如何在c＃中将字符串从koi8-r转换为windows-1251？

Question

  

可能重复：
  Cyrillic encoding in C#

我有什么

string str = "уРБУЙВП ЧБН ЪБ ЧБЫХ РПНПЭШ"; //cyrillic
//I check this on 
site == http://2cyr.com/decode/?lang=en

str == koi8-r
if ( I decoder from the site)
  readeble text == windows-1251;

如何将koi8转换为windows-1251？