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

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 开发者_如何学Pythondoes not?

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

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.

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.

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.

Strings are immutable. Any operation changing them has to create a new string.

A StringBuilder supports the inline Replace method.

Use the StringBuilder if you need to do a lot of string manipulation.

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 that alter the content of that string - you must always assign the results of an operation back to a string reference somewhere.

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

String.Replace is a shared function of string class that returns a new string. It is not an operator on the current object. b.Replace("a","b") would be similar to a line that only has c+1. So just like c=c+1 actually sets the value of c+1 to c, b=b.Replace("a","b") sets the new string returned to b.

As everyone above had said, strings are immutable.

This means that when you do your replace, you get a new string, rather than changing the existing string.

If you don't store this new string in a variable (such as in the variable that it was declared as) your new string won't be saved anywhere.

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

Just to be more explicit. string.Replace("X","Y") returns a new string...but since you are not assigning the new string to anything the new string is lost.