WCF service example with DTOs per request/response pair.

IRegistrator.cs

[ServiceContract]
public interface IRegistrator
{
        [OperationContract]
        RegisterResponse Register(RegisterRequest request);
}

RegisterRequest.cs

[DataContract]
public class RegisterRequest
{
	[DataMember]
	public string Username { get; set; }

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

RegisterResponse.cs

[DataContract]
public class RegisterResponse
{
	[DataMember]
	public Guid UserId { get; set; }
}

Registrator.cs

public class Registrator : IRegistrator
{
	public RegisterResponse Register(RegisterRequest request)
	{
		throw new NotImplementedException();
	}
}