Is possible in WCF service: method return IList<object> when object can be Person class?

Question

Is possible in WCF service: method return IList when object can be Person class?

Ex: in IServiceContract.cs

[ServiceContract]
public interface IDAS
{
    [OperationContract]
    void DoWork();

    [OperationContract]
    List<object> GetAnyClass();

}

And class:

public class DAS : IDAS
{
    public void DoWork()
    {
    }

    public List<object> GetAnyClass()
    {
        List<Person> a = new List<Person>();
        a.Add(new Person());
        return a;
    }
}

The problem at runtime is:

System.ServiceModel.CommunicationException: The server did not provide a meaningful reply; this might be caused by a contract mismatch, a premature session shutdown or an internal server error

Answer 1

Theoretically yes, although you need to tell the service that it might be expecting a Person object using the KnownTypeAttribute on your method.

[OperationContract]
[KnownType(typeof(Person))]
List<object> GetAnyClass();

I would really think twice about doing this in practice though - instead declare different method signatures for the objects you're expecting to return:

[OperationContract]
IList<Person> GetPeople();

[OperationContract]
Person GetPerson();

[OperationContract]
IList<Book> GetBooks();

[OperationContract]
Book GetBook();

etc.

It's supposed to be a contract, i.e. concrete, so if you suddenly change the type of class you return it can really mess the clients up.

Also in your example you were returning a concrete List class - this should be avoided, instead use either IList<> or Collection<>.

Answer 2

1. Yes it is possible, you need to update the reference in Visual Studio (or whatever you are using to generate the proxy class with) and change the collection type returned. There is an option in 'Configure Service Reference' and you can select Generic.List in there (right click your WCF service reference).
2. The mismatch is because you have changed your service on the server end and not got a new proxy. So change it to return a Generic.List and then regenerate using the steps in 1.

Hope that helps

Ryan

Answer 3

You can return an IList but it's definitly not a good approach to take.

When you expose your services you need people at the other end of the service to know what they are getting.

IList<Person> would be clearer for everybody that use the services or that code in the services.

If you need a method that can return different type of object just split them out in multiple operations.

IList<Person> GetPersons(...)
IList<Animal> GetAnimals(...)

My 2 cents.

Answer 4

Cheva (et al),

There is nothing stopping you from calling a single method to fill in the collection(s) you return from the service calls.

IList<Person> GetPersons(...)
IList<Animal> GetAnimals(...)

Both GetPersons() and GetAnimals() can certainly call an internal method e.g.

  IList<Animal> GetAnimals(...)
  {
    // get list of objects of a given type
    internalIList<Object> genericResults = GetItems( 
        ItemType.Persons|ItemType.Animals );

    ...

    IList<Animal> results;

    // convert to specific type
    results = new IList<Animal>(genericResults);

    return results;
  }

That should work, but I didn't test it or anything. YMMV. ; )

-Scott