Add choice to SPFieldChoice from an Eventreceiver in sandboxed

Question

I am trying to make a sandboxed solution that adds a couple of itemeventreceivers to a list, that add the items to the choices of a choicefield. The result would be something like a lookup field, without the referential integrity.

The code below works perfectly in a regular solution - the choice gets added to the field as expected, but in a sandboxed solution I get no results. The event receiver does fire, no exceptions occur and and all code is reached when stepping through, but the item just isn't added. I think I'm doing nothing here that is impossible in a sandboxed solution. Is there something I need to do differently because this code runs sandboxed?

    public override void ItemAdding(SPItemEventProperties properties)
    {
        base.ItemAdding(properties);
        string itemTitle = properties.AfterProperties["Title"].ToString();
        SPField _field = properties.Web.Fields["Klant"];
        if (_field == null)
        {
            properties.Cancel = true;
            return;
        }
        SPFieldChoice KlantField = (SPFieldChoice)_field;
        foreach (string choice in KlantField.Choices)
        {
            if (itemTitle.ToLower() == choice.ToLower())
            {
                properties.Cancel = true;
                return;
            }
        }
        KlantField.Choices.Add(itemTitle);
        KlantField.Update(true);
        properties.Web.Update();
    }

Background: I am running VS2010 to debug, I have a Sharepoint Foundation Server to test on, and I also tried the sandboxed solution on Office365, the environment where we want to use this solution. The sandboxed and non-sandboxed solutions I'm talking about are separate solutions, and not changed afterwards. They are identical except for the description and GUIDs and that one was made as a sandboxed solution and the other as a farm solution one.

Answer 1

All credit goes to Xiang Zeng for providing the answer, many thanks!

This is a bug in Sharepoint. However, there is a workaround.

Directly modifying the SchemaXml property to change the choices is allowed:

    public override void ItemAdding(SPItemEventProperties properties)
    {
        base.ItemAdding(properties);
        string itemTitle = properties.AfterProperties["Title"].ToString();
        SPField _field = properties.Web.Fields["Klant"];
        if (_field == null)
        {
            properties.Cancel = true;
            return;
        }
        SPFieldChoice KlantField = (SPFieldChoice)_field;
        foreach (string choice in KlantField.Choices)
        {
            if (itemTitle.ToLower() == choice.ToLower())
            {
                properties.Cancel = true;
                return;
            }
        }
        string originalXml = KlantField.SchemaXml;
        string newXml = originalXml.Replace("</CHOICES>", String.Format("<CHOICE>{0}</CHOICE></CHOICES>", itemTitle));
        KlantField.SchemaXml = newXml;
    }

Note that calling the Update method is not necessary: http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spfield.schemaxml.aspx

ItemUpdating and ItemDeleting can be done in the same way. I tested it both on my own server and in office365, and it works like a charm!