System.Text.Encoding iso_8859_1 = System.Text.Encoding.GetEncoding("ISO-8859-1");
System.Text.Encoding utf_8 = System.Text.Encoding.UTF8;

// Unicode string.
string s_unicode = "abcéabc";

// Convert to ISO-8859-1 bytes.
byte[] isoBytes = iso_8859_1.GetBytes(s_unicode);

// Convert to UTF-8.
byte[] utf8Bytes = System.Text.Encoding.Convert(iso_8859_1, utf_8, isoBytes);

string utf8str = utf_8.GetString(utf8Bytes);