using (DirectoryEntry entry = new DirectoryEntry("LDAP://XX.XX.XX.Xx"))  //line of  erorr!! :( !!
using (DirectorySearcher search = new DirectorySearcher())
{
    var accountDN = string.Format("CN=Users,DC=aduam,DC=com", userName);
    entry.AuthenticationType = AuthenticationTypes.None;
    entry.Path = StrPath;
    entry.Username = accountDN;
    entry.Password = password;
    search.SearchRoot = entry;
    search.FindOne();

    entry.Close();
    entry.Dispose();
}