Is it possible to change the type of T in a method that takes a generic parameter into any kind of object when inherited. For example, I have this interface:
public interface IMethod
{
void Add<T>(T obj);
List<T> Get<T>();
void Update<T>(T obj);
void Delete<T>(T obj);
}
And I have 4 classes that are Book, Bookcase, Shelf. For each of them I have another class where I implement the methods, so I have the functionality there. Here is the Bookcasecatalog clas.
public class BookcaseCatalog: IMethod
{
private ObservableCollection<Bookcase> obsCase;
public string ConnectionString { get; set; }
public void Add(Bookcase obj)
{
}
public void Add<T>(T obj) where T : Bookcase
{
//Do smth
}
}
And when I'm done here, inherit it the interface in another class and T is a Book for example.
As you have it right now, the user can decide what kind of T he uses when calling the method Add (your constraint limits that, but thats not the idea of how to use them, they shouldn't be used at implementation.).
If you can, make your interface generic. This will allow you to decide what T is when implementing the class. Example:
public interface IMethod<T>
{
void Add<T>(T obj);
List<T> Get<T>();
void Update<T>(T obj);
void Delete<T>(T obj);
}
This will make all of your T the same type as the T in the functions
You can use it like this:
public class BookcaseCatalog: IMethod<Bookcase>
{
private ObservableCollection<Bookcase> obsCase;
public string ConnectionString { get; set; }
public void Add(Bookcase obj)
{
//Do smth
}
}
I think what you need here is a generic interface!
You should change your IMethod to this:
public interface IMethod<T> {
void Add(T obj);
List<T> Get();
void Update(T obj);
void Delete(T obj);
}
Now you get it? When you implement the interface, you are going to specify a type:
public class BookcaseCatalog : IMethod<Bookcase> {
//...
}
Then all the T's in the interface will be replaced by Bookcase:
public class Bookcase : IMethod<Bookcase> {
public void Add(Bookcase obj) {/*Do something*/}
public List<Bookcase> Get() {return something}
public void Update(Bookcase obj) {/*Do something*/}
public void Delete(Bookcase obj) {/*Do something*/}
}
That's it!
And I think it's better to learn some terminologies here. IMethod<Bookcase> is called a closed type, and IMethod<T> is called an open type.
Please note that if a method needs a parameter of an IMethod<T>, you can pass it an IMethod<Bookcase>. But if it wants an IMethod<Bookcase>, you cannot give it an IMethod<SomethingElse>. This means that closed types can be converted their open counterparts but closed types cannot be converted to other closed types unless the rules of contra- and co-variance apply.
Related
I want to implement an interface in C# and there is a function called Testfunction.
For example:
public interface Test
{
void Testfunction(type filter);
}
But I want to make all the classes inherit this interface can implement this Testfunction with all different type of parameters, is it possible?
You can try using generics, e.g.:
public interface ITest<T>
{
void TestFunction(T filter);
}
then you can put:
// Here filter is of type int
public class MyTestInt : ITest<int> {
public void TestFunction(int filter) {
...
}
}
// And here filter is of type string
public class MyTestString : ITest<string> {
public void TestFunction(string filter) {
...
}
}
etc.
Yes, with Generics.
public interface Test<T>
{
void TestFunction(T filter);
}
Implementers would then do
public class Foo : Test<Bar>
{
public void TestFunction(Bar filter)
{
// ...
}
}
You can even add constraints to your generic parameter (T), as documented here, for example you could restrict it so that any type used as a type parameter must implement another specific interface, and a bunch more stuff
Maybe this is a dumb question. But, I don't get the point what I am missing.
Given the following class-definition
public abstract class AbstractBaseClass
{
public abstract void Create(AnotherAbstractClass param1);
}
Wheras AnotherAbstractClass is defined
public abstract class AnotherAbstractClass
{
}
with a concrete implementation
public class AnotherConcreteImplementation : AnotherAbstractClass
{
}
I want to be able to have the override of the Create method to use a concrete type:
public class ConcreteImplementation : AbstractBaseClass
{
public override void Create(AnotherConcreteImplementation param1) <-- There is no suitable method for override
{
// param1 is an instance of the concrete implementation
}
public override void Create(AnotherAbstractClass param1) <-- this is working but I'll have to cast on each implementation
{
// param1 is an instance of the abstract class and needs a cast
}
}
Is this simply not possible or is there some way I'm not aware of? Maybe using generics?
Edit #1 (added more context)
I'm trying to achieve/enforce that in a concrete implementation there is only one parameter valid.
Think of it like it's a database-layer. The Create method will create a new entry in the database. As of each table has different values, the create-parameter also has.
The casting inside smells (in my opinion) as of it can be called with any concrete implementation of AnotherAbstractClass.
public class AddressTable : AbstractBaseClass
{
public override void Create(AnotherAbstractClass param1)
{
// cast to concrete instance
var casted = (ConcreteAddressCreate)param1;
}
}
public class CityTable : AbstractBaseClass
{
public override void Create(AnotherAbstractClass param1)
{
// cast to concrete instance
var casted = (ConcreteCityCreate)param1;
}
}
Having an instance of AddressTable I can call
addressIntance.Create(new ConcreteAddressCreate()); // would be okay
on the other hand I can call it
addressIntance.Create(new ConcreteCityCreate()); // would be okay but will fail at runtime with InvalidCastException
Edit #2 (additional info)
It should also be possible to extend the AbstractBaseClass class with more abstract methods later.
So, for me it's more likely to have generic methods instead of an concrete class-implemenation with 200 generic parameters for each method to implement.
It violates the Liskov Substitution Principle so it makes perfect sense you can't do this. Namely, you can't just "have" covariance like this for free:
AbstractBaseClass bcl = new ConcreteImplementation();
bcl.Create(new DifferentImplementationWithoutSecondAbstract());
The contract AbstractBaseClass defines makes Create have to work with any implementation of AbstractBaseClass passed in - if you give a constraint on what can be passed in you've violated the contract it defines.
Like you assumed - you can use generics:
// notice the recursive definition, we require the generic parameter
// to be a generic parameter of itself - allowing AbstractBaseClass
// to not be aware of its subclasses like in the other answers.
public abstract class AbstractBaseClass<T> where T : AbstractBaseClass<T>
{
public abstract void Create(T param1);
}
public class Concrete : AbstractBaseClass<Concrete>
{
public override void Create(Concrete param1)
{
Console.WriteLine("Hello!");
}
}
Yes, you can do that using generics:
public abstract class AbstractBaseClass<T>
where T : AnotherAbstractClass
{
public abstract void Create(T param1);
}
public class ConcreteImplementation : AbstractBaseClass<AnotherConcreteImplementation>
{
public override void Create(AnotherConcreteImplementation param1)
{
}
}
Generics is indeed the way to do it.
public abstract class AbstractBaseClass<TDerivedClass> where TDerivedClass : AnotherAbstractClass
{
public abstract void Create(TDerivedClass param1);
}
And then you can do:
public class ConcreteImplementation : AbstractBaseClass<AnotherConcreteImplementation>
{
public override void Create(AnotherConcreteImplementation param1) // Works because TDerivedClass = AnotherConcreteImplementation
{
...
}
}
I am getting this error while compiling the code and not able to figure out the exact reason:
Class does not implement interface member
This is my code:
interface IReview<T> where T : Review
{
IEnumerable<T> Reviews { get; set; }
void AddReview<T>(T item);
}
class ReviewCollection : IReview<Review>
{
IEnumerable<Review> _reviews;
public IEnumerable<Review> Reviews
{
get { return _reviews; }
set { _reviews = value; }
}
public void AddReview(Review item)
{
}
}
What is the issue with it?
Your definition of AddReview in the interface is wrong. It should read:
void AddReview(T item);
The generic type argument T is already provided by the class, and you don't want to deviate in your method (in this case). You now changed the meaning of T to be a local type parameter, not to use the one available on the class level.
You didn't implement the generic method:
public void AddReview<Review>(Review item)
{
}
Or you should change the signature of the method in your interface:
void AddReview(T item);
You might have to change this method:
public void AddReview(Review item)
To this:
public void AddReview<Review>(Review item)
I have a problem with a typed method in C#.
I want to invoke a inherited method of an object.
This method invokes a static method with "this" as parameter.
The parameter of the static method is generic.
I now want the generic type of this parameter to be the type of the first object.
But the parameter has always the type of the abstract class.
Here is the example:
abstract class AbstractClass
{
bool update()
{
Connection.Update(this);
}
}
class Entity : AbstractClass
{
}
class Connection
{
public static void Update<T>(T obj)
{
someMethod<T>()
}
}
If I try to do:
Entity foo = new Entity();
foo.update();
Connection.Update will look in the Debugger like this:
public static void Update<AbstractClass>(AbstractClass obj)
{
someMethod<AbstractClass>()
}
But I want this:
public static void Update<Entity>(Entity obj)
{
someMethod<Entity>()
}
Is there any possibility to something like
someMethod<typeof(obj)>()
or anything else to solve my problem?
You can declare the base class as a generic one, here is the example:
abstract class AbstractClass<T>
{
bool update()
{
Connection.Update<T>(this as T);
}
}
class Entity : AbstractClass<Entity>
{
}
class Connection
{
public static void Update<T>(T obj)
{
someMethod<T>()
}
}
Compiler tries to infer the type of T parameter, in the base class it does not have information to infer Entity type. So you should provide information of the child type while you want to call the Update function.#Alexandr answer is a good one just an improvement adding type constraint to T type parameter to restrict it be a child of AbstractClass
abstract class AbstractClass<T>
where T: AbstractClass<T> //restrict T as a child of AbstractClass<T>
{
bool update()
{
Connection.Update<T>(this as T);
}
}
class Entity : AbstractClass<Entity>
{
}
class Connection
{
public static void Update<T>(T obj)
{
someMethod<T>()
}
}
If you want a quick fix then cast this to dynamic which will delay the type evaluation to runtime. However, consider using the visitor pattern.
public bool update()
{
Connection.Update((dynamic)this);
return true;
}
Visitor pattern:
public interface IEntityVisitor
{
void Visit(EntityBase entity);
void Visit(Entity entity);
}
public interface IEntity
{
void Accept(IEntityVisitor visitor);
}
public abstract class EntityBase : IEntity
{
public virtual void Accept(IEntityVisitor visitor)
{
visitor.Visit(this);
}
}
public class Entity : EntityBase
{
public override void Accept(IEntityVisitor visitor)
{
visitor.Visit(this);
}
}
var genericType = typeof(Connection<>);
var specificType = genericType.MakeGenericType(typeof(Entity));
var conn = Activator.CreateInstance(specificType);
Any thing like someMethod<typeof(obj)>() does not exist as it would break the type safety of C#, because the compiler cannot know the runtime type of a parameter. However, you can get the true type object by invoking obj.GetType(). You can use this type object to call methods dynamically using Reflection.
One solution on this:
abstract class AbstractClass
{
bool update<T>()
{
Connection.Update(T);
}
}
then,
Entity foo = new Entity();
foo.update<Entity>();
I've made a generic class that saves and queries objects from a flat file. I keep having to change the method arguments to objects so I can cast them and I'm wondering if I'm going about this the right way...
'T' will always inherit 'FlatFileRecord'
This does not compile:
public class FlatFile<T>
{
public void Save(T record)
{
FlatFileRecord castedRecord = (FlatFileRecord)record;
castedRecord.RecordNumber...
}
}
This compiles but seems to defeat the whole point of a strongly typed generic class:
public class FlatFile<T>
{
public void Save(object record)
{
FlatFileRecord castedRecord = (FlatFileRecord)record;
castedRecord.RecordNumber...
}
}
If T is always going to be derived from FlatFileRecord, then constrain it that way:
public class FlatFile<T> where T : FlatFileRecord
{
public void Save(T record)
{
FlatFileRecord flatRecord = record;
flatRecord.RecordNumber...
}
}
If you need to make do without the constraint for some reason, you can cast to object and then back down again:
public class FlatFile<T>
{
public void Save(T record)
{
FlatFileRecord flatRecord = (FlatFileRecord)(object)record;
flatRecord.RecordNumber...
}
}