How to: Project an Anonymous Type

In some cases you might want to project a query to a new type, even though you know you will only use this type for a short while. It is a lot of extra work to create a new type just to use in the projection. A more efficient approach in this case is to project to an anonymous type. Anonymous types allow you to define a class, then declare and initialize an object of that class, without giving the class a name.

Anonymous types are the C# implementation of the mathematical concept of a tuple. The mathematical term tuple originated from the sequence single, double, triple, quadruple, quintuple, n-tuple. It refers to a finite sequence of objects, each of a specific type. Sometimes this is called a list of name/value pairs. For example, the contents of an address in the Typical Purchase Order XML document could be expressed as follows:

Name: Ellen Adams
Street: 123 Maple Street
City: Mill Valley
State: CA
Zip: 90952
Country: USA

When you create an instance of an anonymous type, it is convenient to think of it as creating a tuple of order n. If you write a query that creates a tuple in the select clause, the query returns an IEnumerable of the tuple.

Example

In this example, the select clause projects an anonymous type. The example then uses var to create the IEnumerable object. Within the foreach loop, the iteration variable becomes an instance of the anonymous type created in the query expression.

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

XElement custOrd = XElement.Load("CustomersOrders.xml");
var custList =
    from el in custOrd.Element("Customers").Elements("Customer")
    select new {
        CustomerID = (string)el.Attribute("CustomerID"),
        CompanyName = (string)el.Element("CompanyName"),
        ContactName = (string)el.Element("ContactName")
    };
foreach (var cust in custList)
    Console.WriteLine("{0}:{1}:{2}", cust.CustomerID, cust.CompanyName, cust.ContactName);

Dim custOrd As XElement = XElement.Load("CustomersOrders.xml")
Dim custList = _
    From el In custOrd.<Customers>.<Customer> _
    Select New With { _
        .CustomerID = el.@<CustomerID>, _
        .CompanyName = el.<CompanyName>.Value, _
        .ContactName = el.<ContactName>.Value _
    }
For Each cust In custList
    Console.WriteLine("{0}:{1}:{2}", cust.CustomerID, cust.CompanyName, cust.ContactName)
Next

This code produces the following output:

GREAL:Great Lakes Food Market:Howard Snyder
HUNGC:Hungry Coyote Import Store:Yoshi Latimer
LAZYK:Lazy K Kountry Store:John Steel
LETSS:Let's Stop N Shop:Jaime Yorres

See Also

Concepts

Projections and Transformations (LINQ to XML)

Build Date:

2012-08-02