C#•15mo ago

❔ Defining base impl ctor in interface

So currently for some testing I have the following code:

    interface HumanInterface
    {
        public string Name { get; }
        public string Description { get; }

        public void walk() { Console.WriteLine("Default implementation"); }

        // Forcing implementing class to define scream()
        public void scream();
    }

    class Foo : HumanInterface
    {
        Foo(string name, string description)
        {
            Name = name;
            Description = description;
        }

        public string Name { get; }
        public string Description { get; }
        void HumanInterface.scream() { Console.WriteLine("Screams"); }
    }

    interface HumanInterface
    {
        public string Name { get; }
        public string Description { get; }

        public void walk() { Console.WriteLine("Default implementation"); }

        // Forcing implementing class to define scream()
        public void scream();
    }

    class Foo : HumanInterface
    {
        Foo(string name, string description)
        {
            Name = name;
            Description = description;
        }

        public string Name { get; }
        public string Description { get; }
        void HumanInterface.scream() { Console.WriteLine("Screams"); }
    }

my ctor Foo is pretty generic, in fact I'd probably want all my derived classes to have that exact ctor. How'd I achieve this?

91 Replies

jorenOP•15mo ago

   interface HumanInterface
   {
       public string Name { get; init; }
       public string Description { get; init; }

       public void walk() { Console.WriteLine("Default implementation"); }

       // Forcing implementing class to define scream()
       public void scream();
   }

   class Foo : HumanInterface
   {

       public string Name { get; init; }
       public string Description { get; init; }
       void HumanInterface.scream() { Console.WriteLine("Screams"); }
   }

   interface HumanInterface
   {
       public string Name { get; init; }
       public string Description { get; init; }

       public void walk() { Console.WriteLine("Default implementation"); }

       // Forcing implementing class to define scream()
       public void scream();
   }

   class Foo : HumanInterface
   {

       public string Name { get; init; }
       public string Description { get; init; }
       void HumanInterface.scream() { Console.WriteLine("Screams"); }
   }

Foo a = new Foo { Name = "John", Description = "A person" };

Angius•15mo ago

You can use the : base()

class Bar : Foo
{
    public Bar(string name, string desc) : base(name, desc) {}
}

class Bar : Foo
{
    public Bar(string name, string desc) : base(name, desc) {}
}

Or an abstract class ig

jorenOP•15mo ago

abstract class is the same as in C++ I assume? at least one pure virtual method and its abstract?

Angius•15mo ago

There's abstract modifier You can have an empty abstract class just as well

jorenOP•15mo ago

ah right

MODiX•15mo ago

Angius

REPL Result: Failure

abstract class Foo {}
new Foo();

abstract class Foo {}
new Foo();

Exception: CompilationErrorException

- Cannot create an instance of the abstract type or interface 'Foo'

- Cannot create an instance of the abstract type or interface 'Foo'

Compile: 454.425ms | Execution: 0.000ms | React with ❌ to remove this embed.

jorenOP•15mo ago

mhm fair enough do ppl even use the interface keyword often seems terribly limited honestly

Angius•15mo ago

Yes, interfaces are very common C# has no multiple inheritance But a class can implement many interfaces

jorenOP•15mo ago

unless its an interface no? yy eh, that blows, what about my init; approach, would that be a acceptable approach? or do people avoid this altogether

Angius•15mo ago

Yeah, initializer syntax is commonly used And yeah, you can use required

jorenOP•15mo ago

required as in? there's a keyword?

Angius•15mo ago

class Foo
{
    public required int Thing { get; set; }
}

class Foo
{
    public required int Thing { get; set; }
}

jorenOP•15mo ago

ah so we force them to set it since i have a non-nullable type its actually pretty relevant

MODiX•15mo ago

Angius

REPL Result: Failure

class Foo
{
    public required int Thing { get; set; }
}
new Foo()

class Foo
{
    public required int Thing { get; set; }
}
new Foo()

Exception: CompilationErrorException

- Required member 'Foo.Thing' must be set in the object initializer or attribute constructor.

- Required member 'Foo.Thing' must be set in the object initializer or attribute constructor.

Compile: 478.418ms | Execution: 0.000ms | React with ❌ to remove this embed.

MODiX•15mo ago

Angius

REPL Result: Success

class Foo
{
    public required int Thing { get; set; }
}
new Foo { Thing = 420 }

class Foo
{
    public required int Thing { get; set; }
}
new Foo { Thing = 420 }

Result: Foo

{
  "thing": 420
}

{
  "thing": 420
}

Compile: 522.218ms | Execution: 62.802ms | React with ❌ to remove this embed.

jorenOP•15mo ago

required does not work within an interface it facepalm

there goes my plan

interface HumanInterface
{
    public string Name { get; init; }
    public string Description { get; init; }

    public void walk() { Console.WriteLine("Default implementation"); }

    public void scream();
}

class Foo : HumanInterface
{
    public required string Name { get; init; }
    public required string Description { get; init; }
    void HumanInterface.scream() { Console.WriteLine("Screams"); }
}

interface HumanInterface
{
    public string Name { get; init; }
    public string Description { get; init; }

    public void walk() { Console.WriteLine("Default implementation"); }

    public void scream();
}

class Foo : HumanInterface
{
    public required string Name { get; init; }
    public required string Description { get; init; }
    void HumanInterface.scream() { Console.WriteLine("Screams"); }
}

you know what bothers me, the fact that this has so much duplicate code, specifically the members are a bother to me

    public string Name { get; init; }
    public string Description { get; init; }

    public string Name { get; init; }
    public string Description { get; init; }

so what exactly is this, member declaration with definitions (much like setters/getters)?

Angius•15mo ago

$getsetdevolve

MODiX•15mo ago

class Foo
{
    private int _bar;

    public int GetBar()
    {
        return _bar;
    }

    public void SetBar(int bar)
    {
        _bar = bar;
    }
}

class Foo
{
    private int _bar;

    public int GetBar()
    {
        return _bar;
    }

    public void SetBar(int bar)
    {
        _bar = bar;
    }
}

can be shortened to

class Foo
{
    private int _bar;

    public int GetBar() => _bar;

    public void SetBar(int bar) => _bar = bar;
}

class Foo
{
    private int _bar;

    public int GetBar() => _bar;

    public void SetBar(int bar) => _bar = bar;
}

can be shortened to

class Foo
{
    private int _bar;
    public int Bar {
        get { return _bar; }
        set { _bar = value; }
    }
}

class Foo
{
    private int _bar;
    public int Bar {
        get { return _bar; }
        set { _bar = value; }
    }
}

can be shortened to

class Foo
{
    private int _bar;
    public int Bar {
        get => _bar;
        set => _bar = value;
    }
}

class Foo
{
    private int _bar;
    public int Bar {
        get => _bar;
        set => _bar = value;
    }
}

can be shortened to

class Foo
{
    public int Bar { get; set; }
}

class Foo
{
    public int Bar { get; set; }
}

jorenOP•15mo ago

though, in practice the get; and set; are much like changing the visiblity, rather than utilizing a setter/getter

Angius•15mo ago

They're literally getter and setter See above

jorenOP•15mo ago

ye with some magic bcs when you are actually setting the value its literally obj.val = 34; not obj.SetVal(34), I dont really see the whole setter/getter back in practice

Angius•15mo ago

Well, yes, with some magic We ain't writing Java, we don't write useless code lol

jorenOP•15mo ago

its literally just like a accessibility modifier in C++ you'd mark it public and call it a day because thats essentially what it is

Angius•15mo ago

It's about encapsulation You expose the internal state of the object via this proxy

jorenOP•15mo ago

I assume it has no additional checks for set;?

Angius•15mo ago

Well, it can if you want You can have { private set; protected get; } if you want Or you can implement some checks in the setter or getter

jorenOP•15mo ago

class A
{
public:
  A(const int a) : a(a) {}
  int a;
private:
  ...
};

A a(23);
a.a;

class A
{
public:
  A(const int a) : a(a) {}
  int a;
private:
  ...
};

A a(23);
a.a;

what's the difference? Pratically none

Angius•15mo ago

Yeah, you're exposing the internal state directly

jorenOP•15mo ago

so it get; no? only difference is that you cannot set;, its split up and one protects the other from happening

Angius•15mo ago

And if you wanted to not have the ability to set it, or not have the ability to get it, or return 0 if the member is < 0 you will have to break the contract by introducing getter/setter methods Not the case with C#

jorenOP•15mo ago

Fair guess it has some use, a fany accesibility modifier I'll call it protected doing what in this context is it so only the derived can use the get; i assume that actually, on second thought

Angius•15mo ago

private makes the setter accessible only in this class, while protected makes the getter accessible in this class and all derived classes

jorenOP•15mo ago

its private by default I assume

Angius•15mo ago

It's whatever the property is, by default

public int Foo { get; private set; } // get is public

public int Foo { get; private set; } // get is public

jorenOP•15mo ago

which is private unless you make it public? y

interface HumanInterface
{
    // Cannot use required here...
    public required string Name { get; init; }
    public required string Description { get; init; }

    public void walk() { Console.WriteLine("Default implementation"); }

    public void scream();
}

class Foo : HumanInterface
{

 
    void HumanInterface.scream() { Console.WriteLine("Screams"); }
}

interface HumanInterface
{
    // Cannot use required here...
    public required string Name { get; init; }
    public required string Description { get; init; }

    public void walk() { Console.WriteLine("Default implementation"); }

    public void scream();
}

class Foo : HumanInterface
{

 
    void HumanInterface.scream() { Console.WriteLine("Screams"); }
}

you see, this is what i'd want however this wouldnt work, but it got rid of the dumb double declarations of the Name and Description, how'd you write this?

Angius•15mo ago

I'd honestly just use an abstract class instead

jorenOP•15mo ago

Guess I will have to start making more proper distinctions between interfaces and abstract classes in my decision making lol oh well, not the worst, safe to say im not a fan of interfaces in C#.

Angius•15mo ago

Interfaces are best treated as literally just a contract

jorenOP•15mo ago

Yeah, they're very limited sadly guess Ill have to respect their use case more and stop trying to reinvent their use-case @angius So having members (or fields whatever they're called), is typicall not something you'd see back very often in an interface they're purely just: methods one has to implement in its child classes, maybe some default methods that can be overrided if wanted) and thats it

Angius•15mo ago

Fields are an implementation detail, as they're usually private Properties and methods you would absolutely see in an interface

jorenOP•15mo ago

properties that have to be re-declared a tenfold? might as well omit them in the interface no? I suppose they enforce you to define them in the child classes

Angius•15mo ago

Well... anything an interface declares has to be redeclared in the class that implements it

jorenOP•15mo ago

but its duplicate code disgusting, might as well opt for a abstract class instantly?

Angius•15mo ago

Unless you use the default implementation, which is probably one of the biggest mistakes C# did lol

jorenOP•15mo ago

why's that, whats the tea here

Angius•15mo ago

An interface should be just that, a contract It should contain no implementation detail Implementation is up to the class

jorenOP•15mo ago

fair actually considering they have abstract class too

Angius•15mo ago

Default implementation was added so it's easier to refactor code, you can throw a NotImplementedException for example But people abuse it for more that that

jorenOP•15mo ago

// Define an interface for entities that can make a sound. It's basically a contract the child classes have to implement.
interface ISoundMaker
{
  // You can define a default impl, but its frowned upon.
  // Cannot define ctor's, so dont.

  // Telling child class, this property has to be defined.
  public string Name { get; }

  // Has to be implemented
  void MakeSound();
}

// Implement the interface in a class.
class Dog : ISoundMaker
{
  public string Name { get; init; }

  public void MakeSound()
  {
      Console.WriteLine("Dog barks.");
  }
}

class Cat : ISoundMaker
{
  public string Name { get; init; }

  public void MakeSound()
  {
      Console.WriteLine("Cat meows.");
  }
}

internal class Program
{
  static void Main()
  {
      ISoundMaker dog = new Dog() { Name="Doggo" };
      ISoundMaker cat = new Cat() { Name = "Catto" };

      dog.MakeSound(); // Output: Dog barks.
      cat.MakeSound(); // Output: Cat meows.
  }
}

// Define an interface for entities that can make a sound. It's basically a contract the child classes have to implement.
interface ISoundMaker
{
  // You can define a default impl, but its frowned upon.
  // Cannot define ctor's, so dont.

  // Telling child class, this property has to be defined.
  public string Name { get; }

  // Has to be implemented
  void MakeSound();
}

// Implement the interface in a class.
class Dog : ISoundMaker
{
  public string Name { get; init; }

  public void MakeSound()
  {
      Console.WriteLine("Dog barks.");
  }
}

class Cat : ISoundMaker
{
  public string Name { get; init; }

  public void MakeSound()
  {
      Console.WriteLine("Cat meows.");
  }
}

internal class Program
{
  static void Main()
  {
      ISoundMaker dog = new Dog() { Name="Doggo" };
      ISoundMaker cat = new Cat() { Name = "Catto" };

      dog.MakeSound(); // Output: Dog barks.
      cat.MakeSound(); // Output: Cat meows.
  }
}

any other important details im missing I assume thats all, pretty straight forward since its so limited yet simple. I suppose this'd be a proper use-case, my example. I assume dtor's are also not possible in the interface, right?

Angius•15mo ago

LGTM

jorenOP•15mo ago

Just to be sure, this whole interface contract goes as deep as the compiler right? There's no case of vtables being generated, polymorpish, etc?

Angius•15mo ago

And no, I don't think deconstructors (or do you mean destructors – finalizers?) are possible in an interface

jorenOP•15mo ago

you can create instance of ISoundMaker interface as I tested, this makes me suspect there's smth going on destructors, and I cant I just tested! Apologies I forget to share my finding.

Angius•15mo ago

Not sure about the deep workings, but you cannot instantiate interfaces

MODiX•15mo ago

Angius

REPL Result: Failure

interface IFoo {}
var f = new IFoo();

interface IFoo {}
var f = new IFoo();

Exception: CompilationErrorException

- Cannot create an instance of the abstract type or interface 'IFoo'

- Cannot create an instance of the abstract type or interface 'IFoo'

Compile: 448.304ms | Execution: 0.000ms | React with ❌ to remove this embed.

jorenOP•15mo ago

No, just save its child classes inside it whats the command? to compile ;compile

interface IFoo {}
var f = new IFoo();

interface IFoo {}
var f = new IFoo();

Angius•15mo ago

!e

MODiX•15mo ago

joren

REPL Result: Success

interface IFoo {}
class A : IFoo{}
IFoo f = new A();

interface IFoo {}
class A : IFoo{}
IFoo f = new A();

Compile: 472.861ms | Execution: 57.902ms | React with ❌ to remove this embed.

jorenOP•15mo ago

ye , and then when you'd call a method on f, it'll call the respective method of the underlying type thats polyphism concepts applied without vtables, virtual methods?

Angius•15mo ago

Well, you're not creating ain instance of an IFoo

jorenOP•15mo ago

yes

Angius•15mo ago

It's still an A

jorenOP•15mo ago

so it just treats it as A, but allows it to be represented by IFoo?

MODiX•15mo ago

Angius

REPL Result: Success

interface IFoo {}
class A : IFoo{}
IFoo f = new A();

f is A

interface IFoo {}
class A : IFoo{}
IFoo f = new A();

f is A

Result: bool

True

True

Compile: 445.172ms | Execution: 33.085ms | React with ❌ to remove this embed.

Angius•15mo ago

Yep

jorenOP•15mo ago

that

Angius•15mo ago

It's an upcast And is is for pattern matching It's extremely useful, believe me Especially with list patterns and all

jorenOP•15mo ago

but how do virtual methods differ in C# do they create vtables as in C++ or is that knowlegde not applicable

Angius•15mo ago

A virtual method is an explicit signal that it's expected to be overridden

jorenOP•15mo ago

this is absurd, the moment one would ISoundMaker dog = new Dog() { Name = "Doggo" }; you'd get a compiler error in C++ unless its a pointer

Angius•15mo ago

Well Dog is a reference type So kinda-sorta a pointer lol

jorenOP•15mo ago

that would also not compile in C++ its weird to me that ISoundMaker dog = new Dog() { Name = "Doggo" }; is basically treated as a pointer but if that were the case are there vtables generated, I assume not? it just holds an alias to Dog() instance and allows it since its a reference type and its a child class of ISoundMaker ..?

Angius•15mo ago

Honestly, not sure about the nitty-gritty of how it works People in #allow-unsafe-blocks might have a better idea about vtables and what not

jorenOP•15mo ago

I'll have a look at that channel after I've read the whole polymorpishm post on the officials docs for now my knowledge of interfaces should suffice I guess im just being pragmatic for no reason, it matters when I have to dive deep which isnt soon

MODiX•15mo ago

joren

REPL Result: Failure

    interface ISoundMaker
    {
        public string Name;
    }

    interface ISoundMaker
    {
        public string Name;
    }

Exception: CompilationErrorException

- Interfaces cannot contain instance fields

- Interfaces cannot contain instance fields

Compile: 408.788ms | Execution: 0.000ms | React with ❌ to remove this embed.

jorenOP•15mo ago

why's this though, since its just a contract you'd think just declaring it would be ok

Angius•15mo ago

Well, yeah, it's a field. An implementation detail You'll need to make it a property

jorenOP•15mo ago

my terminology is not up-to-date with the csharp conventions

Angius•15mo ago

$structure

MODiX•15mo ago

namespace Namespace;

[Attribute]
public class Class
{
  public string PublicField;
  private bool _privateField;
  protected double protectedField;    

  public int PublicProperty { get; set; }

  public Class() {} // Constructor

  public void Method(int parameter)
  {
    var localVariable = parameter; 
  } 
}

namespace Namespace;

[Attribute]
public class Class
{
  public string PublicField;
  private bool _privateField;
  protected double protectedField;    

  public int PublicProperty { get; set; }

  public Class() {} // Constructor

  public void Method(int parameter)
  {
    var localVariable = parameter; 
  } 
}

For C# versions older than 10, see $StructureOld

Angius•15mo ago

Here's a cheat sheet

jorenOP•15mo ago

so a field is basically a class member

Angius•15mo ago

Everything inside of a class is this class's member

jorenOP•15mo ago

a property is basically a class member with certain properties, like get; and set;? by C++'s terminology you have: class members and class methods and thats it

Angius•15mo ago

Right, yeah

jorenOP•15mo ago

its just to compare for me, sorry! a property just being a class member with extra utility so to say? properties being set; get; init; etc? basically when the { ... } is introduced

Angius•15mo ago

Yeah

jorenOP•15mo ago

    interface ISoundMaker
    {
        public string Name { get; }
        void MakeSound();
    }

    class Dog : ISoundMaker
    {
        public string Name { get; init; }

        public void MakeSound()
        {
            Console.WriteLine("Dog barks.");
        }
    }

    interface ISoundMaker
    {
        public string Name { get; }
        void MakeSound();
    }

    class Dog : ISoundMaker
    {
        public string Name { get; init; }

        public void MakeSound()
        {
            Console.WriteLine("Dog barks.");
        }
    }

so in this case the property Name in ISoundMaker tells us that you have to define get; in its child classes correct, and then we can add others like init ourselves if we seem fit

MODiX•15mo ago

joren

REPL Result: Failure

    interface ISoundMaker
    {
        public string Name { set; }
        void MakeSound();
    }

    class Dog : ISoundMaker
    {
        public string Name { get; init; }

        public void MakeSound()
        {
            Console.WriteLine("Dog barks.");
        }
    }

    interface ISoundMaker
    {
        public string Name { set; }
        void MakeSound();
    }

    class Dog : ISoundMaker
    {
        public string Name { get; init; }

        public void MakeSound()
        {
            Console.WriteLine("Dog barks.");
        }
    }

Exception: CompilationErrorException

- 'Dog' does not implement interface member 'ISoundMaker.Name.set'. 'Dog.Name.init' cannot implement 'ISoundMaker.Name.set'.

- 'Dog' does not implement interface member 'ISoundMaker.Name.set'. 'Dog.Name.init' cannot implement 'ISoundMaker.Name.set'.

Compile: 664.426ms | Execution: 0.000ms | React with ❌ to remove this embed.

jorenOP•15mo ago

shouldnt work nice ok, makes sense

Accord•15mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Defining base impl ctor in interface