All .NET Strings are immutable - which means that once they are created, they cannot be changed in any way.
Every time you add or remove a character from a string, a new copy is created, and the previous version is unchanged.
An example:
string orig = "Hello there!";
string copy = orig;
string replaced = orig.Replace("there", "World");
orig += " This is a test.";
Console.WriteLine(replaced);
Console.WriteLine(orig);
Console.WriteLine(copy);
The output is:
Hello World!
Hello there! This is a test.
Hello there!