LINQ to Entities，基于方法查询，查询表达式语法

初次学习LINQ to Entities，给出两种查询的示例代码。

基于方法的查询：

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet<Contact> contacts = AWEntities.Contacts;
    ObjectSet<SalesOrderHeader> orders = AWEntities.SalesOrderHeaders;

    var query =
    contacts.SelectMany(
        contact => orders.Where(order =>
            (contact.ContactID == order.Contact.ContactID)
                && order.OrderDate >= new DateTime(2002, 10, 1))
            .Select(order => new
            {
                ContactID = contact.ContactID,
                LastName = contact.LastName,
                FirstName = contact.FirstName,
                OrderID = order.SalesOrderID,
                OrderDate = order.OrderDate
            }));

    foreach (var order in query)
    {
        Console.WriteLine("Contact ID: {0} Name: {1}, {2} Order ID: {3} Order date: {4:d} ",
            order.ContactID, order.LastName, order.FirstName,
            order.OrderID, order.OrderDate);
    }
}

查询表达式：

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet<Contact> contacts = AWEntities.Contacts;
    ObjectSet<SalesOrderHeader> orders = AWEntities.SalesOrderHeaders;

    var query =
        from contact in contacts
        from order in orders
        let total = order.TotalDue
        where contact.ContactID == order.Contact.ContactID
            && total >= 10000.0M
        select new
        {
            ContactID = contact.ContactID,
            LastName = contact.LastName,
            OrderID = order.SalesOrderID,
            total
        };

    foreach (var order in query)
    {
        Console.WriteLine("Contact ID: {0} Last name: {1} Order ID: {2} Total: {3}",
            order.ContactID, order.LastName, order.OrderID, order.total);
    }
}