Contact.cs类:

public class Contact
    {
        public int Id { get; set; }

        public string FirstName { get; set; }

        public string LastName { get; set; }

        public string Company { get; set; }

        public string City { get; set; }

        public string Telephone { get; set; }

        public string Fax { get; set; }

        public string Email { get; set; }
    }

LinqXmlContactsBLManager.cs

using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Xml.Linq;

namespace LinqXmlContactsBLL
{
    public class LinqXmlContactsBLManager
    {
        public List<Contact> GetAllContacts()
        {
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            IEnumerable<XElement> xmlContacts = xmlDocument.Element("Contacts")
                .Elements("Contact");

            List<Contact> contacts = new List<Contact>();

            //获取联系人列表
            foreach (XElement i in xmlContacts)
            {
                Contact contact = new Contact();

                contact.Id = int.Parse(i.Attribute("Id").Value);
                contact.FirstName = i.Element("FirstName").Value;
                contact.LastName = i.Element("LastName").Value;
                contact.Company = i.Element("Company").Value;
                contact.City = i.Element("City").Value;
                contact.Telephone = i.Element("Telephone").Value;
                contact.Fax = i.Element("Fax").Value;
                contact.Email = i.Element("Email").Value;

                contacts.Add(contact);
            }

            return contacts;
        }

        public Contact GetContactById(int id)
        {
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            XElement xmlContact = xmlDocument.Element("Contacts").Elements("Contact")
                .Where(a => a.Attribute("Id").Value == id.ToString()).FirstOrDefault();

            Contact contact = new Contact();


            contact.Id = int.Parse(xmlContact.Attribute("Id").Value);
            contact.FirstName = xmlContact.Element("FirstName").Value;
            contact.LastName = xmlContact.Element("LastName").Value;
            contact.Company = xmlContact.Element("Company").Value;
            contact.City = xmlContact.Element("City").Value;
            contact.Telephone = xmlContact.Element("Telephone").Value;
            contact.Fax = xmlContact.Element("Fax").Value;
            contact.Email = xmlContact.Element("Email").Value;

            return contact;
        }

        public void AddContact(Contact contact)
        {
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            //将值插入联系人xml文档
            xmlDocument.Element("Contacts").Add(
                new XElement("Contact",new XAttribute("Id",contact.Id),new XElement("FirstName",contact.FirstName),new XElement("LastName",contact.LastName),new XElement("Company",contact.Company),new XElement("City",contact.City),new XElement("Telephone",contact.Telephone),new XElement("Fax",contact.Fax),new XElement("Email",contact.Email)
                    ));

            xmlDocument.Save("Contacts.xml");
        }

        public void UpdateContact(Contact contact)
        {
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            //通过ID更新联系人
            XElement xmlContact = xmlDocument.Element("Contacts").Elements("Contact")
                .Where(x => x.Attribute("Id").Value == contact.Id.ToString())
                .FirstOrDefault();

            //更新联系人
            xmlContact.SetElementValue("FirstName",contact.FirstName);
            xmlContact.SetElementValue("LastName",contact.LastName);
            xmlContact.SetElementValue("Company",contact.Company);
            xmlContact.SetElementValue("City",contact.City);
            xmlContact.SetElementValue("Telephone",contact.Telephone);
            xmlContact.SetElementValue("Fax",contact.Fax);
            xmlContact.SetElementValue("Email",contact.Email);

            xmlDocument.Save("Contacts.xml");
        }

        public void DeleteContact(int id)
        {
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            //得到联系人并删除他
            xmlDocument.Root.Elements().Where(x => x.Attribute("Id").Value == id.ToString())
                .FirstOrDefault().Remove();

            xmlDocument.Save("Contacts.xml");
        }

        public int GetNextId()
        {
            int currentId;
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            //获取id的集合
            IEnumerable<int> contactIds = xmlDocument.Element("Contacts").Elements("Contact")
                .Select(x => int.Parse(x.Attribute("Id").Value));

            //如果集合为空，则将变量设置为0，否则将该变量设置为最高ID值
            if (contactIds.Count() == 0)
                currentId = 0;
            else
                currentId = contactIds.Max();

            return currentId + 1;
        }

        public void SetXmlFile()
        {
            //如果xml文件丢失，则使用根元素创建它
            if (!(File.Exists("Contacts.xml")))
            {
                XDocument xmlDocument = new XDocument(
                    new XDeclaration("1.0","utf-8","yes"),new XElement("Contacts"));

                xmlDocument.Save("Contacts.xml");
            }
        }

        public bool IdExists(int id)
        {
            XDocument xmlDocument = XDocument.Load("Contacts.xml");

            return xmlDocument.Element("Contacts").Elements("Contact")
                .Select(x => int.Parse(x.Attribute("Id").Value))
                .Where(y => y == id).Count() > 0;
        }
    }
}

主窗体frmMain.cs

public partial class frmMain : Form
    {
        LinqXmlContactsBLManager blManager = new LinqXmlContactsBLManager();

        public frmMain()
        {
            InitializeComponent();

            //如果xml文件丢失，创建它
            blManager.SetXmlFile();
            dataGridView1.DataSource = blManager.GetAllContacts();
        }

        private void btnViewContact_Click(object sender,EventArgs e)
        {
            if (txtId.Text == "")
            {
                MessageBox.Show("请输入ID","消息");
                txtId.Focus();
                return;
            }
            int x;
            if (!int.TryParse(txtId.Text,out x))
            {
                MessageBox.Show("请输入有效的ID","消息");
                txtId.Focus();
                txtId.SelectAll();
                return;
            }
            if (!blManager.IdExists(int.Parse(txtId.Text)))
            {
                MessageBox.Show("id " + txtId.Text + " 不存在","消息");
                txtId.Focus();
                txtId.SelectAll();
                return;
            }

            //通过id获得一个联系人
            Contact contact = blManager.GetContactById(int.Parse(txtId.Text));

            //show the contact properties on the controls
            lblId.Text = contact.Id.ToString();
            txtFirstName.Text = contact.FirstName;
            txtLastName.Text = contact.LastName;
            txtCompany.Text = contact.Company;
            txtCity.Text = contact.City;
            txtTelephone.Text = contact.Telephone;
            txtFax.Text = contact.Fax;
            txtEmail.Text = contact.Email;

            txtId.Clear();
        }

        private void btnNewContact_Click(object sender,EventArgs e)
        {
            frmAdd frmAdd1 = new frmAdd();
            frmAdd1.ShowDialog();
            dataGridView1.DataSource = blManager.GetAllContacts();
        }

        private void btnUpdateContact_Click(object sender,EventArgs e)
        {
            if (lblId.Text == "")
            {
                MessageBox.Show("未选择联系人","消息");
                txtId.Focus();
                txtId.SelectAll();
                return;
            }
            if (txtFirstName.Text == "")
            {
                MessageBox.Show("请输入姓氏","消息");
                txtFirstName.Focus();
                return;
            }
            if (txtLastName.Text == "")
            {
                MessageBox.Show("请输入名","消息");
                txtLastName.Focus();
                return;
            }
            if (txtCompany.Text == "")
            {
                MessageBox.Show("请输入公司","消息");
                txtCompany.Focus();
                return;
            }
            if (txtCity.Text == "")
            {
                MessageBox.Show("请输入城市","消息");
                txtCity.Focus();
                return;
            }
            if (txtTelephone.Text == "")
            {
                MessageBox.Show("请输入电话","消息");
                txtTelephone.Focus();
                return;
            }
            if (txtFax.Text == "")
            {
                MessageBox.Show("请输入传真","消息");
                txtFax.Focus();
                return;
            }
            if (txtEmail.Text == "")
            {
                MessageBox.Show("请输入邮件","消息");
                txtEmail.Focus();
                return;
            }

            //创建一个联系人，并从控件值设置他的属性
            Contact contact = new Contact()
            {
                Id = int.Parse(lblId.Text),FirstName = txtFirstName.Text,LastName = txtLastName.Text,Company = txtCompany.Text,City = txtCity.Text,Telephone = txtTelephone.Text,Fax = txtFax.Text,Email = txtEmail.Text
            };

            //更新联系人
            blManager.UpdateContact(contact);
            dataGridView1.DataSource = blManager.GetAllContacts();
        }

        private void btnDeleteContact_Click(object sender,"消息");
                txtId.Focus();
                txtId.SelectAll();
                return;
            }

            Contact contact = blManager.GetContactById(int.Parse(lblId.Text));

            string messageToPrint = "The following contact will get deleted:nn" +
                "Id : " + contact.Id.ToString() + "n" +
                "First name : " + contact.FirstName + "n" +
                "Last name : " + contact.LastName + "n" +
                "Company : " + contact.Company + "n" +
                "City : " + contact.City + "n" +
                "Telephone : " + contact.Telephone + "n" +
                "Fax : " + contact.Fax + "n" +
                "Email : " + contact.Email + "nn" +
                "Are you sure?";

            DialogResult dialogResult = MessageBox.Show(messageToPrint,"Message",MessageBoxButtons.YesNo);

            //如果用户点击“否”，则取消删除操作
            if (dialogResult == DialogResult.No)
            {
                MessageBox.Show("操作被取消","消息");
                return;
            }

            //如果用户点击“是”，删除联系人
            blManager.DeleteContact(contact.Id);

            dataGridView1.DataSource = blManager.GetAllContacts();

            //清除控件值
            lblId.Text = "";
            txtFirstName.Clear();
            txtLastName.Clear();
            txtCompany.Clear();
            txtCity.Clear();
            txtTelephone.Clear();
            txtFax.Clear();
            txtEmail.Clear();
        }

        private void btnClose_Click(object sender,EventArgs e)
        {
            Application.Exit();
        }
    }

增加窗体frmAdd.cs

public partial class frmAdd : Form
    {
        LinqXmlContactsBLManager blManager = new LinqXmlContactsBLManager();

        public frmAdd()
        {
            InitializeComponent();
        }

        private void btnAdd_Click(object sender,EventArgs e)
        {
            if (txtFirstName.Text == "")
            {
                MessageBox.Show("请输入姓氏","消息");
                txtFirstName.Focus();
                return;
            }
            if (txtLastName.Text == "")
            {
                MessageBox.Show("请输入名字","消息");
                txtLastName.Focus();
                return;
            }
            if (txtCompany.Text == "")
            {
                MessageBox.Show("请输入公司","消息");
                txtCompany.Focus();
                return;
            }
            if (txtCity.Text == "")
            {
                MessageBox.Show("请输入城市","消息");
                txtCity.Focus();
                return;
            }
            if (txtTelephone.Text == "")
            {
                MessageBox.Show("请输入电话","消息");
                txtTelephone.Focus();
                return;
            }
            if (txtFax.Text == "")
            {
                MessageBox.Show("请输入传真","消息");
                txtFax.Focus();
                return;
            }
            if (txtEmail.Text == "")
            {
                MessageBox.Show("请输入电子邮件","消息");
                txtEmail.Focus();
                return;
            }

            //创建联系人,并从控件中为属性设置值
            int id = blManager.GetNextId();
            Contact contact = new Contact()
            {
                Id = id,FirstName = txtFirstName.Text,LastName = txtLastName.Text,Company = txtCompany.Text,City = txtCity.Text,Telephone = txtTelephone.Text,Fax = txtFax.Text,Email = txtEmail.Text
            };

            //添加联系人
            blManager.AddContact(contact);

            //清除控件值
            txtFirstName.Clear();
            txtLastName.Clear();
            txtCompany.Clear();
            txtCity.Clear();
            txtTelephone.Clear();
            txtFax.Clear();
            txtEmail.Clear();

            txtFirstName.Focus();
        }

        private void btnClose_Click(object sender,EventArgs e)
        {
            Close();
        }
    }

运行结果如图: