private void button1_Click(object sender, EventArgs e)
    {
      string s = textBox1.Text;
      int i = GetLength(s);
      MessageBox.Show(i.ToString());
    }

    public static int GetLength(string str)
    {
      if (str.Length == 0)
        return 0;
      ASCIIEncoding ascii = new ASCIIEncoding();
      int tempLen = 0; 
      byte[] s = ascii.GetBytes(str);
      for (int i = 0; i < s.Length; i++)
      {
        if ((int)s[i] == 63)
        {
          tempLen += 2;
        }
        else
        {
          tempLen += 1;
        }
      }
      return tempLen;
    }

int i = System.Text.Encoding.Default.GetBytes(s).Length;