String Concatenation using '+' operator

Question

Looking at the string class metadata, I only see the operators == and != overloaded. So how is it able to perform concatenation for the '+' operator?

Edit:

Some interesting notes from Eric Lippert on string concatenation:

Part 1

Part 2

There is also a super article from Joel referred in part 2 (http://www.joelonsoftware.com/articles/fog0000000319.html)

Answer 1

It doesn't - the C# compiler does :)

So this code:

string x = "hello"; string y = "there"; string z = "chaps"; string all = x + y + z; 

actually gets compiled as:

string x = "hello"; string y = "there"; string z = "chaps"; string all = string.Concat(x, y, z); 

(Gah - intervening edit removed other bits accidentally.)

The benefit of the C# compiler noticing that there are multiple string concatenations here is that you don't end up creating an intermediate string of x + y which then needs to be copied again as part of the concatenation of (x + y) and z. Instead, we get it all done in one go.

EDIT: Note that the compiler can't do anything if you concatenate in a loop. For example, this code:

string x = ""; foreach (string y in strings) {     x += y; } 

just ends up as equivalent to:

string x = ""; foreach (string y in strings) {     x = string.Concat(x, y); } 

... so this does generate a lot of garbage, and it's why you should use a StringBuilder for such cases. I have an article going into more details about the two which will hopefully answer further questions.