented May 10, 2019 at 18:12

poke · Accepted Answer · 2016-08-23 22:25:28Z

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.