Develop Reference

Passing a member-access function in the constructor

Let's say I have a helper class like
public class Selection<T, W> : ISelection<T,W> where W : ICollection<T>
{
public Selection(Func<W> selectableItemsProvider)
{
...
}
}
As C# does not support delegating interface implementation, I thought I'd save myself a bit of boilerplate and just extend the Selection class...
public class MyFoo : Selection<Blah, List<Blah>>
{
private List<Blah> _blahs = new List<Blah>();
public MyFoo() : base(() => _blahs)
{
...
}
}
Except I cannot do that, can I, because
Cannot access non-static property '_blahs' in static context
even though I already know the provider won't be invoked until after object construction.
Is there a way around this or am I stuck with adding boilerplate code?

One somewhat roundabout way to do this is to pass a lambda that returns the same list every time to the base class constructor. Then call that lambda to initialise blahs:
public class Selection<T, W>: ISelection<T, W> where W : ICollection<T> {
// assuming you have a property like this that is initialised by the constructor...
protected Func<W> SelectableItemsProvider { get; }
public Selection(Func<W> selectableItemsProvider)
{
SelectableItemsProvider = selectableItemsProvider;
}
}
public class MyFoo : Selection<Blah, List<Blah>>
{
private List<Blah> _blahs;
public MyFoo() : base(new List<Blah>().CaptureInFunc())
{
_blahs = SelectableItemsProvider();
}
}
CaptureInFunc is an extension method declared like this:
public static Func<T> CaptureInFunc<T>(this T t) => () => t;
This gives us a lambda that returns t every time.

One approach might be to first introduce base abstract class. This class will hold all the logic:
public abstract class SelectionBase<T, W> where W : ICollection<T>
{
protected SelectionBase() {
}
protected abstract Func<W> GetSelectableItemsProvider();
}
Then change your current Selection to inherit from it and accept selector in constructor, that's the whole implementation:
public sealed class Selection<T, W> : SelectionBase<T, W> where W: ICollection<T> {
private readonly Func<W> _selectableItemsProvider;
public Selection(Func<W> selectableItemsProvider) {
_selectableItemsProvider = selectableItemsProvider;
}
protected override Func<W> GetSelectableItemsProvider() {
return _selectableItemsProvider;
}
}
Now if you want to extend selection in a way mentioned in your question:
public class SpecificSelection : SelectionBase<string, List<string>> {
private List<string> _blah;
public SpecificSelection() {
}
protected override Func<List<string>> GetSelectableItemsProvider() {
return () => _blah;
}
}

You can't refer to members of 'this' in the call to a base constructor because 'this' does not exist yet.
I suggest composition instead of inheritance. The class may still implement the same interface as the helper and then delegate all calls to the real implementation.
public class MyFoo : ISelection<Blah, List<Blah>>
{
private List<Blah> _blahs;
private ISelection<Blah, List<Blah>> _implementation;
public MyFoo()
{
_blahs = new List<Blah>();
_implementation = new Selection<Blah, List<Blah>>(() => _blahs);
}
public void DoSomethingWithSelection()
{
_implementation.DoSomethingWithSelection();
}
}

Casting to a generic interface [duplicate]

I have the following classes
public abstract class BaseViewPresenter { }
public abstract class BaseView<T> : UserControl
where T : BaseViewPresenter { }
public class LoginPresenter : BaseViewPresenter { }
public partial class LoginView : BaseView<LoginPresenter> { }
I have a method that looks like this (simplified)
public BaseView<BaseViewPresenter> Resolve(BaseViewPresenter model)
{
var type = model.GetType();
var viewType = _dataTemplates[type];
// Correctly creates BaseView object
var control = Activator.CreateInstance(viewType);
// Fails to cast as BaseView<BaseViewPresenter> so returns null
return control as BaseView<BaseViewPresenter>;
}
When I call this using an instances of LoginPresenter
var login = new LoginPresenter();
var ctl = Resolve(login);
The line Activator.CreateInstance(viewType) correctly resolves into a new instances of my LoginView, however control as BaseView<BaseViewPresenter> can't do the cast correctly so returns null.
Is there a way to correctly cast the control into BaseView<BaseViewPresenter> without using specific type generics?
Since LoginView inherits from BaseView<LoginPresenter>, and LoginPresenter inherits from BaseViewPresenter, I would assume there's a way to convert LoginView to BaseView<BaseViewPresenter>.
I am stuck with using .Net 3.5

This is a very frequently asked question. Let's rename your types:
abstract class Fruit { } // was BaseViewPresenter
abstract class FruitBowl<T> where T : Fruit // was BaseView
class Apple : Fruit { } // was LoginPresenter
class BowlOfApples : FruitBowl<Apple> { } // was LoginView
Your question now is:
I have a BowlOfApples, which inherits from FruitBowl<Apple>. Why can I not use it as a FruitBowl<Fruit>? An apple is a fruit, so a bowl of apples is a bowl of fruit.
No, it isn't. You can put a banana in a bowl of fruit, but you can't put a banana in a bowl of apples, and therefore a bowl of apples is not a bowl of fruit. (And by similar argument, a bowl of fruit is not a bowl of apples either.) Since the operations you can legally perform on the two types are different, they cannot be compatible.
Here is a photo of StackOverflow legend Jon Skeet demonstrating this fact:
The feature you want is called generic contravariance, and it is supported only on interfaces and delegate types when the compiler can prove that the variance is safe, and when the varying type is a reference type. For example, you can use an IEnumerable<Apple> in a context where IEnumerable<Fruit> is needed because the compiler can verify that there is no way that you can put a Banana into a sequence of fruit.
Do a search on "C# covariance and contravariance" on this site or on the web and you'll find many more details about how this feature works. In particular, my series of articles on how we designed and implemented this feature in C# 4 starts here: http://blogs.msdn.com/b/ericlippert/archive/2007/10/16/covariance-and-contravariance-in-c-part-one.aspx

I accepted Eric's answer since it provides a great explanation of why what I wanted wasn't possible, but I also thought I'd share my solution in case anyone else runs into this same problem.
I removed the generic type parameter from my original BaseView class, and created a 2nd version of the BaseView class that included the generic type parameter and specifics for it.
The first version is used by my .Resolve() method or other code that doesn't care about the specific types, and the second version is used by any code that does care, such as the implentation of a BaseView
Here's an example of how my code ended up looking
// base classes
public abstract class BaseViewPresenter { }
public abstract class BaseView : UserControl
{
public BaseViewPresenter Presenter { get; set; }
}
public abstract class BaseView<T> : BaseView
where T : BaseViewPresenter
{
public new T Presenter
{
get { return base.Presenter as T; }
set { base.Presenter = value; }
}
}
// specific classes
public class LoginPresenter : BaseViewPresenter { }
public partial class LoginView : BaseView<LoginPresenter>
{
// Can now call things like Presenter.LoginPresenterMethod()
}
// updated .Resolve method used for obtaining UI object
public BaseView Resolve(BaseViewPresenter presenter)
{
var type = model.GetType();
var viewType = _dataTemplates[type];
BaseView view = Activator.CreateInstance(viewType) as BaseView;
view.Presenter = presenter;
return view;
}

You're expecting to treat the type as being covariant with respect to the generic argument. Classes can never be covariant; you'd need to use an interface rather than (or in addition to) an abstract class to make it covariant with respect to T. You'd also need to be using C# 4.0.

My usual solution to this problem is to create an intermediary class that has access to the type-parametric class's methods through delegates. Fields can also be accessed through getters/setters.
The general pattern goes:
public abstract class Super {}
public abstract class MyAbstractType<T> where T : Super {
public MyGeneralType AsGeneralType() {
return MyGeneralType.Create(this);
}
// Depending on the context, an implicit cast operator might make things
// look nicer, though it might be too subtle to some tastes.
public static implicit operator MyGeneralType(MyAbstractType<T> t) {
return MyGeneralType.Create(t);
}
public int field;
public void MyMethod1() {}
public void MyMethod2(int argument) {}
public abstract bool MyMethod3(string argument);
}
public delegate T Getter<T>();
public delegate void Setter<T>(T value);
public delegate void MyMethod1Del();
public delegate void MyMethod2Del(int argument);
public delegate bool MyMethod3Del(string argument);
public class MyGeneralType {
public Getter<int> FieldGetter;
public Setter<int> FieldSetter;
public MyMethod1Del MyMethod1;
public MyMethod2Del MyMethod2;
public MyMethod3Del MyMethod3;
public static MyGeneralType Create<T>(MyAbstractType<T> t) where T : Super {
var g = new MyGeneralType();
g.FieldGetter = delegate { return t.field; };
g.FieldSetter = value => { t.field = value; };
g.MyMethod1 = t.MyMethod1;
g.MyMethod2 = t.MyMethod2;
g.MyMethod3 = t.MyMethod3;
return g;
}
public int field {
get { return FieldGetter(); }
set { FieldSetter(value); }
}
}
The above exemplifies getting all the methods and fields but normally I only need a few of them. This is a general solution to the problem and one could feasibly write a tool to generate these intermediary classes automatically, which I might at some point.
Try it here: https://dotnetfiddle.net/tLkmgR
Note that this is enough for all my cases, but you can be extra hacky with this:
public abstract class MyAbstractType<T> where T : Super {
// ... Same everything else ...
// data fields must become abstract getters/setters, unfortunate
public abstract int field {
get;
set;
}
public static implicit operator MyAbstractType<Super>(MyAbstractType<T> t) {
return MyGeneralType.Create(t);
}
}
public class MyGeneralType : MyAbstractType<Super> {
// ... same constructors and setter/getter
// fields but only keep method fields
// that contain the method references for
// implementations of abstract classes,
// and rename them not to clash with the
// actual method names ...
public MyMethod3Del myMethod3Ref;
// Implement abstract methods by calling the corresponding
// method references.
public override bool MyMethod3(string argument) {
return myMethod3Ref(argument);
}
// Same getters/setters but with override keyword
public override int field {
get { return FieldGetter(); }
set { FieldSetter(value); }
}
}
And there you go, now you can literally cast a MyAbstractType<Sub> where Sub : Super to a MyAbstractType<Super>, although it's no longer the same object anymore, but it does retain the same methods and data, it's sort of a complex pointer.
public class Sub : Super {}
public class MySubType : MyAbstractType<Sub> {
public int _field;
public override int field {
get { return _field; }
set { _field = value; }
}
public override bool MyMethod3(string argument) {
Console.WriteLine("hello " + argument);
return argument == "world";
}
}
public class MainClass {
public static void Main() {
MyAbstractType<Sub> sub = new MyAbstractType<Sub>();
MyAbstractType<Super> super = sub;
super.MyMethod3("hello"); // calls sub.MyMethod3();
super.field = 10; // sets sub.field
}
}
This isn't as good in my opinion, the other version of MyGeneralType is a more straighforward layer over the concrete types, plus it doesn't require rewriting the data fields, but it does actually answer the question, technically. Try it here: https://dotnetfiddle.net/S3r3ke
Example
Using these abstract classes:
public abstract class Animal {
public string name;
public Animal(string name) {
this.name = name;
}
public abstract string Sound();
}
public abstract class AnimalHouse<T> where T : Animal {
List<T> animals;
public AnimalHouse(T[] animals) {
this.animals = animals.ToList();
}
public static implicit operator GeneralAnimalHouse(AnimalHouse<T> house) {
return GeneralAnimalHouse.Create(house);
}
public List<string> HouseSounds() {
return animals.Select(animal => animal.Sound()).ToList();
}
}
We make this "general" variant:
public delegate List<string> HouseSoundsDel();
public class GeneralAnimalHouse {
public HouseSoundsDel HouseSounds;
public static GeneralAnimalHouse Create<T>(AnimalHouse<T> house) where T : Animal {
var general = new GeneralAnimalHouse();
general.HouseSounds = house.HouseSounds;
return general;
}
}
And finally with these inheritors:
public class Dog : Animal {
public Dog(string name) : base(name) {}
public override string Sound() {
return name + ": woof";
}
}
public class Cat : Animal {
public Cat(string name) : base(name) {}
public override string Sound() {
return name + ": meow";
}
}
public class DogHouse : AnimalHouse<Dog> {
public DogHouse(params Dog[] dogs) : base(dogs) {}
}
public class CatHouse : AnimalHouse<Cat> {
public CatHouse(params Cat[] cats) : base(cats) {}
}
We use it like this:
public class AnimalCity {
List<GeneralAnimalHouse> houses;
public AnimalCity(params GeneralAnimalHouse[] houses) {
this.houses = houses.ToList();
}
public List<string> CitySounds() {
var random = new Random();
return houses.SelectMany(house => house.HouseSounds())
.OrderBy(x => random.Next())
.ToList();
}
}
public class MainClass {
public static void Main() {
var fluffy = new Cat("Fluffy");
var miu = new Cat("Miu");
var snuffles = new Cat("Snuffles");
var snoopy = new Dog("Snoopy");
var marley = new Dog("Marley");
var megan = new Dog("Megan");
var catHouse = new CatHouse(fluffy, miu, snuffles);
var dogHouse = new DogHouse(snoopy, marley, megan);
var animalCity = new AnimalCity(catHouse, dogHouse);
foreach (var sound in animalCity.CitySounds()) {
Console.WriteLine(sound);
}
}
}
Output:
Miu: meow
Snoopy: woof
Snuffles: meow
Fluffy: meow
Marley: woof
Megan: woof
Notes:
I added names so it's clear that the method references carry their owner's data with them, for those unfamiliar with delegates.
The required using statements for this code are System, System.Collections.Generic, and System.Linq.
You can try it here: https://dotnetfiddle.net/6qkHL3#
A version that makes GeneralAnimalHouse a subclass of AnimalHouse<Animal> can be found here: https://dotnetfiddle.net/XS0ljg

Mapping classes c#

I am writing a tranformer that takes some input and gives an output.I need to call a specific tranformer based on my input type.
public static myentrypoint( template t);
{
//I could do something like this.
switch(t)
{
case t1:
transformt1(..);
case t2:
transformt1(..);
....
}
}
Trasform1 : Itransform
{
tranform1(...);
}
Trasform2 : Itransform
{
tranform2(...);
}
I need to map which function to call based on what my template is. I can do a switch but are there more cleaner ways to do this using some design patterns ? I was thinking a of writing a static dictionary. I am new to OOP so any suggestions would be great.

If template is a class, and each template potentially has a different transform, then why not just include the transform function inside of your template class?
public static myentrypoint( ITemplate t);
{
t.transform();
}

The way that I do these types of situations is through the use of Generics. (Shameless self-promotion of a blog post)
Basically, you'll have your base class set up like this:
public abstract class Transformer<T>
where T : Template
{
public abstract void Transform(T item);
}
Then you derive for each of your types like this:
public class Transformer1 : Tansformer<Template1>
{
public void Transform(Template1 item)
{
}
}
public class Transformer2 : Transformer<Template2>
{
public void Transform(Template2 item)
{
}
}
Then you'll just need a factory to give you the correct Transformer.
public class TransformFactory
{
public Transformer<T> GetTransformer<T>(T item)
{
if (item is Template1)
return new Transformer1();
else if (item is Template2)
return new Transformer2();
// ...
}
}
The benefit of this approach is that you'll be able to encapsulate all behavior on that specific type in the concrete implementations. If there is any common behavior on them all, you can do that in the abstract base.

Invoking methods based on a parameter without switch-case statements in C#
In OOP, based on the [open/close principle] which says that software entities such as classes and functions should be open for extension, but closed
for modification.
Methods which use switch-case statement would call this principle into question. In order to implement this principle inside the codes without
causing changes in their functionality.
We use a pattern named "Delegate Dictionary Pattern".
For example, we have an entity named Template that keep input values as well as some of Transform classes for processing this Template.
Template class for keeping input value
public class Template
{
public int TransformNo { get; set; }
public string Title { get; set; }
}
ITransform interface for transform abstract
public interface ITransform
{
void Do(Template template);
}
Transform1 as a concrete class of ITransform
public class Transform1 : ITransform
{
public void Do(Template template)
{
Console.WriteLine($"Transform : {template.TransformNo}, TemplateTitle : { template.Title}");
}
}
Transform2 as a concrete class of ITransform
public class Transform2 : ITransform
{
public void Do(Template template)
{
Console.WriteLine($"Transform : {template.TransformNo}, TemplateTitle : { template.Title}");
}
}
TransformCordinator class for coordinating template of *ITransformer**
public class TransformCordinator
{
Dictionary<int, Action<Template>> transformMap = new Dictionary<int, Action<Template>>();
public TransformCordinator()
{
transformMap.Add(1, x => new Transform1().Do(x));
transformMap.Add(2, x => new Transform2().Do(x));
}
public void Do(Template template)
{
transformMap[template.TransformNo](template);
}
}
// example
class Program
{
static void Main(string[] args)
{
var transformCordinator = new TransformCordinator();
transformCordinator.Do(new Template() { TransformNo = 1, Title = "Hi!" });
Console.ReadLine();
}
}

C# - Access property in inheriting class

I'm trying to access a generic typed property in a child class. In the below example I recreated my problem. Is there a workaround for this problem, or is it simply not possible? Thanks in advance!
EDIT: It's not possible to declare the collection as A<Model> or A<T>.
public abstract class Model {
public int Id { get; }
}
public interface I<T> where T: Model {
ICollection<T> Results { get; }
}
public abstract class A { }
public class A<T> : A, I<T> where T : Model {
public ICollection<T> Results { get; }
}
public class Example {
A[] col;
void AddSomeModels() {
col = new A[] {
new A<SomeModel>(),
new A<SomeOtherModel>()
}
}
void DoSomethingWithCollection() {
foreach (var a in col) {
// a.Results is not known at this point
// is it possible to achieve this functionality?
}
}
}

You can't do what you intend without some compromises.
First of all, you need to make your interface I<T> covariant in T:
public interface I<out T> where T : Model
{
IEnumerable<T> Results { get; }
}
The first compromise is therefore that T can only be an output. ICollection<T> isn't covariant in T so you'd need to change the type of Results to IEnumerable<T>.
Once you do this, the following is type safe and therefore allowed:
public void DoSomethingWithCollecion()
{
var genericCol = col.OfType<I<Model>>();
foreach (var a in genericCol )
{
//a.Results is now accessible.
}
}

C# Generics and Collections

I have an two interfaces defined as follows:
public interface IFoo
{
...
}
Public interface IFooWrapper<T> where T : IFoo
{
T Foo {get;}
}
I want to be able to declare a collection of IFooWrappers but I don't want to specify the implementation of IFoo.
Ideally I want to do something like the:
IList<IFooWrapper<*>> myList;
I can't figure out a way around this.

public interface IFoo
{
...
}
public interface IFooWrapper : IFoo
{
...
}
public interface IFooWrapper<T> : IFooWrapper
where T : IFoo
{
...
}
IList<IFooWrapper> myList;
this is a way to do what you want

What's wrong with
IList<IFooWrapper<IFoo>> myList?

public class FooWrapper : IFooWrapper<IFoo>

What I'm about to suggest is overkill for most situations, since usually you can create an interface higher up in the hierarchy that you can use. However, I think this is the most flexible solution in some ways, and the most faithful representation of what you want:
public interface IFooWrapperUser<U> {
U Use<T>(IFooWrapper<T> wrapper);
}
public interface IFooWrapperUser {
void Use<T>(IFooWrapper<T> wrapper);
}
public interface IExistsFooWrapper {
U Apply<U>(IFooWrapperUser<U> user);
void Apply(IFooWrapperUser user);
}
public class IExistsFooWrapper<T> : IExistsFooWrapper {
private IFooWrapper<T> wrapper;
public IExistsFoo(IFooWrapper<T> wrapper) {
this.wrapper = wrapper;
}
public U Apply<U>(IFooWrapperUser<U> user) {
return user.Use(foo);
}
public void Apply(IFooWrapperUser user) {
user.Use(foo)
}
}
Now you can create an instance of an IList<IExistsFooWrapper> which can be used as if it's an IList<IFooWrapper<*>>. The downside is you'll need to create a class to encapsulate the logic you want to run on each element:
private class FooPrinter : IFooWrapperUser<string> {
public string Apply<T>(IFooWrapper<T> wrapper) {
return wrapper.Foo.ToString();
}
}
...
IFooWrapperUser<string> user = new FooPrinter();
foreach (IExistFooWrapper wrapper in list) {
System.Console.WriteLine(wrapper.Apply(user));
}
...