[ServiceContract]
public interface IService
{
    [OperationContract]
    void Initialize();

    [OperationContract]
    Settings GetSettings(); // returns custom type Settings

    [OperationContract]
    void SetSettings(Settings settings);

    [OperationContract]
    bool SettingsAccepted();
}

[DataContract]
public enum ServiceStatus
{
    [EnumMember]
    NormalWork = 0,

    [EnumMember]
    TimeOut,

    [EnumMember]
    DenialOfService
};

[ServiceContract]
public interface IService
{
    [OperationContract]
    (ServiceStatus) Initialize();

    [OperationContract]
    (ServiceStatus, Settings) GetSettings();

    [OperationContract]
    (ServiceStatus) SetSettings(Settings settings);

    [OperationContract]
    (ServiceStatus, bool) SettingsAccepted();
}

[DataContract]
public class ServiceResponse<T>
{
  [DataMember]
  ServiceStatus Status { get; set; }

  [DataMember]
  T Payload { get; set; }
}

[OperationContract]
ServiceResponse<bool> SettingsAccepted();

[DataContract]
public class ServiceResultBase
{
	[DataMember]
	public ServiceStatus Status { get; set; }
}

[DataContract]
public class ServiceResult_Settings : ServiceResultBase
{
	[DataMember]
	public Settings Settings { get; set; }
}

[DataContract]
public class ServiceResult_SettingsAccepted : ServiceResultBase
{
	[DataMember]
	public bool SettingsAccepted { get; set; }
}

Generic types are handled in exactly the same way as non-generic types. There are no special requirements for generic parameters. For example, consider the following type.