string.Replace(oldStr, newStr)???!!!!

That is because that the contents of a string object cannot be altered. When you replace some characters using Replace -method, you can see that the method returns a string, that is a altered copy of the original. All operations of the System.String-class work the same way.

If you want to have a "alterable" string, you can use System.Text.StringBuilder class. Its contects can be direclty changed, thus it is more efficient in those cases, when you need to do lots of string manipulation.