public class MyController : Controller
{
    public bool userExists(string user, string pass)
    {
        DirectoryEntry directory  = new DirectoryEntry();
        DirectorySearcher directorySearch  = new DirectorySearcher(directory);

        directorySearch.Filter = "(&(uid=" + user + ")(userPassword=" + pass + "))";

        return directorySearch.FindOne() != null;
    }
}