In this example we will create a combobox with 1000 items.
We will start with the normal creation of window stuff: 
#define _GNU_SOURCE
#include <string.h>
#include <Elementary.h>
static void
_combobox_clicked_cb(void *data EINA_UNUSED, Evas_Object *obj EINA_UNUSED,
                     void *event_info EINA_UNUSED)
{
   printf("Hover button is clicked and 'clicked' callback is called.\n");
}
static void
_combobox_selected_cb(void *data EINA_UNUSED, Evas_Object *obj EINA_UNUSED,
                      void *event_info)
{
   const char *txt = elm_object_item_text_get(event_info);
   printf("'selected' callback is called. (selected item : %s)\n", txt);
}
static void
_combobox_dismissed_cb(void *data EINA_UNUSED, Evas_Object *obj EINA_UNUSED,
                       void *event_info EINA_UNUSED)
{
   printf("'dismissed' callback is called.\n");
}
static void
_combobox_expanded_cb(void *data EINA_UNUSED, Evas_Object *obj EINA_UNUSED,
                       void *event_info EINA_UNUSED)
{
   printf("'expanded' callback is called.\n");
}
static void
_combobox_item_pressed_cb(void *data EINA_UNUSED, Evas_Object *obj,
                      void *event_info)
{
   const char *txt = elm_object_item_text_get(event_info);
   printf("'item,pressed' callback is called. (selected item : %s)\n", txt);
   elm_object_text_set(obj, txt);
}
static char *
gl_text_get(void *data, Evas_Object *obj EINA_UNUSED, const char *part EINA_UNUSED)
{
   char buf[256];
   snprintf(buf, sizeof(buf), "Item # %i", (int)(uintptr_t)data);
   return strdup(buf);
}
static Eina_Bool gl_state_get(void *data EINA_UNUSED,
                               Evas_Object *obj EINA_UNUSED,
                               const char *part EINA_UNUSED)
{
   return EINA_FALSE;
}
static Eina_Bool
gl_filter_get(void *data, Evas_Object *obj EINA_UNUSED, void *key)
{
   
   if (!strlen((char *)key)) return EINA_TRUE;
   char buf[256];
   snprintf(buf, sizeof(buf), "Item # %i", (int)(uintptr_t)data);
   if (strcasestr(buf, (char *)key))
     return EINA_TRUE;
   
   return EINA_FALSE;
}
EAPI_MAIN int
elm_main(int argc EINA_UNUSED, char **argv)
{
   Evas_Object *win, *bg;
   evas_object_size_hint_weight_set(bg, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(bg, EVAS_HINT_FILL, EVAS_HINT_FILL);
   evas_object_show(bg);
Next we will create a box. 
   evas_object_size_hint_weight_set(bx, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_show(bx);
And now we create our combobox and set some of it's properties. We set win as its parent, set a text "A Simple List" (which acts as a placeholder). We pack the combobox in box. 
   evas_object_size_hint_weight_set(combobox, EVAS_HINT_EXPAND, 0);
   evas_object_size_hint_align_set(combobox, EVAS_HINT_FILL, 0);
   evas_object_show(combobox);
Next we create a new genlist item class and sets its properties: item_style as deafult , callback for text_get and set others as NULL. 
   itc->item_style = "default";
   itc->func.text_get = gl_text_get;
   itc->func.content_get = NULL;
   itc->func.state_get = gl_state_get;
   itc->func.filter_get = gl_filter_get;
   itc->func.del = NULL;
Next we will append 1000 items to the combobox, this is similar to appending items to the genlist 
   for (int i = 0; i < 1000; i++)
                             NULL, ELM_GENLIST_ITEM_NONE, NULL,
                             (void*)(uintptr_t)(i * 10));
We also set a pair of callbacks to be called whenever any item is selected or pressed. when the combobox is activated, dismissed, expanded : 
   evas_object_smart_callback_add(combobox, "clicked",
                                  _combobox_clicked_cb, NULL);
   evas_object_smart_callback_add(combobox, "selected",
                                  _combobox_selected_cb, NULL);
   evas_object_smart_callback_add(combobox, "dismissed",
                                  _combobox_dismissed_cb, NULL);
   evas_object_smart_callback_add(combobox, "expanded",
                                  _combobox_expanded_cb, NULL);
   evas_object_smart_callback_add(combobox, "item,pressed",
                                  _combobox_item_pressed_cb, NULL);
And then ask that our combobox be shown and run the main loop: 
   evas_object_resize(win, 300, 500);
   evas_object_show(win);
   return 0;
}
We now have the callback for setting text in the each item of genlist: 
Next we have the callback which is called when the combobox is clicked: 
Next we have the callback that is called whenever an item is selected and text of that item is set on combobox: 
Next we have the callback that is called whenever an item is pressed and text of that item is set on combobox and the hover is closed: 
Next we have the callback that is called whenever an item is double-clicked or pressing (enter|return|spacebar) on an item also the text(event_info) of that item is set on combobox and the hover is closed: 
And the callback that is called when the hover,genlist are closed. 
And finally the callback is called when hover,genlist are shown. 
Our example will initially look like this: