I've build a generic (abstract) builder which will provide basic implementation
for entity builders that will be used during testing.
This is the entity base class:
public abstract class Entity : IObjectState
{
[NotMapped]
public ObjectState ObjectState { get; set; }
}
This is the IKey interface:
public interface IKey
{
int Id { get; set; }
}
This is the Builder class:
public abstract class Builder<T> where T : Entity, IKey, new()
{
protected int _id { get; set; }
protected ObjectState _objectState { get; set; }
public Builder()
{
_objectState = ObjectState.Added;
}
public virtual Builder<T> WithId(int id)
{
this._id = id;
return this;
}
public virtual Builder<T> HavingObjectState(ObjectState objectState)
{
_objectState = objectState;
return this;
}
public static implicit operator T(Builder<T> builder)
{
return new T
{
Id = builder._id,
ObjectState = builder._objectState
};
}
}
This is a sample UnitBuilder implementation:
public class UnitBuilder : Builder<Unit>
{
private string _shortDescription;
private string _longDescription;
public UnitBuilder WithShort(string shortDescription)
{
_shortDescription = shortDescription;
return this;
}
public UnitBuilder WithLong(string longDescription)
{
_longDescription = longDescription;
return this;
}
public static implicit operator Unit(UnitBuilder builder)
{
return new Unit
{
Id = builder._id,
ObjectState = builder._objectState,
Short = builder._shortDescription,
Long = builder._longDescription
};
}
}
And this is the problem I'm having:
The error:
Error CS1061 'Builder' does not contain a definition for
'WithShort' and no extension method 'WithShort' accepting a first
argument of type 'Builder' could be found (are you missing a
using directive or an assembly reference?)
I understand what is going on but I would like a better (more elegant) solution than thirdUnit.
UPDATE:
As per suggestion I added the following to the UnitBuilder class:
public new UnitBuilder WithId(int id)
{
return (UnitBuilder)base.WithId(id);
}
public new UnitBuilder WithObjectState(ObjectState objectState)
{
return (UnitBuilder)base.WithObjectState(objectState);
}
But now I don't see any point in the base class... This has to be a
general generic base class problem, how do other people handle this?
Maybe the thirdUnit solution IS elegant but I'm just being difficult about it? :)
The answer is simple, your base class builder methods have to be called last and cannot be chained with your more specific builder classes as it returns the generic. So simply change your code to:
Unit secondUnit = new UnitBuilder()
.WithShort("ShortDesc")
.WithId(10);
That's it!
This is my working final solution:
public abstract class Builder<TEntity, TBuilder>
where TEntity : Entity, IKey, new()
where TBuilder : Builder<TEntity, TBuilder>, new()
{
protected int _id { get; set; }
protected ObjectState _objectState { get; set; }
public Builder()
{
_objectState = ObjectState.Added;
}
public virtual Builder<TEntity, TBuilder> WithId(int id)
{
this._id = id;
return this;
}
public virtual Builder<TEntity, TBuilder> WithObjectState(ObjectState objectState)
{
this._objectState = objectState;
return this;
}
public static implicit operator TEntity(Builder<TEntity, TBuilder> builder)
{
return new TEntity
{
Id = builder._id,
ObjectState = builder._objectState
};
}
}
Related
I have a singleton class, holding several list of data. I want to have a function to return one of the list based on the requested data type
public interface IRentable
{
long Id { get; set; }
}
public class Book : IRentable
{
private long _id;
public long Id
{
get { return _id; }
set { _id = value; }
}
public string Name { get; set; }
public int Pages { get; set; }
}
public class DVD : IRentable
{
private long _id;
public long Id
{
get { return _id; }
set { _id = value; }
}
public string Name { get; set; }
public TimeSpan Length { get; set; }
}
public class DBReferenceSingleton
{
private List<Book> _bookList;
private List<DVD> _dvdList;
public IEnumerable<Entity> GetEntities<Entity>() where Entity : IRentable
{
switch(typeof(Entity).Name)
{
case nameof(Book):
return _bookList;
case nameof(DVD):
return _dvdList;
}
return null;
}
This is not working, since a conversion is needed. I wonder if there is any elegant solution (without serialize all elements or implement IConvertible)?
Background: I was looking into the implementation of Repository pattern implementation from Microsoft document website and a couple of YouTube videos. Their base class implementations are bounded with DBContext,
[from Microsoft doc]
public class GenericRepository<TEntity> where TEntity : class
{
internal SchoolContext context;
internal DbSet<TEntity> dbSet;
[from youtube]
public class Repository<TEntity> : IRepository<TEntity> where TEntity : class
{
protected readonly DbContext Context;
public Repository(DbContext context)
{
Context = context;
}
public IEnumerable<TEntity> GetAll()
{
return Context.Set<TEntity>().ToList();
}
So, this means this base class cannot be used for databases without EntityFramework. I'm trying to remove DBContext in my base class
public class InMemoryRepository<TEntity> : IRepository<TEntity> where TEntity : IRentable
{
private IEnumerable<TEntity> _entities;
public IEnumerable<TEntity> GetAll()
{
if (_entities == null)
{
_entities = DBReferenceSingleton.GetInstance.GetEntities<TEntity>();
}
return _entities;
}
public TEntity Get(long id)
{
GetAll();
if (_entities == null)
{
return default(TEntity);
}
return _entities.FirstOrDefault(t => t.Id == id);
}
(I was wrong in my previous code, as I was using "class" instead of "IRentable" as I thought it will be generic for all classes. After implementing the Get(long id) function. I found I have to have a IRentable interface so I made the changes. And the answer from The Lemon work perfectly.
Thanks
It doesn't look like generics seem the right tool here; not least because Book and DVD don't have a common base class.
I think you should have two methods:
public List<Book> GetBooks() => _bookList;
public List<DVD> GetDvds() => _dvdList;
Have you tried doing a Cast?
public class Book
{
public string Name { get; set; }
public int Pages { get; set; }
}
public class DVD
{
public string Name { get; set; }
public TimeSpan Length { get; set; }
}
public class DBReferenceSingleton
{
private List<Book> _bookList;
private List<DVD> _dvdList;
public IEnumerable<Entity> GetEntities<Entity>() where Entity : class
{
switch(typeof(Entity).Name)
{
case nameof(Book):
return _bookList as List<Entity>;
case nameof(DVD):
return _dvdList as List<Entity>;
}
return null;
}
}
the "as List< Entity >" will return null if the object is the wrong type, or the object as the type if it's the right type. The A = B as C pattern works for inherited types as well, just remember to check for null returned values for cases where your type isn't as well known as in this situation
A side comment on the usefulness of having a generic method in this case: In this method you're forced to set the type of entity each time explicitly, which means your method is functionally non-generic - so you might as well make two explicit methods.
One case where a generic method like the one you have might be more useful, is if book and dvd both inherited from a base class, and you had some follow up methods that needed to operate off a list of . For example, you might end up wanting to do something like this instead in your code:
public class Book : Rentable
{
public int Pages { get; set; }
}
public class DVD : Rentable
{
public TimeSpan Length { get; set; }
}
public class Rentable
{
public string Name { get; set; }
public string borrowedBy { get; set; }
}
public class DBReferenceSingleton
{
private List<Book> _bookList;
private List<DVD> _dvdList;
public enum RentableType { Book, DVD }
public IEnumerable<Rentable> GetEntities(RentableType entityType)
{
switch (entityType)
{
case RentableType.Book:
return _bookList.ToList<Rentable>();
case RentableType.DVD:
return _dvdList.ToList<Rentable>();
default:
throw new NotImplementedException($"Entity {entityType} not supported");
}
return null;
}
}
I have 2 classes:
public class GenericClass<T>
{
public T Item {get;set;}
}
public class StringClass
{
public string Item {get;set;}
}
now i have a GenericClass object and i need to cast it to StringClass object:
var genericObj = new GenericClass<string>();
var stringObj = (StringClass)genericObj; // <---
How to cast / convert from generic class to specific one?
You can't cast one type to the other because the types are unrelated.
You could, however, define a conversion operator:
public class StringClass
{
public string Item { get; set; }
public static explicit operator StringClass(GenericClass<string> generic)
=> new StringClass { Item = generic.Item };
}
Which would allow this syntax:
var genericObj = new GenericClass<string>();
var stringObj = (StringClass)genericObj;
You can't. You would need common inherited type or implement an interface.
With inheritance:
public class GenericClass<T>
{
public T Item {get;set;}
}
public class StringClass : GenericClass<string>
{
}
if your really need it, you can do this way for examle
var stringObj = new StringClass(genericObj);
public class StringClass
{
public string Item { get; set; }
public StringClass(GenericClass<string> genericClass)
{
Item=genericClass.Item;
}
public StringClass(){}
}
or this is more flexible
public interface IGenericClass<T>
{
public T Item { get; set; }
}
public class GenericClass<T>:IGenericClass<T>
{
public T Item { get; set; }
}
public class StringClass
{
public string Item { get; set; }
public StringClass(IGenericClass<string> genericClass)
{
Item=genericClass.Item;
}
public StringClass(){}
}
Using this answer:
var genericObj = new GenericClass<string>();
var stringObj = (StringClass)Convert.ChangeType(genericObj, typeof(StringClass));
Finally i solved using ICloneable,
Here i have a base class named GenericClass, a generic class named GenericClassT, and a class named StringClass.
Inheritance is:
GenericClass <- GenericClassT <- StringClass
Using ICloneable implementation on GenericClass and GenericClassT, adding a CreateObject and CopyTo methods i reach the final goal:
var genericObj = new GenericClass<string>();
var stringObj = (StringClass)genericObj.Clone<StringClass>();
class definitions:
public class GenericClass: ICloneable
{
public string Id {get;set;}
protected virtual ApiRequestResult CreateObject()
{
return new GenericClass();
}
protected virtual void CopyTo(GenericClass obj)
{
obj.Id = Id;
}
public virtual object Clone()
{
var obj = CreateObject();
CopyTo(obj);
return obj;
}
public virtual object Clone<T>() where T: GenericClass
{
var obj = (GenericClass)Activator.CreateInstance(typeof(T));
CopyTo(obj);
return obj;
}
}
public class GenericClass<T>: GenericClass
{
public T Data {get; set;}
protected override GenericClass CreateObject()
{
return new GenericClass<T>();
}
protected override void CopyTo(GenericClass obj)
{
base.CopyTo(obj);
((GenericClass<T>)obj).Data = Data;
}
}
public class StringClass: GenericClass<string>
{
}
Suppose I have two classes and both contain the same fields
Class A
{
public string Name { get; set; }
public int Designaton { get; set; }
}
Class B
{
public string Name { get; set; }
public int Designation { get; set; }
}
And I have one interface and two classes which are inherited from interface
public interface IDeprt
{
object BindData();
}
And two extractor classes:
public classAItem : IDeprt
{
public object BindData()
{
return new A()
{
// mapping operation
}
}
}
public classBItem : IDeprt
{
public object BindData()
{
return new B()
{
//same mapping operation
}
}
}
My question, how can I implement this in generic way using <T> .
Both classes are doing same operation only return type change. If I am doing in the above way there is lot of duplication of code.
Make your ITem interface and also BindData generic make them use the same generic parameter.
public interface IItem<T>
{
T BindData();
}
Then implement the subclasses like below :
public class AItem : ITem<A>
{
public A BindData(){
return new A(){
// mapping operation
}
}
}
public class BItem : ITem<B>
{
public B BindData(){
return new B(){
//same mapping operation
}
}
}
Edit : As the question evolves.
Make a shared base class for A and B classes.
public abstract class CommonItem
{
public string Name { get; set; }
public int Designaton { get; set; }
}
class A : CommonItem
{
}
class B : CommonItem
{
}
Then make class with a method that accepts a generic parameter with new and CommonItem constraints.
public class Binder
{
public T BindData<T>() where T: CommonItem, new()
{
return new T()
{
// you can access the properties defined in ICommonItem
}
}
}
Usage :
var binder = new Binder();
var boundA = binder.BindData<A>();
var boundB = binder.BindData<B>();
I have the following interfaces, one for the entity and one for some logic:
public interface IItem
{
int Id { get; set; }
}
public interface IGenerator
{
IList<IItem> Generate();
}
and implementation:
public class ItemA : IItem
{
public int Id { get; set; }
public string Name { get; set; }
}
public class ItemAGenerator : IGenerator
{
public IList<ItemA> Generate()
{
// do stuff
return List<ItemA>;
}
}
That implementation did not work, it says that it does not have the matching return type, so I also tried:
public class ItemAGenerator : IGenerator
{
public IList<IItem> Generate()
{
// do stuff
return List<ItemA>;
}
}
it does not work as well, it says: cannot implicitly convert type List<IItem> to List<ItemA>.
How to make it work? what am I missing here.
Just create the list as a List<IItem> but add ItemA's to it.
public class ItemAGenerator : IGenerator
{
public IList<IItem> Generate()
{
var list = new List<IItem>();
list.Add(new ItemA());
return list;
}
}
Just make IGenerator generic. Then you can specify the type that will be returned.
public interface IGenerator<T> where T : IItem
{
IList<T> Generate();
}
public class ItemAGenerator : IGenerator<ItemA>
{
public IList<ItemA> Generate()
{
// do stuff
return List<ItemA>;
}
}
I've written a code as below. In this code I want to put a constraint on ServiceResult and BaseService classes so that T needs to implement IBaseEntity interface.
Here is the code:
public interface IBaseEntity
{
int Id { get; set; }
}
public class Photo : IBaseEntity
{
public int Id { get; set; }
public float FileSize { get; set; }
public string Url { get; set; }
}
public class ServiceResult<T> where T : class, IBaseEntity, new()
{
public bool Succeed { get; set; }
private T data;
public T Data
{
get
{
if (data == null)
data = new T();
return data;
}
set
{
data = value;
}
}
}
public abstract class BaseService<T> where T : class, IBaseEntity, new()
{
public abstract ServiceResult<List<T>> GetAll();
public abstract ServiceResult<T> GetById(int Id);
}
public class PhotoService : BaseService<Photo>
{
public override ServiceResult<List<Photo>> GetAll()
{
throw new Exception();
}
public override ServiceResult<Photo> GetById(int Id)
{
throw new Exception();
}
}
public class Program
{
public static void Main(string[] args)
{
Console.ReadKey();
}
}
In the code I get the error as below (error refers to GetAll() methods)
Error 3 The type
'System.Collections.Generic.List' cannot be used
as type parameter 'T' in the generic type or method
'FOC.Session04.ServiceResult'. There is no implicit reference
conversion from 'System.Collections.Generic.List'
to 'FOC.Session04.IBaseEntity'. G:\Courses\ASP.NET MVC5\Session4
960803\FOC.Session04\FOC.Session04\Program.cs 55 52 FOC.Session04`
But when I remove the interface constraint IBaseEntity from ServiceResult class and let it remain after BaseService I will get no error and the code compiles without error.
Can anybody explain me why I can't add constraint after ServiceResult class?
What's the reason? Or which part of code need to be changed in order to compile error less in this case?
Thanks all
I think you want rather
public abstract List<ServiceResult<T>> GetAll();
instead of
public abstract ServiceResult<List<T>> GetAll();
List<T> does not even match your constraints (hence the compiler error)
What you really need is for GetAll to return a List of ServiceResults, like so
public interface IBaseEntity
{
int Id { get; set; }
}
public class Photo : IBaseEntity
{
public int Id { get; set; }
public float FileSize { get; set; }
public string Url { get; set; }
}
public class ServiceResult<T> where T : class, IBaseEntity, new()
{
public bool Succeed { get; set; }
private T data;
public T Data
{
get
{
if (data == null)
data = new T();
return data;
}
set
{
data = value;
}
}
}
public abstract class BaseService<T> where T : class, IBaseEntity, new()
{
public abstract List<ServiceResult<T>> GetAll();
public abstract ServiceResult<T> GetById(int Id);
}
public class PhotoService : BaseService<Photo>
{
public override List<ServiceResult<Photo>> GetAll()
{
throw new Exception();
}
public override ServiceResult<Photo> GetById(int Id)
{
throw new Exception();
}
}
public class Program
{
public static void Main(string[] args)
{
Console.ReadKey();
}
}
I made a look at the source again and understood what is going on.
Putting IBaseEntity constraint on BaseService class has no problem. Because T represents a single class here (PhotoService : BaseService<Photo>). So T is Photo and Photo implements IBaseEntity.
But for GetAll() method in BaseService class the return type is ServiceResult<List<T>>. Therefore in ServiceResult class T will be something like List<Photo> and List<Photo> doesn't implement IBaseEntity. Therefore it raises an error.
Removing IBaseEntity constraint from ServiceResult class solves the problem.