总结：Upate field which lookups from Content Types

1. when updating field which lookups from Content Types, (for example: set Required to Yes), it needs update Content Types first.

    please read this article: http://www.novolocus.com/2010/10/28/programmatically-making-a-field-required-in-sharepoint-2007/

2. Configuration:

    A. Set "Allow management of content types" to "Yes";   

    B. Click respond content type, then it can edit every content type.

3. Coding:

    Helpful link:

    http://www.3guysonsharepoint.com/?p=265