why string type is working as string builder

[HunainHafeez]

Like i heard that string is immutable mean cannot be changed once created so i tried:

String name = "Hunain";
Console.Write("String_replaced="+  name.Replace('i', 'y') + "\n");

according to someone, string.Replace('','') throws a new object and doesn't modify existing string but it did like in above example, neither i stored it in another object nothing, why ? then what's the difference between string type and string builder ?

[Nguyễn Đình Đại]

I think this link will help you: http://stackoverflow.com/questions/3069416/difference-between-string-and-stringbuilder-in-c-sharp