var Mystring = "钓鱼岛是中国的 I love China";

var MyDate = new ASCIIEncoding();

byte[] MyBytes = MyDate.GetBytes(Mystring);

var myLength = 0;

for (int i = 0; i <= MyBytes.Length - 1; i++)

{

  if (MyBytes[i] == 63)//判断是否为汉字或全脚符号

  {

   myLength++;

  }

  myLength++;

}