I have a question regarding a problem with L2S, Autogenerated DataContext and the use of Partial Classes. I have abstracted my datacontext and for every table I use, I'm implementing a class with an interface. In the code below you can see I have the Interface and two partial classes. The first class is just there to make sure the class in the auto-generated datacontext inherets Interface. The other autogenerated class makes sure the method from Interface is implemented.
namespace PartialProject.objects
{
public interface Interface
{
Interface Instance { get; }
}
//To make sure the autogenerated code inherits Interface
public partial class Class : Interface { }
//This is autogenerated
public partial class Class
{
public Class Instance
{
get
{
return this.Instance;
}
}
}
}
Now my problem is that the method implemented in the autogenerated class gives the following error:
-> Property 'Instance' cannot implement property from interface 'PartialProject.objects.Interface'. Type should be 'PartialProjects.objects.Interface'. <-
Any idea how this error can be resolved? Keep in mind that I can't edit anything in the autogenerated code.
Thanks in advance!
You can solve this by implementing the interface explicitly:
namespace PartialProject.objects
{
public interface Interface
{
Interface Instance { get; }
}
//To make sure the autogenerated code inherits Interface
public partial class Class : Interface
{
Interface Interface.Instance
{
get
{
return Instance;
}
}
}
//This is autogenerated
public partial class Class
{
public Class Instance
{
get
{
return this.Instance;
}
}
}
}
Return types aren't covariant in C#. As you can't change the auto-generated code the only solution I see is to change the interface:
public interface Interface<T>
{
T Instance { get; }
}
And change your partial class accordingly:
public partial class Class : Interface<Class> { }
Related
Given the following classes and hierarchy:
public abstract class AbsractClass<T>
{
public T Id { get; private set; }
}
public class ImplementationA : AbsractClass<Guid> { }
public class ImplementationB : AbsractClass<int> { }
What constraint should I be using in where clause/constraint to specify that generic T must be implementation of AbsractClass. Which would mean that Entity would accept either ImplementationA or ImplementationB.
public abstract class Entity<T> // where : T ?
{
}
Is this something that is possible or should I be using interfaces instead?
You need to add two generic parameters to make it work:
public abstract class Entity<T, TKey> where T : AbstractClass<TKey>
{
}
Generic interfaces would behave the same way.
I have an interface
public interface IStrategy<T> where T : BaseModel
{
T GetModel(Guid userId);
}
and a concrete class inheriting the interface specifying that it should be a ConcreteModel
public class ConcreteStrategy: IStrategy<ConcreteModel>
{
ConcreteModel GetModel(Guid userId) { ... }
}
Now in the following method I can pass a new instance of ConcreteStrategy and everything works
public class Manager
{
public TModel GetContentModel<TModel>(IStrategy<TModel> strategy, Guid userId)
where TModel : ModelBase
{
return strategy.GetContentModel(userId);
}
}
But if I try to assign it to a property like this I get an error
public class Strategies
{
public static IStrategy<ModelBase> MyStrategy { get; set; }
}
Strategies.MyStrategy = new ConcreteStrategy();
Is there a way I can achieve this in C# ?
I want to be able to make a factory method that encapsulates the logic for which strategy to use and just return an instance of some type of strategy class (like ConcreteStrategy).
The error I am getting is:
Cannot implicitly convert type IStrategy<ModelBase> to IStrategy<ConcreteModel>
You need to make your interface covariant:
public interface IStrategy<out T> where T : BaseModel
Note that it will work only if T only appears in an output position in the interface (which is the case in the code you have shown, but I don't know if it's your real code).
i have another question open here on SO and after thinking about it, i may be approaching this in the wrong way.
i have 4 classes, that have the same properties and methods.
some of the classes, have their own properties and methods ( not overrides of the existing ones ).
currently i create each class as:
public class ClassOne
{
public ClassOne()
{
}
public int ID {get;set;}
// More properties here
public void Set(){
// Do Stuff to save this
}
// More Methods here
}
cant i create one class that will generate all of the 4 classes?
and in the classes themselfs i only create specific properties/methods for that class?
repeating the code seems very odd to me, im sure there must be a way to do this, just dont know how.
Your situation is one of the main reasons why inheritance was invented. So with that, you can write
public class Base
{
// Properties and methods common to all
}
public class ClassOne : Base
{
// Properties and methods specific to ClassOne
}
public class ClassTwo : Base
{
// Properties and methods specific to ClassTwo
}
public class ClassThree : Base
{
// Properties and methods specific to ClassThree
}
public class ClassFour : Base
{
// Properties and methods specific to ClassFour
}
As requested, more code, using interfaces and abstract classes:
An interface is just a blueprint, defining what properties and methods are required to be compatible with other "BaseClasses"
public interface IBaseClass
{
public int ID {get;set;}
public void Set();
}
Abstract classes can contain code, but can not be instantiated, they are form of starting point for a class, but not a complete class themselves.
public abstract class ABaseClass : IBaseClass
{
public int ID {get;set;}
public void Set(){
// Do Stuff to save
}
}
Each class inherits from the abstract class and can then override and implement whatever it wants, customizing it however is necessary.
public class ClassOne : ABaseClass
{
}
public class ClassTwo : ABaseClass
{
}
public class ClassThree : ABaseClass
{
}
public class ClassFour : ABaseClass
{
}
ps. not entirely sure if my syntax is 100% correct
Could you simply make a base class with your properties and inherit from that class?
Why not use inheritance??
public class ClassOne
{
public ClassOne()
{
}
public virtual int ID {get;set;}
// More properties here
public virtual void Set(){
// Do Stuff to save this
}
// More Methods here }
public class ClassTwo : ClassOne
{
public string ClassTwoString { get; set; }
}
public class ClassThree : ClassOne
{
public string ClassThreeString { get; set; }
}
Can you make them all inherit off of the same class? If so, that sounds ideal.
Barring the possibility of making them inherit, you could write an interface that describes the methods and properties which each of them use. Then you can call each instance of the class through the same interface.
Barring again that possibility, you could write a reflective assignor/accessor. But you shouldn't do that.
In this post I talked about using a generic base class to enable me to create repository classes without duplicating loads of basic plumbing code.
Each Repository is accessed through an interface. In the code below, I will only show one of the methods for the sake of brevity:
Interface:
IQueryable<Suggestion> All { get; }
Generic base class
public IQueryable<T> All
{
get { return _unitOfWork.GetList<T>(); }
}
Concrete class (implements the interface and extends the generic base class)
public IQueryable<Suggestion> All
{
get { return _unitOfWork.GetList<Suggestion>(); }
}
I anticipated that I would be able to simply strip the method out of the concrete class, and the compiler would use the generic base class implementation instead and work out that was intended to satisfy the interface. But no!
When I strip the method out I get the old 'does not implement interface member' error.
If I can't do this, have my efforts to use a generic base class not been pointless? Or is there a way around this?
Can you make the interface itself generic then implement a typed version in your concrete class?
public interface IRepository<T>
{
List<T> All { get; }
}
public class Repository<T>
{
public List<T> All
{
get { return new List<T>(); }
}
}
public class SuggestionRepository : Repository<Suggestion>, IRepository<Suggestion>
{ }
I'd still suggest using the generic interface since it will save you from repeating yourself, but this works too.
public interface ISuggestionRepository
{
List<Suggestion> All { get; }
}
public class Repository<T>
{
public List<T> All
{
get { return new List<T>(); }
}
}
public class SuggestionRepository : Repository<Suggestion>, ISuggestionRepository
{ }
Use the virtual keyword and put your interface on your concrete implementation..
public interface IMyInterface<T>
{
IQueryable<T> All { get; }
}
public abstract class MyBaseClass<T> : IMyInterface<T>
{
public virtual IQueryable<T> All
{
get { return _unitOfWork.GetList<T>(); ; }
}
}
public class MyClass : MyBaseClass<Suggestion>, IMyInterface<Suggestion>
{
}
I have a two generic abstract types: Entity and Association.
Let's say Entity looks like this:
public class Entity<TId>
{
//...
}
and Association looks like this:
public class Association<TEntity, TEntity2>
{
//...
}
How do I constrain Association so they can be of any Entity?
I can accomplish it by the following:
public class Association<TEntity, TId, TEntity2, TId2>
where TEntity : Entity<TId>
where TEntity2: Entity<TId2>
{
//...
}
This gets very tedious as more types derive from Association, because I have to keep passing down TId and TId2. Is there a simpler way to do this, besides just removing the constraint?
This problem is usually solved by having your generic class (Entity<TId>, in this case) inherit from a common non-generic class.
public abstract class EntityBase
{
}
public class Entity<TId> : EntityBase
{
}
This will allow you to do:
public class Association<TEntity, TEntity2>
where TEntity : EntityBase
where TEntity2 : EntityBase
{
}
Edit
If having them inherit from a common class is an issue, then this could be easily done with an interface as well.
If the Id types are important inside the Association definition, you could create an enclosing "context":
public static partial class EntityIds<TId1, TId2> {
public class Association<TEntity1, TEntity2>
where TEntity1 : Entity<TId1>
where TEntity2 : Entity<TId2>
{
// ...
}
}
This way, the Association class declaration is still intelligible, and it retains the necessary type arguments for its type parameters.
A factory method could help you with the normal case:
public static class AssociationFactory {
public static EntityIds<TId1, TId2>.Association<Entity<TId1>, Entity<TId2>> Create<TId1, TId2>(/*params...*/) {
return new EntityIds<TId1, TId2>.Association<Entity<TId1>, Entity<TId2>>(/*params...*/);
}
}
It that looks like too much, and if you don't have entity specializations, you could model the association differently:
public class Association<TId1, TId2>
{
// ...
Entity<TId1> Entity1 { get; set; }
Entity<TId2> Entity2 { get; set; }
// ...
}