private bool ValidateUsingCredentialsProvider()
{
    bool isAuthorized = false;
    try
    {
        ClientFormsAuthenticationMembershipProvider authProvider =
            System.Web.Security.Membership.Provider as
            ClientFormsAuthenticationMembershipProvider;

        // Call ValidateUser with empty strings in order to display the 
        // login dialog box configured as a credentials provider.
        isAuthorized = authProvider.ValidateUser(String.Empty, String.Empty);
    }
    catch (System.Net.WebException)
    {
        MessageBox.Show("Unable to access the authentication service.",
            "Warning", MessageBoxButtons.OK, MessageBoxIcon.Warning);
    }
    if (!isAuthorized)
    {
        MessageBox.Show("Unable to authenticate.", "Not logged in", 
            MessageBoxButtons.OK, MessageBoxIcon.Error);
        Application.Exit();
    }
    return isAuthorized;
}