Thanks for all reply.
Because I have to write a decimal number in a xml file I have find out the problem. In this discussion I have learned that xml file standard use dot for decimal value and this is culture independent.
So my solution is write dot decimal number in a xml file and convert the readed string from the same xml file mystring.Replace(".", ",");
Thanks Agat for suggestion to research the problem in xml context and ? ? ? ? ? ? because I didn't know visual studio doesn't respect the culture settings I have in my code.