csharp #LINQ到#XPO

Posted 2021-05-12

using System.Linq;
using DevExpress.Xpo;

XPQuery<Customer> customers = Session.DefaultSession.Query<Customer>();
// Equivalent definition. 
// XPQuery<Customer> customers = new XPQuery<Customer>(Session.DefaultSession); 

foreach (Customer cust in customers)
    Console.WriteLine(string.Format("{0}", cust.ContactName));
In addition to calling the DevExpress.Xpo.XPQueryExtensions.Query extension method, you can call the DevExpress.Xpo.XPQueryExtensions.QueryInTransaction``1 method to include all in-memory object changes to query results (as if Session.InTransactionMode is enabled). To create an InTransaction XPQuery<T> instance based on an existing XPQuery<T> instance, call its XPQuery<T>.InTransaction method.

 Expanded Sample Expressions
C#
VB
using System.Linq;
using System.Linq.Expressions;
using DevExpress.Xpo;

XPQuery<Customer> customers = Session.DefaultSession.Query<Customer>();
XPQuery<Order> orders = Session.DefaultSession.Query<Order>();
XPQuery<Employee> employees = Session.DefaultSession.Query<Employee>();

// Simple Select with Where and OrderBy clauses 
var list = from c in customers
           where (c.Country == "Germany" && c.ContactTitle == "Sales Representative")
           orderby c.ContactName
           select c;
foreach (Customer cust in list)
    Console.WriteLine(string.Format("{0}\t{1}\t{2}", cust.ContactName,
        cust.Country, cust.ContactTitle));

// Select Top 5 objects 
var list = (from o in orders
            orderby o.ShippedDate descending
            select o).Take(5);
foreach (Order order in list)
    Console.WriteLine(string.Format("{0}\t{1}\t{2}", order.OrderID, order.ShippedDate,
        order.Customer.CompanyName));

// Group Join customers with an aggregation on their Orders 
var list = from c in customers
           join o in orders on c equals o.Customer into oo
           where oo.Count() >= 1
           select new { c.CompanyName, OrderCount = oo.Count() };
foreach (var item in list)
    Console.WriteLine(string.Format("{0}\t{1}", item.CompanyName, item.OrderCount));

// An example of aggregated functions (Count and Average) 
var list = from o in orders
           select o;
int count = list.Count();
Console.WriteLine(string.Format("Orders Row Count: {0}", count));

decimal avg = list.Average(x => x.Freight);
Console.WriteLine(string.Format("Orders Average Freight: {0:c2}", avg));

// Select with Group By 
var list = from c in customers
           group c by c.ContactTitle into cc
           where cc.Count() >= 1
           select new { Title = cc.Key, Count = cc.Count() };
foreach (var item in list)
    Console.WriteLine(string.Format("{0}\t{1}", item.Title, item.Count));

// Any method  
bool result = customers.Any(c => c.Country == "Spain");
Console.WriteLine(string.Format("Is there any customer from Spain? {0}", result ? "Yes" : "No"));

result = customers.Any(c => c.Country == "Monaco");
Console.WriteLine(string.Format("Is there any customer from Monaco? {0}", result ? "Yes" : "No"));
Note
You can find a sample project in the LINQ to XPO Knowledge Base article. You should have Visual Studio 2008 and SQL Server 2005 or 2000 with the Northwind demo database.

To learn how to implement custom functions and criteria, and use them in LINQ to XPO expressions, see How to: Implement Custom Functions and Criteria in LINQ to XPO.

 Expanded LINQ to XPO Limitations
The maximum number of total or group summaries allowed is 14.
The FirstOrDefault method is not supported for property initialization in the Select statement.
Cross Join queries are not supported.