I have crated two content type like A and B .some of field are similar .I don't want to repeat that field .I want to put drop down list on any one of A or B.
YOU should use the entity reference module for the above purpose.
Provides a field type that can reference arbitrary entities:
A generic entity reference field Four widgets: Two implemented on top of the Options module (Select and Radios/Checkboxes), Two autocomplete widgets (one with one text field per entry, one tag-style with comma-separated entries in the same text field) Two formatters: A "Label" formatter, with optional link that displays the label of the referenced entity (the node title, the user name, etc.) A "Rendered entity" formatter, that renders the references entity using a configurable view mode Integration with Views (for both forward and backward references) Integration with Entity Metadata (and as a consequence with Search API and the like) Integration with Devel generate Integration with Feeds and Migrate (>= 2.3) for painless data import This modules depends on Entity API and CTools.
Modules extending Entity reference functionality:
Earlier answer deleted following comments :
If you want to use the existing field then just go to manage fields tab under
Home » Administration » Structure » Content types » A
and use existing field option where you will get the existing field of other content type
Check the image below for existing field option :
Or else you can do it in the following way :
You can get your dropdown list for content type in
and Save that value in
After saving that value you can use that value to insert into another content type in the hook_form_alter function or your custom_form itself.