人员和组自定义字段类型sharepoint

I have created a custom field type as it is there in sharepoint OOTB, the difference is only that the end user does not need to check the name i.e I have replaced it with DropDownList. The dropdownlist suggest the no. of users available in the web site for that I have created a FieldClass which inherits from SPFieldUser and a FieldControlClass which inherits from UserField. It is working fine in all conditions i.e when I create a List or Document Libarary it shows me the DropDownList with respective users after saying OK it creates an item for me. I have overriden a Value property in FieldControlClass as follows,I have created a custom field type as it is the