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Using interfaces in models with SQLite

Let's say I have an interface like this:
public interface IUser
{
int Id { get; }
string Name { get; }
List<IMonthlyBudget> MonthlyBudget { get; }
}
and then I have a model that implements this:
public class User : IUser
{
public int Id { get; set; }
public string Name { get; set; }
public List<IMonthlyBudget> MonthlyBudget { get; set; }
}
and here I have the IMonthlyBudget:
public interface IMonthlyBudget
{
int Id { get; }
float MonthlyMax { get; }
float CurrentSpending { get; }
float MonthlyIncome { get; }
}
Now I have my models. But the issue comes with using SQLite. SQLite can't understand what is the real implementation of IMonthlyBudget. I understand why, but I really don't want remove the interface and expose the real implementation to all the clients that use these models. In my project structure I have a Core project that has all the model interfaces, and the model implementation are in a data access project.
Is there something wrong with how I'm approaching this problem? I assume i'm not the first one to run into a issue like this. Isn't it completely normal practice to keep model interfaces (what repositories etc then use as their return types, parameters and stuff like that) and implement the actual concrete models in a data access project?
And can someone explain why I can't do this:
public class User : IUser
{
public int Id { get; set; }
public string Name { get; set; }
public List<MonthlyBudget> MonthlyBudget { get; set; }
}
MonthlyBudget implements IMonthlyBudget, shouldn't it be completely fine to use the concrete model as the type instead of the the interface when the concrete model actually implements the interface?

A few questions here, so I'll break it down into sections:
Use of Interfaces
It is definitely good practice to interface classes that perform operations. For example, you may have a data service (i.e. data access layer) interface that allows you to do operations to read and modify data in your persistent store. However, you may have several implementations of that data service. One implementation may save to the file system, another to a DBMS, another is a mock for unit testing, etc.
However, in many cases you do not need to interface your model classes. If you're using an anemic business object approach (as opposed to rich business objects), then model classes in general should just be containers for data, or Plain Old CLR Objects (POCO). Meaning these objects don't have any real functionality to speak of and they don't reference any special libraries or classes. The only "functionality" I would put in a POCO is one that is dependent only upon itself. For example, if you have a User object that has a FirstName and LastName property, you could create a read-only property called FullName that returns a concatenation of the two.
POCOs are agnostic as to how they are populated and therefore can be utilized in any implementation of your data service.
This should be your default direction when using an anemic business object approach, but there is at least one exception I can think of where you may want to interface your models. You may want to support for example a SQLite data service, and a Realm (NoSQL) data service. Realm objects happen to require your models to derive from RealmObject. So, if you wanted to switch your data access layer between SQLite and Realm then you would have to interface your models as you are doing. I'm just using Realm as an example, but this would also hold true if you wanted to utilize your models across other platforms, like creating an observable base class in a UWP app for example.
The key litmus test to determining whether you should create interfaces for your models is to ask yourself this question:
"Will I need to consume these models in various consumers and will those consumers require me to define a specific base class for my models to work properly in those consumers?"
If the answer to this is "yes", then you should make interfaces for your models. If the answer is "no", then creating model interfaces is extraneous work and you can forego it and let your data service implementations deal with the specifics of their underlying data stores.
SQLite Issue
Whether you continue to use model interfaces or not, you should still have a data access implementation for SQLite which knows that it's dealing with SQLite-specific models and then you can do all your CRUD operations directly on those specific implementations of your model. Then since you're referring to a specific model implementation, SQLite should work as usual.
Type Compatibility
To answer your final question the type system does not see this...
List<IMonthlyBudget> MonthlyBudget
as being type-compatible with this...
List<MonthlyBudget> MonthlyBudget
In our minds it seems like if I have a list of apples, then it should be type-compatible with a list of fruit. The compiler sees an apple as a type of fruit, but not a list of apples as a type of a list of fruit. So you can't cast between them like this...
List<IMonthlyBudget> myMonthlyBudget = (List<IMonthlyBudget>) new List<MonthlyBudget>();
but you CAN add a MonthlyBudget object to a list of IMonthlyBudget objects like this...
List<IMonthlyBudget> myMonthlyBudget = new List<IMonthlyBudget>();
myMonthlyBudget.Add(new MonthlyBudget());
Also you can use the LINQ .Cast() method if you want to cast an entire list at once.
The reason behind this has to do with type variance. There's a good article on it here that can shed some light as to why:
Covariance and Contravariance
I hope that helps! :-)

“Do not use Abstract Base class in Design; but in Modeling/Analysis”

I am newbie to SOA though I have some experience in OOAD.
One of the guidelines for SOA design is “Use Abstract Classes for Modeling only. Omit them from Design”. The use of abstraction can be helpful in modeling (analysis phase).
During analysis phase I have come up with a BankAccount base class. The specialized classes derived from it are “FixedAccount” and “SavingsAccount”. I need to create a service that will return all accounts (list of accounts) for a user. What should be the structure of service(s) to meet the requirement?
Note: It would be great if you can provide code demonstration using WCF.

It sounds like you are trying to use SOA to remotely access your object model. You would be better of looking at the interactions and capabilities you want your service to expose and avoid exposing inheritance details of your services implementation.
So in this instance where you need a list of user accounts your interface would look something like
[ServiceContract]
interface ISomeService
{
[OperationContract]
Collection<AccountSummary> ListAccountsForUser(
User user /*This information could be out of band in a claim*/);
}
[DataContract]
class AccountSummary
{
[DataMember]
public string AccountNumber {get;set;}
[DataMember]
public string AccountType {get;set;}
//Other account summary information
}
if you do decide to go down the inheritance route, you can use the KnownType attribute, but be aware that this will add some type information into the message being sent across the wire which may limit your interoperability in some cases.
Update:
I was a bit limited for time earlier when I answered, so I'll try and elaborate on why I prefer this style.
I would not advise exposing your OOAD via DTOs in a seperate layer this usually leads to a bloated interface where you pass around a lot of data that isn't used and religously map it into and out of what is essentially a copy of your domain model with all the logic deleted, and I just don't see the value. I usually design my service layer around the operations that it exposes and I use DTOs for the definition of the service interactions.
Using DTOs based on exposed operations and not on the domain model helps keep the service encapsulation and reduces coupling to the domain model. By not exposing my domain model, I don't have to make any compromises on field visibility or inheritance for the sake of serialization.
for example if I was exposing a Transfer method from one account to another the service interface would look something like this:
[ServiceContract]
interface ISomeService
{
[OperationContract]
TransferResult Transfer(TransferRequest request);
}
[DataContract]
class TransferRequest
{
[DataMember]
public string FromAccountNumber {get;set;}
[DataMember]
public string ToAccountNumber {get;set;}
[DataMember]
public Money Amount {get;set;}
}
class SomeService : ISomeService
{
TransferResult Transfer(TransferRequest request)
{
//Check parameters...omitted for clarity
var from = repository.Load<Account>(request.FromAccountNumber);
//Assert that the caller is authorised to request transfer on this account
var to = repository.Load<Account>(request.ToAccountNumber);
from.Transfer(to, request.Amount);
//Build an appropriate response (or fault)
}
}
now from this interface it is very clear to the conusmer what the required data to call this operation is. If I implemented this as
[ServiceContract]
interface ISomeService
{
[OperationContract]
TransferResult Transfer(AccountDto from, AccountDto to, MoneyDto dto);
}
and AccountDto is a copy of the fields in account, as a consumer, which fields should I populate? All of them? If a new property is added to support a new operation, all users of all operations can now see this property. WCF allows me to mark this property as non mandatory so that I don't break all of my other clients, but if it is mandatory to the new operation the client will only find out when they call the operation.
Worse, as the service implementer, what happens if they have provided me with a current balance? should I trust it?
The general rule here is to ask who owns the data, the client or the service? If the client owns it, then it can pass it to the service and after doing some basic checks, the service can use it. If the service owns it, the client should only pass enough information for the service to retrieve what it needs. This allows the service to maintain the consistency of the data that it owns.
In this example, the service owns the account information and the key to locate it is an account number. While the service may validate the amount (is positive, supported currency etc.) this is owned by the client and therefore we expect all fields on the DTO to be populated.
In summary, I have seen it done all 3 ways, but designing DTOs around specific operations has been by far the most successful both from service and consumer implementations. It allows operations to evolve independently and is very explicit about what is expected by the service and what will be returned to the client.

I would go pretty much with what others have said here, but probably needs to add these:
Most SOA systems use Web Services for communication. Web Services expose their interface via WSDL. WSDL does not have any understanding of inheritance.
All behaviour in your DTOs will be lost when they cross the wire
All private/protected fields will be lost when they cross the wire
Imagine this scenario (case is silly but illustrative):
public abstract class BankAccount
{
private DateTime _creationDate = DateTime.Now;
public DateTime CreationDate
{
get { return _creationDate; }
set { _creationDate = value; }
}
public virtual string CreationDateUniversal
{
get { return _creationDate.ToUniversalTime().ToString(); }
}
}
public class SavingAccount : BankAccount
{
public override string CreationDateUniversal
{
get
{
return base.CreationDateUniversal + " UTC";
}
}
}
And now you have used "Add Service Reference" or "Add Web Reference" on your client (and not re-use of the assemblies) to access the the saving account.
SavingAccount account = serviceProxy.GetSavingAccountById(id);
account.CreationDate = DateTime.Now;
var creationDateUniversal = account.CreationDateUniversal; // out of sync!!
What is going to happen is the changes to the CreationDate will not be reciprocated to the CreationDateUniversal since there is no implementation crossed the wire, only the value of CreationDateUniversal at the time of serialization at the server.

Design Patterns for Objects in REST API's?

I've built a REST API with the WCF Web API Preview and I wanna build a library with the classes that you pass to this API (Just to make the .Net developers life easier). The should be simple POCO classes without much functionality.
But on the receiver side it would make sense for me to add some functionality to these classes. I have an example below:
[WebInvoke(UriTemplate = "", Method = "POST")]
public Supertext.API.Order Create(Supertext.API.Order apiOrder)
{
And this is an example POCO class:
public class Order
{
public string Service { get; set; }
public string OrderTitle { get; set; }
public string Currency { get; set; }
}
Now, what's a good way to extend this class on the server side?
I guess using a subclass would not work.
Delegates?
Actually have two different versions of the class? One for clients and one for the server?
What do other people do?

The problem with adding extra functionality to this POCO class is you are turning it into a domain object. The nature of this domain object will now be constrained by the fact that, essentially, this class acts as the definition of the interface into the operation. Changing details about this class will potentially break clients.
It is a far cleaner model to keep this class purely as a Data Transfer Object whose single responsibility is aiding the bridging of the wire format to objects and use a mapper such as AutoMapper to map the data from the DTO to a real domain object. The real domain object is fully under your control and you can happily refactor it without threatening a cascading effect to your service consumers

WCF - multiple service contracts using pretty same data contracts

I have a new question for WCF gurus.
So, I have a class User which is close to the 'User' representation from the DB which I use for database operations. Now, I would like to have 2 different service contracts that use this class as data contract, but each in their own way... I mean,
public class DBLayer
{
void InsertUsers(List<User> userList)
{
// both 'PropertyVisibleForService1' and 'PropertyVisibleForService2'
// are used HERE to be inserted into their columns
}
}
[DataContract]
public class User
{
[DataMember] public string PropertyVisibleOnlyForService1{...}
[DataMember] public string PropertyVisibleOnlyForService2{...}
}
[ServiceContract]
public interface IService1
{
List<User> GetUsers(); // user with 'PropertyVisibleOnlyForService1' inside
}
[ServiceContract]
public interface IService2
{
List<User> GetUsers(); // user with 'PropertyVisibleOnlyForService2' inside
}
So, the idea is that each service will get a different kind of user, subset of 'User'. Keeping in mind that I want to use the 'User' as is for DB operations, what would be my options to achieve this? Do I really need to create different data contracts or is there another smarter way?
Best would be to not only give me the solution, but also to explain me some best practices and alternatives.
Thank you in advance.
EDIT1:
I added a dummy DBLayer class here for a better overview and why I think the inheritance may not be good in this case.
A solution would be of having another 'UserForService1' and 'UserForService2' as data contracts which would map at the end from/into an 'User' but I wanted some other points of view.
EDIT2: Very good article which helped me in this case: http://bloggingabout.net/blogs/vagif/archive/2009/03/29/iextensibledataobject-is-not-only-for-backward-compatibility.aspx

You could create separate DTO's for each service but your case would actually be ideal for a Decorator pattern:
[DataContract]
public class UserForService1 : User
{
private User mUser;
public UserForService1(User u)
{
mUser = u;
}
//expose only properties you'd like the user of this data contract to see
[DataMember]
public string SomeProperty
{
get
{
//always call into the 'wrapped' object
return mUser.SomeProperty;
}
set
{
mUser.SomeProperty = value;
}
}
// etc...
}
and for Service2 similar code, that exposes only what you care for there...

If they are designed to represent different types of users, they should be different classes. I agree with phoog in the comments, you should derive the type you want from the shared User class and add the specific service properties to the derived classes.
Why don't you think inheritance would be good in this case? If you give us some more details, we could try to revise the suggestions to suit your actual problem.

As suggested in the comment, you can have two classes deriving from a base User then using Data Contract Known Types, you can accomplish your desired goal. See the following links for more examples.
http://www.freddes.se/2010/05/19/wcf-knowntype-attribute-example/
http://footheory.com/blogs/bennie/archive/2007/07/28/handling-data-contract-object-hierarchies-in-wcf.aspx

If you don't want to use inheritance, something like:
[DataContract]
public class User
{
}
[DataContract]
public class Service1User : User
{
[DataMember] public string PropertyVisibleOnlyForService1{...}
}
[DataContract]
public class Service2User : User
{
[DataMember] public string PropertyVisibleOnlyForService2{...}
}
[ServiceContract]
public interface IService1
{
List<Service1User> GetUsers(); // user with 'PropertyVisibleOnlyForService1' inside
}
[ServiceContract]
public interface IService2
{
List<Service2User> GetUsers(); // user with 'PropertyVisibleOnlyForService2' inside
}
Then I'm not sure what you would do. Your sortof breaking the principals of an type declaration at that point. Think of it in a normal .NET way; if you define "User" in your application, then it is the same type everywhere. Some properties cant be hidden from certain other classes or methods.
WCF is also going to pack this type information into the generated WSDL, and it is only going to define the User type once, so it needs to know what properties are there.
Now, if all you care about is the actual SOAP message that is constructed, and you don't care about the WSDL or what any clients generated off the WSDL will see, then technically you can have it not emit that property into the SOAP message when it is null, by doing:
[DataMember(EmitDefaultValue=false)]
Then when that property is null, it wont be included in the serialization. That would make no real difference if the client was generated from the WSDL though, as its User type would still have to contain both properties. It would just change the serialization so that instead of sending the client something like:
<User>
<PropertyVisibleOnlyForService1 nil="true" />
<PropertyVisibleOnlyForService2>something</PropertyVisibleOnlyForService2>
</User>
it would instead send:
<User>
<PropertyVisibleOnlyForService2>something</PropertyVisibleOnlyForService2>
</User>

Huge object in web application

I am writing web application and I have huge class that have more then 40 members, 3 types of arrays and have many methods.
This class represents a task with all task's ingrediants including pricing of task.
The problem is that I need to pass the client list of 40 first tasks therefore I pass 40 complete task items which turns to be very big and weight a lot of KB.
How can I reduce this object??..
Here is a pic of the class diagram:
http://www.imagehousing.com/image/624350

The first thing I would tackle before even thinking of sending anything is that you refactor that class into more manageable subcomponents. Take for instance properties like DeliveryNote, DeliveryNoteId, DeliveryNoteSerial could be housed in a Delivery class. The same could be said for the Group, Payback and other similarly named properties. When designing properties of a class, you need to ask yourself whether the properties are specific to the parent model, or are specific to a smaller problem domain.
Once you've refactored that class, you then need to determine what data the client needs. Do they need every article of data in that class. If not, why not produce a view class based on what the client needs and send only that.
If you don't think a partial view is appropriate for your data, you can use DataContractAttribute and DataMemberAttribute attributes to control which parts of your model actually represent the data contract to be passed to the client. E.g.:
[DataContract]
public class Task
{
[DataMember]
public string PropertyA { get; set; }
[DataMember]
public string PropertyB { get; set; }
public string PropertyC { get; set; }
}
In the above example, using these attributes I can enforce that PropertyA and PropertyB form component parts of the data contract. PropertyC will not be part of the contract, so will not be serialised. This is of course dependent using the DataContractSerializer or a WCF-service (which uses that serialiser).
This also limits a single-data contract view of the model.

In first glance, I think you need to reduce/simplify your class, that 40 members ain't really necessary to be direct members, e.g.:
Discount, DiscountTypeCode, DiscountTypeId, DiscountTypeSign can all becomes a class ->
class Discount {
float rate;
int typeCode, typeId;
string sign;
}
Simpily group every member with same prefix to a class. This is needed not only to trim the size, but better maintenance. To reduce size, it's up to you whether you need to include those sub objects to send to the client, since it's a web application, I don't think everytime your client need everything in the object, so while you return your object, consider return just a partial of it, create that in another view model.

I think you have you to follow some basic rule.
Having big class create problem to understand the code and maintain it.
List down all the nouns and Verbs
Group common nouns and related verbs and create a class
Repeat same procedure untill there is no noun and verbs remain.
Now you to give serious thought to created classes and their methods
a. Findout Generic and Specialise Classes
b. If there is need of design pattern then think about and create relationship like
Generalisation and Specialisation, Association, Dependency and realization
The above step will automatic give better shape to classe.