[DllImport("advapi32.dll", CharSet = CharSet.Auto)]
    public static extern int LogonUser(String lpszUserName,
    String lpszDomain,
    String lpszPassword,
    int dwLogonType,
    int dwLogonProvider,
    ref IntPtr phToken);

    public const int LOGON32_LOGON_INTERACTIVE = 2;
    public const int LOGON32_PROVIDER_DEFAULT = 0;
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    public static bool UserAdLogin(string UserName, string UserDomain, string UserPass)
    {
        IntPtr token = IntPtr.Zero;

        if (LogonUser(UserName, UserDomain, UserPass, LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, ref token) != 0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }