public partial class MasterPage : System.Web.UI.MasterPage
{
    protected void Page_Load(object sender, EventArgs e)
    {
        TextBox userName = MasterLoginView.FindControl( "txtUserName" ) as TextBox;
        TextBox password = MasterLoginView.FindControl( "txtPassword" ) as TextBox;

        if ( userName != null && password != null )
        {
            if ( Membership.ValidateUser( userName.Text, password.Text ) )
            {
                String usernameString = null;
                MembershipUser usr = Membership.GetUser( userName.Text );

                if ( usr != null )
                    usernameString = usr.UserName;
                if ( String.IsNullOrEmpty( usernameString ) )
                    usernameString = userName.Text;

                FormsAuthentication.RedirectFromLoginPage( usernameString, false );
            }
        }
    }
}