This question already has an answer here:
C# using Generics both in the Interface and its implementor class
(1 answer)
Closed 3 years ago.
I have a class that has type constraints:
public class DataManager<TEntity> : IDisposable where TEntity : LogRecord, new()
{
public TEntity GetRecord()
{
...
}
}
It has a derived class which uses ServerLogRecord derived LogRecord as TEntity:
public class ServerDataManager : DataManager<ServerLogRecord>
{
}
I want to abstract DataManager's public functions into an interface, so I can make it unittestable and decouple the implementation with client code, and because the public function returns TEntity, so I have to put the type constraints in interface:
public interface IDataManager<out TEntity> where TEntity : LogRecord, new()
{
TEntity GetRecord();
}
Then I have a question, how should I declare my implantation class to keep it generic? I tried this solution suggested:
public class DataManager<TEntity> : IDataManager<TEntity>, IDisposable
However I got this error:
error CS0314: The type 'TEntity' cannot be used as type parameter 'TEntity' in the generic type or method 'IDataManager<TEntity>'. There is no boxing conversion or type parameter conversion from 'TEntity' to 'LogRecord'.
but reapply constraints works:
public class DataManager<TEntity> : IDataManager<TEntity>, IDisposable where TEntity : LogRecord, new()
So is "rewriting type constraints in implementation class" the only way to keep DataManager generic?
If I understand what was asked, Just have the class implement the interface
public class DataManager<TEntity> : IDataManager<TEntity>, IDisposable { ... }
If the intention is not to have the implementation as a generic as well and you have you entity then
public class DataManager : IDataManager<MyClass>, IDisposable { ... }
Provided MyClass satisfies the constraint.
when implementing interfaces, you are expected to provide the real classes which are going to be used
in the implementation not generic types because most times, you would not be calling the implementation but
the interfaces.
consider this code
//Where K is key
//where T is a generic entity which may be a programming language
public interface ICoder<K, T>
{
Task Program();
Task LearnNewLanguage(T Language);
K GetCoderId();
}
and your implementation
public class CoderImplementation<TDbContext> : ICoder<string, ProgrammingLanguage> where TDbContext : DbContext
{
private readonly TDbContext _edu;
public CoderImplemtation(TDbContext edu)
{
_edu = edu;
}
public Task Program()
{
...start programming by 12 to 6
}
public Task LearnNewLanguage(string Name)
{
_edu.AddLanguage(Name);
}
}
By telling it that TDbContext is of type DbContext you can be able to access methods which a DbContext exposes
even though that DbContext is not known
Related
This question already has answers here:
Why an inherited interface can't be converted to its base interface in generic context?
(2 answers)
Closed 3 years ago.
There is an interface which inherits the generic interface.
Here is generic interface
public interface IBaseService<TEntity> where TEntity : BaseEntity
{
}
Interface which inherits generic interface
public interface IChatService :IBaseService<Messages>
{
}
Then i have a class which takes the generic interface in the constructor as a parameter.
public class BaseApi
{
IBaseService<BaseEntity> _baseService;
public BaseApi(IBaseService<BaseEntity> baseService)
{
_baseService = baseService;
}
}
When i pass the interface object which implements the generic interface it doesn't allow me to pass the object of the interface, it asks to pass generic interface.
Here is error:
Any solution?
Thanks in advance, sorry for bad english!
This is quite logical. Let's simplify matters a bit by renaming our types and creating a new example:
public class BaseApi
{
public BaseApi(IBucket<Fruit> baseService) { ... }
}
And our IBucket<Messages> represents a concrete type of fruit (apple):
public interface IChatService :IBucket<Apple>
{
}
A bucket of apples is not the same as a bucket of fruit because you can put an orange in the latter and you cannot do this with the former.
To deal with this you could either change the signature of IChatService:
public interface IChatService :IBaseService<BaseEntity>
{
}
Or pass the expected IBaseService<Messages> type to BaseApi's ctor.
You can fix this with covariance. But only if your argument can be covariant
public interface IBaseService<out TEntity> where TEntity : BaseEntity
{
}
https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance
This question already has answers here:
'T' does not contain a definition
(4 answers)
Closed 4 years ago.
I have a class as follows
public class EntityBase
{
// other code
public virtual void SetDataContext(IDataContext context) { throwNotImplemented("SetDefaultDataContext"); }
protected void throwNotImplemented( string mname )
{
throw new Exception( String.Format( "Method {0} is not implemented") );
}
}
The purpose of throwNotImplemented is to force derived classes to implement the virtual methods as if they were abstract.
Then I have a class that references EntityBase, as follows
public class BasePresenter<IDataContextFactory, IDataContext, EntityBase>
where IDataContextFactory : class, new() where IDataContext : class where EntityBase : class
{
public IDataContext dataContext;
public BasePresenter( IDataContext dc, EntityBase sample)
{
dataContext = dc;
sample.SetDataContext(dataContext);
}
// more code
}
The compiler is stuck in this error:
'EntityBase' does not contain a definition for 'SetDataContext' and no extension method ... etc
Who or what is wrong?
TIA
Inside your generic class, the identifier EntityBase represents a generic parameter. This hides the class named EntityBase from scope. In particular, this means that
I have a class that references EntityBase
is wrong.
Since the only constraint on the generic parameter EntityBase is : class, the only members you can use are those found on object, such as GetHashCode(), GetType(), and ToString().
You use EntityBase as a generic type parameter in the <> and constrain it to be a decendant of the type "class". So any class. Therefore the compiler can not know whether EntityBase is an actual decentant of your EntityBase class. What you probably want to do is simply strip the EntityBase generic type from between the <>.
This question already has answers here:
C# Multiple generic constraints
(7 answers)
Closed 9 years ago.
I want to create a generic class where T parameter can be any instance from the set of other types.
So this code compiles fine:
public interface TestA { }
public interface TestB { }
public class Test<T> where T : TestA, TestB
{
}
However, when I try to use it like this
var a = new Test<TestA>();
var b = new Test<TestB>();
I get two compile errors:
Error 1 The type 'TestA' cannot be used as type parameter 'T' in the
generic type or method 'Test'. There is no implicit reference
conversion from 'TestA' to 'TestB'.
Error 2 The type 'TestB' cannot be used as type parameter 'T' in the
generic type or method 'Test'. There is no implicit reference
conversion from 'TestB' to 'TestA'.
I am confused as to why this doesn't work. How can I workaround this? Ideally, I'd like TestA and TestB classes to have no relation (eg. no inheritance between them).
The type must follow all contraints defined on the generic type (AND, not OR).
If you want to allow both TestA or TestB, you should define a base interface :
public interface TestBase { }
public interface TestA : TestBase { }
public interface TestB : TestBase { }
public class Test<T> where T : TestBase
{
}
Well you specified that the class must implement both interfaces. So you must give it a class that implements both interfaces, not just A or B
public class LikeThis : TestA, TestB
{
// have both A and B's properties, methods, etc.
}
var a = new Test<LikeThis>();
The other option is to make both TestA and TestB inherit from a base interface
public interface IBaseInterface { }
public interface IInterfaceA { }
public interface IInterfaceB { }
public class Test<T> where T : IBaseInterface
{
}
var a = new Test<IBaseInterface>();
But it is hard to tell what you are trying to accomplish here. The Test class could possibly hold a reference to IBaseInterface as a property, but it will not know if it is InterfaceA or InterfaceB.
Maybe if you tell us what you are trying to accomplish we can suggest a better solution.
The constraints on the type parameter are conjunctive (AND), so any type used there must implement both interfaces.
One way, as already mentioned, is to create a super-interface to both interfaces and constrain your type parameter to that (note that I added the usual interface prefix I):
public interface IBase { }
public interface ITestA : TestBase { }
public interface ITestB : TestBase { }
public class Test<T> where T : IBase {
}
That has the disadvantage that it's not just implementers of ITestA or ITestB that can be used as type parameters in Test<T>, but any other type that implements IBase.
Another option is to provide a super class that cannot be inherited from outside your assembly and create two sub-classes from it, one for each desired interface type:
public interface ITestA { }
public interface ITestB { }
public abstract class Test<T> {
internal Test() { }
}
public class Test1<T> : Test<T> where T : ITestA { }
public class Test2<T> : Test<T> where T : ITestB { }
Now the Test<T> superclass cannot be inherited from outside your assembly (it has an internal constructor). Your classes (Test1<T> and Test2<T>) just use the logic of the superclass and each works with one of the desired interfaces as a constraint. Client code would have to choose which one to use based on the interface constraint they'd like to use.
Also, if you have common code in your interfaces that you'd like to use inside Test<T>, you should extract that to a super-interface and have your super-class be constrained by it, resulting in a hybrid of both approaches:
public interface IBase { }
public interface ITestA : IBase { }
public interface ITestB : IBase { }
public abstract class Test<T> where T : IBase {
internal Test() { }
}
public class Test1<T> : Test<T> where T : ITestA { }
public class Test2<T> : Test<T> where T : ITestB { }
If I have this code:
public interface IThing<T> where T : class
{
// ...
}
public class BaseThing<T> : IThing<T> where T : class
{
// ...
}
public class ThingA : BaseThing<string>
{
// ...
}
public class ThingB : BaseThing<Uri>
{
// ...
}
This code fails:
List<IThing<object>> thingList = new List<IThing<object>>();
thingList.Add(new ThingA());
thingList.Add(new ThingB());
Even though ThingA (indirectly) inherits from (and should be an instance of) IThing<T>. Why? Is ThingA/ThingB not an instance of IThing<T>?
This would require your interface to be covariant. For details, see Covariance and Contravariance in Generics.
In this case, you can make this work by using:
// Add out here
public interface IThing<out T> where T : class
{
}
Note that this does place limitations on the interface and what you can do with it, however, as it requires that the type T in the interface be used only as a method return type within the interface, and not used as a type of formal method parameters.
If this is not viable, another option is to create a non-generic IThing interface, and have IThing<T> implement IThing. You could then use List<IThing> for your collection.
Given these base classes and interfaces
public abstract class Statistic : Entity, IStatistic
{
protected abstract IStatisticsRepository<IStatistic> Repository {get;}
...
public class AverageCheckTime : Statistic
...
public interface IStatisticsRepository<T> : IRepository<T> where T : IStatistic
...
public interface IAverageCheckTimeRepository : IStatisticsRepository<AverageCheckTime>
...
public class AverageCheckTimeRepository : StatisticRepository<AverageCheckTime>, IAverageCheckTimeRepository
...
public class RepositoryFactory
{
public static IAverageQueueTimeRepository AverageQueueTimeRepository
{
get { return CurrentServiceLocator.GetInstance<IAverageQueueTimeRepository>(); }
}
Why does AverageCheckTime's implementation throw an invalid cast exception:
protected override IStatisticsRepository<IStatistic> Repository
{
get { return (IStatisticsRepository<IStatistic>)RepositoryFactory.AverageCheckTimeRepository; }
}
How do I cast an instance of IAverageCheckTimeRepository as an IStatisticsRepository<IStatistic> which I assumed it already was?
OK, I've made these changes...which makes me wonder if I've gone over the top with the generics in the first place
public interface IStatisticsHelper
{
void GenerateStatistics();
List<IStatistic> BuildReport();
}
...
public interface IStatisticsRepository<T> : IRepository<T>, IStatisticsHelper where T : IStatistic
{
}
...
public abstract class Statistic : Entity, IStatistic
{
protected abstract IStatisticsHelper Repository { get; }
...
public class AverageCheckTime : Statistic
{
protected override IStatisticsHelper Repository
{
get { return RepositoryFactory.AverageCheckTimeRepository; }
}
No, C# 3 does not support generic variance. C# 4 does, but you would have to declare that IStatisticsRepository is covariant in T:
public interface IStatististicsRepository<out T> : IRepository<T>
where T : IStastistic
Variance isn't safe in general - it depends on how the generic type parameter is used. C# 4 supports both covariance and contravariance for type arguments which are reference types, but only when the generic type involved is an interface or a delegate, and only when the type parameter is used in the appropriate way within the interface/delegate.
Without seeing the declaration for IRepository<T>, we can't tell whether or not it's safe. For example, if IRepository<T> contains a method like this:
void Save(string id, T value);
then it wouldn't be safe, because you'd be able to write:
IStatisticsRepository<IStatistic> repo = RepositoryFactory.AverageCheckTimeRepository;
IStatistic foo = new SomeOtherStastisticType();
repo.Save("Foo", foo);
That would be trying to save a SomeOtherStatisticType value in an AverageCheckTimeRepository, which violates type safety. It's only safe to make the interface covariant in T if values of type T only come "out" of the interface. (There are some wrinkles around exactly what that means, mind you...)
For a lot more information on this, see Eric Lippert's blog series on the topic.