The Membership class provides a method for validating a membership user. If a user has entered his user name and password in a login mask, you can use the ValidateUser() method for programmatically validating the information entered by the user, as follows:
protected void LoginAction_Click(object sender, EventArgs e)
{
if (Membership.ValidateUser(UsernameText.Text, PasswordText.Text))
{
FormsAuthentication.RedirectFromLoginPage(UsernameText.Text, false);
}
else
{
LegendStatus.Text = “Invalid user name or password!”;
}
}