Strange null string concatenation behaviour [duplicate]

Question

This question already has answers here:

Why is it Valid to Concatenate Null Strings but not to Call "null.ToString()"? (11 answers)

Why is the result of adding two null strings not null? [duplicate] (6 answers)

Closed 8 years ago.

I have a code concatenating two string using plus operator. If two strings are null, sum of two strings is not null.

var strLeft = default(string);
var strRight = default(string);

var strSum = strLeft + strRight;

I want to distinguish String.Empty + default(string) and default(string) + default(string). Both concat operation result is String.Empty.

Why results of both total are String.Empty?

Answer 1

It's because null string is treated as empty string MSDN:

In string concatenation operations, the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string

Answer 2

The + operator calls on string arguments are translated by the C# compiler to calls to the string.Concat method. And here's what the documentation of this method states:

An Empty string is used in place of any null argument.

So basically that's by design. The string.Concat method will never return null.