I have used extension methods (http://msdn.microsoft.com/en-us/library/bb383977.aspx) for the entity manager then at the begining of my application I load the content of the citites table in asynchronous query, then when the user use the combobox the query is ready and my method return the items source getCities , in the worst case scenario the query will not be ready and the user must have to wait
public static class EntityManagerExtension
{
public static void LoadDataBaseObjects(this EntityManager manager)
{
var citites = manager.GetQuery<City>();
manager.ExecuteQueryAsync(citites);
}
public static IEnumerable<City> getCities(this EntityManager manager)
{
var citites = manager.GetQuery<City>();
return manager.ExecuteQuery(citites);
}