Override Views Contextual Filter in View Exposed Form Validation I'm using the Content:Nid contextual filter in my view (with exposed filters) and would like to dynamically set the value of this contextual filter from the exposed form validation callback. Is this possible?
Contextual filters work with arguments from the url so for example {drupal site}/search-nid/% where search-nid is the url of your page  and % is the value you wish to submit to the contextual filter.

An exposed filter tends to take it's values from the query string E.G.
{drupal site}/search-nid?field_nid=% (Again search-nid is the page url and % is the value of the nid)  

If you are trying to do what I think you are, (Filter results to a specific nid using a URL to "autofill" the filter) I did this in a hook_form_submit function and not a hook_form_validate function.  The function below assumes your module is mymodule and your form id is advancedsearch_form.  The function looked as follows:

&lt;?php 
/**
 * Implements hook_form_submit()
 * Takes values from submitted form and appends them to query string
 * uses @see drupal_goto for info one $form_state['#redirect']
 */
function mymodule_advancedsearch_form_submit($form, &amp;$form_state) {
  $qsa = array(); //for QSA arguments

  $field_nid = $form_state['values']['field_nid'];
  $qsa['field-nid'] = $field_nid;

  $form_state['redirect'] = array(
    'search-nid', 
    array(
      'query' =&gt; $qsa,
    ),
    302,
  );
}


This may not be best practise (In fact I'm sure it isn't as it was a quick hack I implemented in about 5 mins!) but I hope it does answer your question.