The following is the interface that you want define. [ServiceContract] public interface IMyServiceContract ...{ [OperationContract] string GetData(int value); [OperationContract] CompositeType GetDataUsingDataContract(CompositeType composite); // TODO: Add your service operations here } Now this is the consistent class for this interface. public class Service1 : IMyServiceContract ...{ public string GetData(int value) ...{ return string.Format("You entered: {0}", value); } public CompositeType GetDataUsingDataContract(CompositeType composite) ...{ if (composite.BoolValue) ...{ composite.StringValue += "Suffix"; } return composite; } }