InsertSubTextPart.cs

[HttpPost]
public string InsertSubTextPart(string Name, string BinaryDocument, bool Protected)
{
    byte[] data = null;

    // create a temporary ServerTextControl to create the
    // SubTextPart
    using (ServerTextControl tx = new ServerTextControl())
    {
        tx.Create();

        // load the Selection from the Web.TextControl
        tx.Load(Convert.FromBase64String(BinaryDocument),
            BinaryStreamType.InternalUnicodeFormat);

        tx.SelectAll();
        int iTextLength = tx.Selection.Length;

        // create a new SubTextPart over the complete text
        SubTextPart part = new SubTextPart("txmb_" + Name, 
            Convert.ToInt32(Protected), 1, iTextLength);
        tx.SubTextParts.Add(part);

        // save the complete document
        tx.SelectAll();
        tx.Selection.Save(out data, BinaryStreamType.InternalUnicodeFormat);
    }

    // return the Selection as a Base64 encoded string
    return Convert.ToBase64String(data);
}