/// <summary>
/// Return true if user is authenticated
/// </summary>
/// <param name="strUsername_">User name</param>
/// <param name="strPassword_">User password</param>
/// <returns>True if authenticated</returns>
public static bool IsAuthenticated(string strUsername_, string strPassword_)
{
    using (var pc = new PrincipalContext(ContextType.Domain, DomainManager.DomainName))
        return pc.ValidateCredentials(strUsername_, strPassword_);
}