Why string.Replace(“X”,“Y”) works only when assign

Question

I guess it has to do something with string being a reference type but I dont get why simply string.Replace("X","Y") does not work?

Why do I need to do string A = stringB.Replace("X","Y")? I thought it is just a method to be done on specified instance.

EDIT: Thank you so far. I extend my question: Why does b+="FFF" work but b.Replace does not?

Answer 1

To answer your extended question, b+="FFF" is equivalent to b = b + "FFF", so basically you are creating a new string here also.

Answer 2

Strings are immutable, which means that once they are created, they cannot be changed anymore. This has several reasons, as far as I know mainly for performance (how strings are represented in memory).

See also (among many):

• http://en.wikipedia.org/wiki/Immutable_object
• http://channel9.msdn.com/forums/TechOff/58729-Why-are-string-types-immutable-in-C/

As a direct consequence of that, each string operation creates a new string object. In particular, if you do things like

foreach (string msg in messages)
{
    totalMessage = totalMessage + message;
    totalMessage = totalMessage + "\n";
}

you actually create potentially dozens or hundreds of string objects. So, if you want to manipulate strings more sophisticatedly, follow GvS's hint and use the StringBuilder.

Answer 3

Because strings are immutable. Any time you change a string .net creates creates a new string object. It's a property of the class.

Immutable objects
String Object

Answer 4

Yes its a method of System.String. But you can try a = a.Replace("X","Y");

Answer 5

Why "b+="FFF"works but the b.replace is not

Because the += operator assigns the results back to the left hand operand, of course. It's just a short hand for b = b + "FFF";.

The simple fact is that you can't change any string in .Net. There are no instance methods for strings - you must always assign the results of an operating back to a string reference somewhere.