Take the 2-minute tour ×  I have List_A = { Title, Type} &
List_B = { Title, myLookUpField }  I want myLookUpField = Title,
IF(Type=="Red")  I know....
List_B.Fields.AddLookUp("myLookUpField", List_A.ID, false);
SPFieldLookUp myCol = new
SPFieldLookUp(List_B.Fields,"myLookUpField"); myCol.LookUpField
= List_A.Fields["Title"].InternalName;  Im not sure how to
properly restrict it to only accepting ListItems from List_A
where Type is set to "Red"  Filtered Field? calulated Field?
addDependent lookup? associated field?  share|improve this
question  1 Answer 1  up vote 1 down vote accepted  I would
venture the following solution (there may be an easier solution,
if so I'd love to know it also):  Create a custom field type
with Visual Studio that extends SPFieldLookup (this video helped
me to get started down that path:
http://www.youtube.com/watch?v=VPhYXgKRKq8 ).  Your custom field
type can filter the lookup dropdown by a "related field" value
in the field control class. You can also add validation if you
wish