Linq to sql增删改查

private void button2_Click(object sender, EventArgs e)//增
{
try
{
Customers ct = new Customers();
ct.CustomerID = "test";
ct.CompanyName = "test";
NorthwindDataContext nc = new NorthwindDataContext();
nc.Customers.InsertOnSubmit(ct);
nc.SubmitChanges();
MessageBox.Show("更新成功");
}
catch (Exception e1)
{
MessageBox.Show(e1.ToString());
}
}
private void button3_Click(object sender, EventArgs e)//查
{
NorthwindDataContext nt = new NorthwindDataContext();
var a = from s in nt.Customers where s.CustomerID == "test" select new {s.CompanyName,s.ContactName,s.ContactTitle };
foreach (var b in a)
{
MessageBox.Show(b.ToString());
}

}
private void button4_Click(object sender, EventArgs e)//改
{
try
{
NorthwindDataContext nw = new NorthwindDataContext();
var cityNameQuery =
from cust in nw.Customers
where cust.City.Contains("London")
select cust;
foreach (var customer in cityNameQuery)
{
if (customer.City == "London - Metro")
{
customer.City = "London";
}
}
nw.SubmitChanges();
MessageBox.Show("成功");
}
catch (Exception e1)
{
MessageBox.Show(e1.ToString());
}
}
private void button5_Click(object sender, EventArgs e)//删
{
NorthwindDataContext nw = new NorthwindDataContext();
var a = from s in nw.Customers where s.CustomerID == "jj