Interface defining a constructor signature

Question

It s weird that this is the first time I ve bumped into this problem  but   How do you define a constructor in a C  interface   Edit Some people wanted an example  it s a free time project  so yes  it s a game   IDrawable   Update   Draw  To be able to Update  check for edge of screen etc  and draw itself it will always need a GraphicsDeviceManager  So I want to make sure the object has a reference to it  This would belong in the constructor   Now that I wrote this down I think what I m implementing here is IObservable and the GraphicsDeviceManager should take the IDrawable    It seems either I don t get the XNA framework  or the framework is not thought out very well   Edit There seems to be some confusion about my definition of constructor in the context of an interface  An interface can indeed not be instantiated so doesn t need a constructor  What I wanted to define was a signature to a constructor  Exactly like an interface can define a signature of a certain method  the interface could define the signature of a constructor

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Late  but for someone need  You can using Func lt TResult gt  void Create Func lt IDrawable gt  func      IDrawable result   func Invoke       GraphicsDeviceManager graphicsDeviceManager       xxx Create      gt  new Draw graphicsDeviceManager    yyy Create      gt  new Update graphicsDeviceManager

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One way to solve this problem i found is to seperate out the construction into a seperate factory   For example I have an abstract class called IQueueItem  and I need a way to translate that object to and from another object  CloudQueueMessage    So on the interface IQueueItem i have -   public interface IQueueItem       CloudQueueMessage ToMessage        Now  I also need a way for my actual queue class to translate a CloudQueueMessage back to a IQueueItem - ie the need for a static construction like  IQueueItem objMessage   ItemType FromMessage    Instead I defined another interface IQueueFactory -  public interface IQueueItemFactory lt T gt  where T   IQueueItem       T FromMessage CloudQueueMessage objMessage       Now I can finally write my generic queue class without the new   constraint which in my case was the main issue   public class AzureQueue lt T gt  where T   IQueueItem       private IQueueItemFactory lt T gt   objFactory      public AzureQueue IQueueItemFactory lt T gt  objItemFactory                 objFactory   objItemFactory              public T GetNextItem TimeSpan tsLease                CloudQueueMessage objQueueMessage    objQueue GetMessage tsLease           T objItem    objFactory FromMessage objQueueMessage           return objItem            now I can create an instance that satisfies the criteria for me   AzureQueue lt Job gt  objJobQueue   new JobQueue new JobItemFactory      hopefully this helps someone else out someday  obviously a lot of internal code removed to try to show the problem and solution

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It is not possible to create an interface that defines constructors  but it is possible to define an interface that forces a type to have a paramerterless constructor  though be it a very ugly syntax that uses generics    I am actually not so sure that it is really a good coding pattern    public interface IFoo lt T gt  where T   new       void SomeMethod       public class Foo   IFoo lt Foo gt         This will not compile   public Foo int x               region ITest lt Test gt  Members    public void SomeMethod           throw new NotImplementedException            endregion     On the other hand  if you want to test if a type has a paramerterless constructor  you can do that using reflection   public static class TypeHelper     public static bool HasParameterlessConstructor Object o          return HasParameterlessConstructor o GetType            public static bool HasParameterlessConstructor Type t             Usage  HasParameterlessConstructor typeof SomeType       return t GetConstructor new Type 0      null          Hope this helps

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I was looking back at this question and I thought to myself  maybe we are aproaching this problem the wrong way  Interfaces might not be the way to go when it concerns defining a constructor with certain parameters    but an  abstract  base class is    If you create a base class with a constructor on there that accepts the parameters you need  every class that derrives from it needs to supply them    public abstract class Foo     protected Foo SomeParameter x          this X   x         public SomeParameter X   get  private set      public class Bar   Foo    Bar inherits from Foo     public Bar          base new SomeParameter  etc          Bar will need to supply the constructor param

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As already well noted  you can t have constructors on an Interface   But since this is such a highly ranked result in Google some 7 years later  I thought I would chip in here - specifically to show how you could use an abstract base class in tandem with your existing Interface and maybe cut down on the amount of refactoring needed in the future for similar situations   This concept has already been hinted at in some of the comments but I thought it would be worth showing how to actually do it   So you have your main interface that looks like this so far   public interface IDrawable       void Update        void Draw        Now create an abstract class with the constructor you want to enforce   Actually  since it s now available since the time you wrote your original question  we can get a little fancy here and use generics in this situation so that we can adapt this to other interfaces that might need the same functionality but have different constructor requirements   public abstract class MustInitialize lt T gt        public MustInitialize T parameters                   Now you ll need to create a new class that inherits from both the IDrawable interface and the MustInitialize abstract class   public class Drawable   MustInitialize lt GraphicsDeviceManager gt   IDrawable       GraphicsDeviceManager  graphicsDeviceManager       public Drawable GraphicsDeviceManager graphicsDeviceManager            base  graphicsDeviceManager                 graphicsDeviceManager   graphicsDeviceManager             public void Update                   use  graphicsDeviceManager here to do whatever            public void Draw                   use  graphicsDeviceManager here to do whatever           Then just create an instance of Drawable and you re good to go   IDrawable drawableService   new Drawable myGraphicsDeviceManager     The cool thing here is that the new Drawable class we created still behaves just like what we would expect from an IDrawable   If you need to pass more than one parameter to the MustInitialize constructor  you can create a class that defines properties for all of the fields you ll need to pass in

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I use the following pattern to make it bulletproof    A developer who derives his class from the base can t accidentally create a public accessible constructor The final class developer are forced to go through the common create method Everything is type-safe  no castings are required It s 100  flexible and can be reused everywhere  where you can define your own base  class  Try it out you can t break it without making modifications to the base classes  except  if you define an obsolete flag without error flag set to true  but even then you end up with a warning       public abstract class Base lt TSelf  TParameter gt      where TSelf   Base lt TSelf  TParameter gt   new         protected const string FactoryMessage    Use YourClass Create      instead       public static TSelf Create TParameter parameter                var me   new TSelf            me Initialize parameter            return me              Obsolete FactoryMessage  true       protected Base                      protected virtual void Initialize TParameter parameter                  public abstract class BaseWithConfig lt TSelf  TConfig gt   Base lt TSelf  TConfig gt      where TSelf   BaseWithConfig lt TSelf  TConfig gt   new         public TConfig Config   get  private set          Obsolete FactoryMessage  true       protected BaseWithConfig                   protected override void Initialize TConfig parameter                this Config   parameter           public class MyService   BaseWithConfig lt MyService   string UserName  string Password  gt         Obsolete FactoryMessage  true       public MyService                  public class Person   Base lt Person   string FirstName  string LastName  gt         Obsolete FactoryMessage true       public Person                    protected override void Initialize  string FirstName  string LastName  parameter                this FirstName   parameter FirstName          this LastName   parameter LastName             public string LastName   get  private set         public string FirstName   get  private set          Test  public void FactoryTest         var notInitilaizedPerson   new Person       doesn t compile because of the obsolete attribute      Person max   Person Create   Max    Mustermann         Assert AreEqual  Max  max FirstName        var service   MyService Create   MyUser    MyPassword         Assert AreEqual  MyUser   service Config UserName        EDIT  And here is an example based on your drawing example that even enforces interface abstraction           public abstract class BaseWithAbstraction lt TSelf  TInterface  TParameter gt          where TSelf   BaseWithAbstraction lt TSelf  TInterface  TParameter gt   TInterface  new                  Obsolete FactoryMessage  true           protected BaseWithAbstraction                                protected const string FactoryMessage    Use YourClass Create      instead           public static TInterface Create TParameter parameter                        var me   new TSelf                me Initialize parameter                return me                     protected virtual void Initialize TParameter parameter                                    public abstract class BaseWithParameter lt TSelf  TInterface  TParameter gt    BaseWithAbstraction lt TSelf  TInterface  TParameter gt          where TSelf   BaseWithParameter lt TSelf  TInterface  TParameter gt   TInterface  new                 protected TParameter Parameter   get  private set              Obsolete FactoryMessage  true           protected BaseWithParameter                               protected sealed override void Initialize TParameter parameter                        this Parameter   parameter              this OnAfterInitialize parameter                      protected virtual void OnAfterInitialize TParameter parameter                                  public class GraphicsDeviceManager                  public interface IDrawable               void Update            void Draw               internal abstract class Drawable lt TSelf gt    BaseWithParameter lt TSelf  IDrawable  GraphicsDeviceManager gt   IDrawable          where TSelf   Drawable lt TSelf gt   IDrawable  new                  Obsolete FactoryMessage  true           protected Drawable                                public abstract void Update            public abstract void Draw               internal class Rectangle   Drawable lt Rectangle gt                 Obsolete FactoryMessage  true           public Rectangle                                public override void Update                         GraphicsDeviceManager manager   this Parameter                 TODo  manager                    public override void Draw                         GraphicsDeviceManager manager   this Parameter                 TODo  manager                     internal class Circle   Drawable lt Circle gt                 Obsolete FactoryMessage  true           public Circle                                public override void Update                         GraphicsDeviceManager manager   this Parameter                 TODo  manager                    public override void Draw                         GraphicsDeviceManager manager   this Parameter                 TODo  manager                        Test      public void FactoryTest                    doesn t compile because interface abstraction is enforced          Rectangle rectangle   Rectangle Create new GraphicsDeviceManager                 you get only the IDrawable returned          IDrawable service   Circle Create new GraphicsDeviceManager

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Beginning with C  8 0  an interface member may declare a body  This is called a default implementation  Members with bodies permit the interface to provide a  default  implementation for classes and structs that don t provide an overriding implementation  In addition  beginning with C  8 0  an interface may include   Constants Operators Static constructor  Nested types Static fields  methods  properties  indexers  and events Member declarations using the explicit interface implementation syntax  Explicit access modifiers  the default access is public

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While you can t define a constructor signature in an interface  I feel it s worth mentioning that this may be a spot to consider an abstract class  Abstract classes can define unimplemented  abstract  method signatures in the same way as an interface  but can also have implemented  concrete  methods and constructors   The downside is that  because it is a type of class  it cannot be used for any of the multiple inheritance type scenarios that an interface can

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You can t   Interfaces define contracts that other objects implement and therefore have no state that needs to be initialized   If you have some state that needs to be initialized  you should consider using an abstract base class instead

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You can t  It s occasionally a pain  but you wouldn t be able to call it using normal techniques anyway   In a blog post I ve suggested static interfaces which would only be usable in generic type constraints - but could be really handy  IMO   One point about if you could define a constructor within an interface  you d have trouble deriving classes   public class Foo   IParameterlessConstructor       public Foo      As per the interface                public class Bar   Foo          Yikes  We now don t have a parameterless constructor        public Bar int x

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One way to force some sort of constructor is to declare only Getters in interface  which could then mean that the implementing class must have a method  ideally a constructor  to have the value set  privately  for it

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A very late contribution demonstrating another problem with interfaced constructors   I choose this question because it has the clearest articulation of the problem   Suppose we could have   interface IPerson       IPerson string name      interface ICustomer       ICustomer DateTime registrationDate      class Person   IPerson  ICustomer       Person string name          Person DateTime registrationDate          Where by convention the implementation of the  interface constructor  is replaced by the type name   Now make an instance   ICustomer a   new Person  Ernie      Would we say that the contract ICustomer is obeyed   And what about this   interface ICustomer       ICustomer string address

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The purpose of an interface is to enforce a certain object signature  It should explicitly not be concerned with how an object works internally  Therefore  a constructor in an interface does not really make sense from a conceptual point of view   There are some alternatives though    Create an abstract class that acts as a minimal default implementation  That class should have the constructors you expect implementing classes  to have  If you don t mind the overkill  use the AbstractFactory pattern and declare a method in the factory class interface that has the required signatures  Pass the GraphicsDeviceManager as a parameter to the Update and Draw methods  Use a Compositional Object Oriented Programming framework to pass the GraphicsDeviceManager into the part of the object that requires it  This is a pretty experimental solution in my opinion    The situation you describe is not easy to handle in general  A similar case would be entities in a business application that require access to the database

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If I understood OP correctly  we want to enforce a contract where GraphicsDeviceManager is always initialised by implementing classes  I had a similar problem and I was looking for a better solution   but this is the best I can think of   Add a SetGraphicsDeviceManager GraphicsDeviceManager gdo  to the interface  and that way the implementing classes will be forced to write a logic which will be require a call from constructor

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The generic factory approach still seems ideal  You would know that the factory requires a parameter  and it would just so happen that those parameters are passed along to the constructor of the object being instantiated    Note  this is just syntax verified pseudo code  there may be a run-time caveat I m missing here   public interface IDrawableFactory       TDrawable GetDrawingObject lt TDrawable gt  GraphicsDeviceManager graphicsDeviceManager                 where TDrawable  class  IDrawable  new       public class DrawableFactory   IDrawableFactory       public TDrawable GetDrawingObject lt TDrawable gt  GraphicsDeviceManager graphicsDeviceManager                        where TDrawable   class  IDrawable  new                 return  TDrawable  Activator                  CreateInstance typeof TDrawable                                    graphicsDeviceManager             public class Draw   IDrawable      stub    public class Update   IDrawable       private readonly GraphicsDeviceManager  graphicsDeviceManager       public Update     throw new NotImplementedException           public Update GraphicsDeviceManager graphicsDeviceManager                 graphicsDeviceManager   graphicsDeviceManager           public interface IDrawable         stub   public class GraphicsDeviceManager         stub     An example of possible usage       public void DoSomething                 var myUpdateObject   GetDrawingObject lt Update gt  new GraphicsDeviceManager             var myDrawObject   GetDrawingObject lt Draw gt  null           Granted  you d only want the create instances via the factory to guarantee you always have an appropriately initialized object  Perhaps using a dependency injection framework like AutoFac would make sense  Update   could  ask  the IoC container for a new GraphicsDeviceManager object

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One way to solve this problem is to leverage generics and the new   constraint   Instead of expressing your constructor as a method function  you can express it as a factory class interface  If you specify the new   generic constraint on every call site that needs to create an object of your class  you will be able to pass constructor arguments accordingly   For your IDrawable example   public interface IDrawable       void Update        void Draw       public interface IDrawableConstructor lt T gt  where T   IDrawable       T Construct GraphicsDeviceManager manager       public class Triangle   IDrawable       public GraphicsDeviceManager Manager   get  set        public void Draw               public void Update               public Triangle GraphicsDeviceManager manager                Manager   manager            public TriangleConstructor   IDrawableConstructor lt Triangle gt        public Triangle Construct GraphicsDeviceManager manager                return new Triangle manager              Now when you use it   public void SomeMethod lt TBuilder gt  GraphicsDeviceManager manager    where TBuilder  IDrawableConstructor lt Triangle gt   new            If we need to create a triangle     Triangle triangle   new TBuilder   Construct manager           Do whatever with triangle     You can even concentrate all creation methods in a single class using explicit interface implementation   public DrawableConstructor   IDrawableConstructor lt Triangle gt                                IDrawableConstructor lt Square gt                                IDrawableConstructor lt Circle gt        Triangle IDrawableConstructor lt Triangle gt  Construct GraphicsDeviceManager manager                return new Triangle manager               Square IDrawableConstructor lt Square gt  Construct GraphicsDeviceManager manager                return new Square manager               Circle IDrawableConstructor lt Circle gt  Construct GraphicsDeviceManager manager                return new Circle manager              To use it   public void SomeMethod lt TBuilder  TShape gt  GraphicsDeviceManager manager    where TBuilder  IDrawableConstructor lt TShape gt   new            If we need to create an arbitrary shape     TShape shape   new TBuilder   Construct manager           Do whatever with the shape     Another way is by using lambda expressions as initializers  At some point early in the call hierarchy  you will know which objects you will need to instantiate  i e  when you are creating or getting a reference to your GraphicsDeviceManager object   As soon as you have it  pass the lambda       gt  new Triangle manager     to subsequent methods so they will know how to create a Triangle from then on  If you can t determine all possible methods that you will need  you can always create a dictionary of types that implement IDrawable using reflection and register the lambda expression shown above in a dictionary that you can either store in a shared location or pass along to further function calls

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you don t   the constructor is part of the class that can implement an interface   The interface is just a contract of methods the class must implement

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It would be very useful if it were possible to define constructors in interfaces   Given that an interface is a contract that must be used in the specified way  The following approach might be a viable alternative for some scenarios   public interface IFoo             lt summary gt          Initialize foo           lt  summary gt           lt remarks gt          Classes that implement this interface must invoke this method from         each of their constructors           lt  remarks gt           lt exception cref  InvalidOperationException  gt          Thrown when instance has already been initialized           lt  exception gt      void Initialize int a       public class ConcreteFoo   IFoo        private bool  init   false       public int b          Obviously in this case a default value could be used for the        constructor argument  using overloads for purpose of example      public ConcreteFoo             Initialize 42              public ConcreteFoo int a            Initialize a              public void Initialize int a            if   init              throw new InvalidOperationException             init   true           b   a

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You could do this with generics trick  but it still is vulnerable to what Jon Skeet wrote   public interface IHasDefaultConstructor lt T gt  where T   IHasDefaultConstructor lt T gt   new         Class that implements this interface must have parameterless constructor   public class A   IHasDefaultConstructor lt A gt    Notice A as generic parameter       public A int a        compile time error

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