Introduction to Derived Properties

What is a derived property?

A derived property is a property that does not exist on the EntityType associated with the EntitySet; rather it exists on a type that derives from the base type of the entity set. This feature has been among our customers top asks since the lack of support makes exposing models with rich, well-defined inheritance hierarchies impossible to do. To enable such scenarios WCF Data Services now supports both exposing and consuming models which have properties (primitive, complex & navigation) defined on subtypes of the base type associated with the set

How can I specify derived properties in CSDL?

As you know, WCF Data Services exposes a metadata document ($metadata endpoint) which describes the data model exposed by the service.  Below is an example of a metadata document that defines derived properties. No new extensions have been added to support this functionality. Protocol changes need to support derived properties can be found here

How are derived properties used on the client?

A representation of the above example in a .NET type would be:

public class Person

{

    public int ID { get; set; }

public string FirstName{ get; set; }

public string LastName{ get; set; }

}

public class Employee:Person

{

public string Building{get;set;}

public IList<Employee> Resports { get; set; }

public Employee Manager { get; set; }

}

Two new Linq operators are now supported on the client:

- OfType<T>:  used when the type specification needs to happen in the path segment.

- As: used when the type appears in a query option (e.g. $filter, $select, $orderby)

Below are some examples of how to query derived properties on the client:

PeopleData ctx = newPeopleData(serviceUri);

// returns only Employee instances

foreach (Employee e in  ctx.People.OfType<Employee>() )

{

. . .

}

// returns a narrow projection that mixes content from Person and Employee

var q = from c in ctx.People

select new EmployeeBuilding { Name = c.FirstName, Building = (c as Employee).Building };

foreach (EmployeeBuilding s in q)

{

               . . .

}

// returns only entities that are Employees working in building 18

var q2 = from p in ctx.People

where (p.FirstName==”Bill” && ((p as Employee).Building == "18"))

select p;

foreach (Employee e in q2)

{

. . .

}

// returns entities of type Person, expanding derived Reports navigation property on the employee instances

foreach (Person p in ctx.People.Expand(p => (p as Employee).Reports))

{

. . .

}

Your feedback is appreciated.

Ahmed Moustafa
Program Manager
WCF Data Services

Comments

• Anonymous
  March 21, 2011
  will it support the use of only keyword to return only instances of that type. Ef supports that since version 1. msdn.microsoft.com/.../bb399295.aspx

• Anonymous
  March 27, 2011
  Hello, I'm experiencing an error when using the OfType<> method. I get a DataServiceQueryException with the message "An error occurred while processing this request." The inner exception mentions not being able to find the specified resource. It seems to occur any time I use the OfType<> method in a client querying a WCF Data Service. Any ideas on what I'm doing wrong? Thanks.

• Anonymous
  June 20, 2011
  Michael, Did you ever solve this as I have teh same issue with CTP2 March 2011?

• Anonymous
  November 01, 2011
  Michael and Ian, Did any of you find a solution for this problem? I'm using October 2011 CTP and it still has the "OfType" issue.

• Anonymous
  March 15, 2012
  How can I use this with DataServiceQuery.AddQueryOption  from silverlight?