c# - 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