How to get users/usergroups from an lookup column from SPField

Question

I have a custom list for example "MyCustomListA" I have created a custom ListForm template and overide the ListFieldIterator. I have overide the ListFieldIterator and CreateChildControls to implement some readonly logica depandant of user membership.

One of the fields is a lookup to another custom list (MyCustomListB). In this custom list I am using a custom content type with 2 fields:

(MyCustomListB)

1: Title

2: Person or Group

The lookup shows a dropdownlist with for each item in "MyCustomListB" only the title. I would like to get the "Person or Group" value, someone know how??

public class CustomListFieldIterator : ListFieldIterator
{

    protected override void CreateChildControls()
    {
    // some logic to set readonly depandant by user membership    
    }

    protected override bool IsFieldExcluded(SPField field)
    {
                  // some logic
                  if(field.InternalName.Equals("UsersAndGroups"))
                    {
                        // get users and groups from this field
                        SPUser selectedUsers = field.Some logic...
                        SPGroup selectedUserGroups = field.Some logic...
                        // some logic
                    }

    }

}

Answer 1

This should do it for you:

SPFieldUser userField = (SPFieldUser)item.Fields.GetField("UsersAndGroups");
SPFieldUserValue userFieldValue = (SPFieldUserValue)userField.GetFieldValue(item["UsersAndGroups"].ToString());
SPUser user = userFieldValue.User;

If you want more examples (e.g. when using multi-value selections) see here http://dsen-25.blogspot.ch/2012/07/how-to-get-spuser-or-spgroup-from.html

[EDIT after Comments]: If the UserAndGroups column is actually pointing to another list having one column of type "Person or Group" you need to use something like expressed here Get related items from a parent list item through lookup