Why string.Replace("X","Y") works only when assigned to new string?

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

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 2

Why doesn't stringA.Replace("X","Y") work?
Why do I need to do stringB = stringA.Replace("X","Y"); ?

Because strings are immutable in .NET. You cannot change the value of an existing string object, you can only create new strings. string.Replace creates a new string which you can then assign to something if you wish to keep a reference to it. From the documentation:

Returns a new string in which all occurrences of a specified string in the current instance are replaced with another specified string.

Emphasis mine.

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So if strings are immutable, why does b += "FFF"; work?

Good question.

First note that b += "FFF"; is equivalent to b = b + "FFF"; (except that b is only evaluated once).

The expression b + "FFF" creates a new string with the correct result without modifying the old string. The reference to the new string is then assigned to b replacing the reference to the old string. If there are no other references to the old string then it will become eligible for garbage collection.

Answer 3

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.