GuestResponse

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.ComponentModel;
using System.Text.RegularExpressions;
using System.Net.Mail;
using System.Text;

namespace PartyInvites.Models
{
    public class GuestResponse : IDataErrorInfo
    {
        public string Name       { get; set; }
        public string Email      { get; set; }
        public string Phone      { get; set; }
        public bool?  WillAttend { get; set; }

        public string Error { get { return null; } }

        public string this[string propName]
        {
            get
            {
                if ((propName == "Name") && string.IsNullOrEmpty(Name))
                    return "Please enter your name";
                if ((propName == "Email") && string.IsNullOrEmpty(Email))           // Need to cover them leaving field blank
                    return "Please enter a valid email address";
                if ((propName == "Email") && !Regex.IsMatch(Email, ".+\\@.+\\..+")) // Need to cover input != name@example.com
                    return "Please enter a valid email address";
                if ((propName == "Phone") && string.IsNullOrEmpty(Phone))
                    return "Please enter your phone number";
                if ((propName == "WillAttend") && !WillAttend.HasValue)             // Does the drop down have a value?
                    return "Please specify whether you'll attend";
                return null;
            }
        }

        public void Submit()
        {
            EnsureCurrentlyValid();

            // Send via email
            var message = new StringBuilder();
            message.AppendFormat("Date: {0:yyyy-MM-dd hh:mm}\n", DateTime.Now);
            message.AppendFormat("Email: {0}\n", Email);
            message.AppendFormat("Phone: {0}\n", Phone);
            message.AppendFormat("Can come: {0}\n", WillAttend.Value? "Yes" : "No");

                SmtpClient smtpClient = new SmtpClient("smtp.example.com");
                smtpClient.Send(new MailMessage(
                    "rsvps@example.com",                                          // From
                    "person@example.com",                                         // To
                    Name + (WillAttend.Value? " will attend" : " won't attend"),  // Subject
                    message.ToString()                                            // Body
                    ));
        }

        private void EnsureCurrentlyValid()
        {
            // I'm valid if IDataErrorInfo.this[] returns null for every property
            var propsToValidate = new[] { "Name", "Email", "Phone", "WillAttend" };
            bool isValid = propsToValidate.All(x => this[x] == null); // This is a lambda expression
            if (!isValid)
                throw new InvalidOperationException("Can't submit invalid GuestResponse");
        }

    }
}