How to: Join Two Collections (LINQ to XML) (C#)

An element or attribute in an XML document can sometimes refer to another element or attribute. For example, the Sample XML File: Customers and Orders (LINQ to XML) XML document contains a list of customers and a list of orders. Each Customer element contains a CustomerID attribute. Each Order element contains a CustomerID element. The CustomerID element in each order refers to the CustomerID attribute in a customer.

The topic Sample XSD File: Customers and Orders contains an XSD that can be used to validate this document. It uses the xs:key and xs:keyref features of XSD to establish that the CustomerID attribute of the Customer element is a key, and to establish a relationship between the CustomerID element in each Order element and the CustomerID attribute in each Customer element.

With LINQ to XML, you can take advantage of this relationship by using the join clause.

Note that because there is no index available, such joining will have poor runtime performance.

For more detailed information about join, see Join Operations (C#).

Example

The following example joins the Customer elements to the Order elements, and generates a new XML document that includes the CompanyName element in the orders.

Before executing the query, the example validates that the document complies with the schema in Sample XSD File: Customers and Orders. This ensures that the join clause will always work.

This query first retrieves all Customer elements, and then joins them to the Order elements. It selects only the orders for customers with a CustomerID greater than "K". It then projects a new Order element that contains the customer information within each order.

This example uses the following XML document: Sample XML File: Customers and Orders (LINQ to XML).

This example uses the following XSD schema: Sample XSD File: Customers and Orders.

Note that joining in this fashion will not perform very well. Joins are performed via a linear search. There are no hash tables or indexes to help with performance.

XmlSchemaSet schemas = new XmlSchemaSet();  
schemas.Add("", "CustomersOrders.xsd");  
  
Console.Write("Attempting to validate, ");  
XDocument custOrdDoc = XDocument.Load("CustomersOrders.xml");  
  
bool errors = false;  
custOrdDoc.Validate(schemas, (o, e) =>  
                     {  
                         Console.WriteLine("{0}", e.Message);  
                         errors = true;  
                     });  
Console.WriteLine("custOrdDoc {0}", errors ? "did not validate" : "validated");  
  
if (!errors)  
{  
    // Join customers and orders, and create a new XML document with  
    // a different shape.  
  
    // The new document contains orders only for customers with a  
    // CustomerID > 'K'  
    XElement custOrd = custOrdDoc.Element("Root");  
    XElement newCustOrd = new XElement("Root",  
        from c in custOrd.Element("Customers").Elements("Customer")  
        join o in custOrd.Element("Orders").Elements("Order")  
                   on (string)c.Attribute("CustomerID") equals  
                      (string)o.Element("CustomerID")  
        where ((string)c.Attribute("CustomerID")).CompareTo("K") > 0  
        select new XElement("Order",  
            new XElement("CustomerID", (string)c.Attribute("CustomerID")),  
            new XElement("CompanyName", (string)c.Element("CompanyName")),  
            new XElement("ContactName", (string)c.Element("ContactName")),  
            new XElement("EmployeeID", (string)o.Element("EmployeeID")),  
            new XElement("OrderDate", (DateTime)o.Element("OrderDate"))  
        )  
    );  
    Console.WriteLine(newCustOrd);  
}  

This code produces the following output:

Attempting to validate, custOrdDoc validated  
<Root>  
  <Order>  
    <CustomerID>LAZYK</CustomerID>  
    <CompanyName>Lazy K Kountry Store</CompanyName>  
    <ContactName>John Steel</ContactName>  
    <EmployeeID>1</EmployeeID>  
    <OrderDate>1997-03-21T00:00:00</OrderDate>  
  </Order>  
  <Order>  
    <CustomerID>LAZYK</CustomerID>  
    <CompanyName>Lazy K Kountry Store</CompanyName>  
    <ContactName>John Steel</ContactName>  
    <EmployeeID>8</EmployeeID>  
    <OrderDate>1997-05-22T00:00:00</OrderDate>  
  </Order>  
  <Order>  
    <CustomerID>LETSS</CustomerID>  
    <CompanyName>Let's Stop N Shop</CompanyName>  
    <ContactName>Jaime Yorres</ContactName>  
    <EmployeeID>1</EmployeeID>  
    <OrderDate>1997-06-25T00:00:00</OrderDate>  
  </Order>  
  <Order>  
    <CustomerID>LETSS</CustomerID>  
    <CompanyName>Let's Stop N Shop</CompanyName>  
    <ContactName>Jaime Yorres</ContactName>  
    <EmployeeID>8</EmployeeID>  
    <OrderDate>1997-10-27T00:00:00</OrderDate>  
  </Order>  
  <Order>  
    <CustomerID>LETSS</CustomerID>  
    <CompanyName>Let's Stop N Shop</CompanyName>  
    <ContactName>Jaime Yorres</ContactName>  
    <EmployeeID>6</EmployeeID>  
    <OrderDate>1997-11-10T00:00:00</OrderDate>  
  </Order>  
  <Order>  
    <CustomerID>LETSS</CustomerID>  
    <CompanyName>Let's Stop N Shop</CompanyName>  
    <ContactName>Jaime Yorres</ContactName>  
    <EmployeeID>4</EmployeeID>  
    <OrderDate>1998-02-12T00:00:00</OrderDate>  
  </Order>  
</Root>