Am I correct in thinking that if I have a WCF OperationContract takes in an object and needs to set a property on that object so the client gets the update, I need to declare it to return the object.

e.g. given a datacontract:

[DataContract]
public class CompositeType
{
    [DataMember]
    public int Key { get; set; }

    [DataMember]
    public string Something { get; set; }
}

this will not work with WCF:

public void GetDataUsingDataContract(CompositeType composite)
{
  composite.Key = 42;              
}

this will work:

public CompositeType GetDataUsingDataContract(CompositeType composite)
{
  composite.Key = 42;

  return new CompositeType
  {
    Key = composite.Key,
    Something = composite.Something
  };           
}