I need classes as shown below but I'm unable to override context in Child Class because signature is different
class BaseContext{}
class ChildContext:BaseContext{}
abstract class Base
{
abstract BaseContext context{get;}
...
}
class Child : Base
{
public override ChildContext context{get;}
...
}
Is there any better approach for this?
I would consider using generics here. For example:
// Make the base class generic
public abstract class Base<TContext>
where TContext : BaseContext // constrain the generic type to be a BaseContext
{
public TContext context { get; }
}
public class Child : Base<ChildContext>
{
// And here we no longer have to override the property
}
There is Covariant return types that should make your example work if you just fix the access modifiers to both be public or protected. But this feature is only available in .net 5 and later.
For older versions you can do add a new modifier to create a new property with the same name:
public class BaseContext{}
public class ChildContext : BaseContext{}
public abstract class Base
{
public BaseContext Context => ContextImpl;
protected abstract BaseContext ContextImpl { get; }
}
public class Child : Base
{
public new ChildContext Context { get; }
protected override BaseContext ContextImpl => Context;
}
This patter can be used to provide different levels of access depending on the reference type you have. The pattern is a bit easier to implement with interfaces since you can use Explicit interface implementation to achieve the same thing, no need for a hidden abstract property.
public interface IBase
{
BaseContext Context { get; }
}
public class Child : IBase
{
BaseContext IBase.Context => Context;
public ChildContext Context { get; }
}
Related
How do I force all derived classes of an interface to have a constructor with a signature? This doesn't work:
public interface Constructor<T> where T : Constructor<T>, new()
{
Constructor(T);
}
public interface IParameters
{
}
public interface IRule : Constructor<IParameters>
{
//IRule (IParameters); must exist
}
You can't, not via an interface. But you can sort of get at it with an abstract class. Similar to what the accepted answer here describes, try:
public abstract class MustInitialize<T>
{
public MustInitialize(T parameters)
{
}
}
public class Rule : MustInitialize<IParameters>, IRule
{
IParameters _parameters;
public Rule(IParameters parameters)
: base (parameters)
{
_parameters= parameters;
}
}
You can't force a specific constructor signature.
Even with an abstract class as demonstrated in Mark's answer, you can only force the constructor of the abstract class, but nothing is stopping the author of the derived class to do something like this:
public class Rule : MustInitialize<IParameters>, IRule
{
public Rule()
: base (new Parameters())
{
// Assuming Parameters is a class that implements the IParameters interface
}
}
However, you can force dependency injection by using method (setter) injection:
public interface IMethodInjection<T>
{
void Method(T injected);
}
I think you can design your base class like the following example:
public abstract class MyBase
{
private MyBase()
{
}
public MyBase(string a)
{
}
}
public class MyDerived : MyBase
{
public MyDerived(string a) : base(a)
{
}
}
You can even delete the private constructor if its not needed
Now I use Entity Framework with the following DbContext class:
public class ItemDbContext : DbContext {
...
public DbSet<Item1> Item1s { get; set; }
public DbSet<Item2> Item2s { get; set; }
}
There are some classes need DbContext with either Item11s or Item2s as dependencies. So my purpose is create a layer of abstraction for DbContext with that DbSet types (logically separate DbContext_Item1 and DbContext_Item2). It can be useful for implementation of abstract factory pattern (create either DbContext_Item1 or DbContext_Item2 instance) also.
My thoughts:
1) Interfaces
public interface IDbContext_Item1 {
DbSet<Item1> Item1s { get; set; }
}
public interface IDbContext_Item2 {
DbSet<Item3> Item2s { get; set; }
}
So my abstract factory is able to have API like that:
public abstract ItemFactory {
public abstract IDbContext_Item1 GetItem1Context;
public abstract IDbContext_Item2 GetItem2Context;
}
It would be fine for me. However class which get IDbContext_ItemX instance doesn't treat it as DbContext instance (i.e. can not call method like SaveChanges() etc.). Unfortunately any interface can not be inherited from class like DbContext.
2) Extended interfaces with methods of DbContext
public interface IDbContext_Item1 {
DbSet<Item1> Item1s { get; set; }
void SaveChanges;
// etc
}
In my opinion it's extremely not elegant.
3) Abstract classes for IDbContext_ItemX
But my current DbContext implementation, ItemDbContext, can not be inherited from multiple classes.
4) Just devide current ItemDbContext on DbContext_Item1 and DbContext_Item2 concrete classes.
Yes, it is the decision. But is it only way? It would be great if my concrete factories return instances of ItemDbContext under the hood. Can I achieve it?
As I commented, I'd first challenge the reasoning behind wanting to have a single entity for a given context, but if you decide to go forward with that, I'd go with a generic interface, or even a generic context :
interface IDbContext<T> where T : class
{
IDbSet<T> Set { get; }
}
// Context implement generic Interface
class DbContextItem1 : IDbContext<Item1>
{
IDbSet<Item1> Set { get; private set; }
override void OnModelCreating(DbModelBuilder modelBuilder)
{
// Logic to get relevant mappings,
}
}
// Technically you could also do that.
class DbContextItem1 : IDbContext<Item1>, IDbContext<Item2>
{
IDbSet<Item1> IDbContext<Item1>.Set { get; }
IDbSet<Item2> IDbContext<Item2>.Set { get; }
override void OnModelCreating(DbModelBuilder modelBuilder)
{
// Logic to get relevant mappings,
}
}
// Generic context.
class DbContextGeneric<T> : IDbContext<T>
{
IDbSet<T> Set { get; private set; }
override void OnModelCreating(DbModelBuilder modelBuilder)
{
// Logic to get relevant mappings based on T.
}
}
I think you need to create some base interface (lets call it IDatabaseContext):
public interface IDatabaseContext : IDisposable
{
//base DbContext stuff like SaveChanges here
}
Then you have nested interfaces:
IDbContext_Item1 : IDatabaseContext
IDbContext_Item2 : IDatabaseContext
And your ItemDbContext implements both interfaces.
I agree that writing all DbContext methods in IDatabaseContext interface is not elegant, but you need to do it just one time
I have this base class:
public class BaseEvent
{
public int EventID { get; set; }
public int GetEventID()
{
return EventID;
}
}
And then I have another class inherited from that base one:
public class ValidationResult<T> where T : BaseEvent
{
private void AddEventStatusUpdater(ValidationResult<T> validationResult)
{
validationResult.GetEventID();
}
}
The issue I´m having is that I cannot access the GetEventID() method from the base class.
I think this may happen because I´m using a T generic. Is there any other way to access this method?
public class ValidationResult<T> where T : BaseEvent
Says that T must be a BaseEvent, not that ValidationResult<T> inherits from BaseEvent. That'd be:
public class ValidationResult<T> : BaseEvent
And there T would not have any constraint
Is that what you want?
You have begun to implement a generic interface rather than inheriting from the base class. I think you meant to do the following:
public class ValidationResult : BaseEvent
{
private void AddEventStatusUpdater()
{
var id = this.GetEventID();
}
}
I have a generic class with a class constraint on it.
public class MyContainer<T> where T : MyBaseRow
MyBaseRow is an abstract class which I also want to contain a member of some flavour of MyContainer.
public abstract class MyBaseRow
{
public MyContainer<MyBaseRow> ParentContainer;
public MyBaseRow(MyContainer<MyBaseRow> parentContainer)
{
ParentContainer = parentContainer;
}
}
I am having problems with the constructors of classes inherited from MyBaseRow eg.
public class MyInheritedRowA : MyBaseRow
{
public MyInheritedRowA(MyContainer<MyInheritedRowA> parentContainer)
: base(parentContainer)
{ }
}
Won't allow MyInheritedRowA in the constructor, the compiler only expects and only allows MyBaseRow. I thought the generic class constraint allowed for inheritance? What am I doing wrong here and is there any way I can redesign these classes to get around this?
Many thanks in advance for any responses.
Basically, you can't use generics that way, because the covariance system doesn't work that way with classes. See here: http://geekswithblogs.net/abhijeetp/archive/2010/01/10/covariance-and-contravariance-in-c-4.0.aspx
You can however use an interface like this:
public interface MyContainer<out T> where T : MyBaseRow {
}
And that code will compile.
You can make a covariant generic interface (C#4.0):
public interface IContainer<out T> where T : MyBaseRow
{
}
public class MyContainer<T> : IContainer<T> where T : MyBaseRow
{
}
public abstract class MyBaseRow
{
public IContainer<MyBaseRow> ParentContainer;
public MyBaseRow(IContainer<MyBaseRow> parentContainer)
{
ParentContainer = parentContainer;
}
}
public class MyInheritedRowA : MyBaseRow
{
public MyInheritedRowA(IContainer<MyInheritedRowA> parentContainer)
: base(parentContainer)
{ }
}
in my Silverlight 4 application I started creating and using some generics and now I stumbled upon the following problem:
In a non-generic class, I have a abstract method, that returns a generic class:
public abstract class DTO_Base()
{
public abstract ServiceModelBase<ServiceNodeBase> CreateBusinessObject();
}
The generic class is defined in the following way:
public abstract class ServiceModelBase<RootNodeType> where RootNodeType : ServiceNodeBase
Naturally, from DTO_Base derived classes will have to override the CreateBusinessObject method:
public class DTO_Editor : DTO_Base
{
public override ServiceModelBase<ServiceNodeBase> CreateBusinessObject()
{
// the object to return have to be of type ServiceModelEditor
// which is derived from ServiceModelBase<ServiceNodeEditor>
// public class ServiceModelEditor : ServiceModelBase<ServiceNodeEditor>
// ServiceNodeEditor is derived from ServiceNodeBase
// public class ServiceNodeEditor : ServiceNodeBase
ServiceModelEditor target = new ServiceModelEditor()
...
Functions to populate the 'target'
...
return target;
}
}
The line return target; causes an error, stating that it isn't possible to implicitly convert the type ServiceModelEditor in ServiceModelBase<ServiceNodeBase>. Also, an explicit conversion via target as ServiceModelBase<ServiceNodeBase> doesn't work.
How would I have to implement this method to work?
Try this:
public interface IDTO<Node> where Node : ServiceNodeBase
{
ServiceModelBase<Node> CreateBusinessObject();
}
public abstract class DTO_Base<Model,Node> : IDTO<Node>
where Model : ServiceModelBase<Node>
where Node : ServiceNodeBase
{
public abstract Model CreateBusinessObject();
#region IDTO<Node> Members
ServiceModelBase<Node> IDTO<Node>.CreateBusinessObject()
{
return CreateBusinessObject();
}
#endregion
}
public class DTO_Editor : DTO_Base<ServiceModelEditor, ServiceNodeEditor>
{
public override ServiceModelEditor CreateBusinessObject()
{
// the object to return have to be of type ServiceModelEditor
// which is derived from ServiceModelBase<ServiceNodeEditor>
// public class ServiceModelEditor : ServiceModelBase<ServiceNodeEditor>
// ServiceNodeEditor is derived from ServiceNodeBase
// public class ServiceNodeEditor : ServiceNodeBase
ServiceModelEditor target = new ServiceModelEditor();
return target;
}
}
I have faced a similar problem before and the only thing reasonable to do is to make the core base class generic also. You can remove the Model generic parameter (and the interface) and it will look a little less scary, but you loose visibility on the functionality of ServiceModelEditor outside of the method.
As it is, you've got to return a ServiceModelBase<ServiceNodeBase>. One option is to make your base class generic:
public abstract class DtoBase<T> where T : RootNodeType
{
public abstract ServiceModelBase<T> CreateBusinessObject();
}
Then:
public class DtoEditor : DtoBase<ServiceNodeBase>
{
public override ServiceModelBase<ServiceNodeBase> CreateBusinessObject()
{
...
}
}
If you are using .Net 4.0 I suggest you use interfaces to define your ServiceModelBase and specify an out variance modifier on that interface generic type:
class ServiceNodeBase { }
class ServiceNodeEditor : ServiceNodeBase {/*implementation*/}
//
interface IServiceModelBase<out RootNodeType>
where RootNodeType : ServiceNodeBase {
}
class ServiceModelEditor : IServiceModelBase<ServiceNodeEditor> {
/*implementation*/
}
//
abstract class DTO_Base {
public abstract IServiceModelBase<ServiceNodeBase> CreateBusinessObject();
}
class DTO_Editor : DTO_Base {
public override IServiceModelBase<ServiceNodeBase> CreateBusinessObject() {
return new ServiceModelEditor();
}
}