Implement an interface with a default property, in cli/c++

Question

I'm trying to declare several indexed properties as part of an interface in C#. The aim being able to write something like: int v=obj.Field1[4]+obj.Field2[4];

The implementation is done in C++/CLI.

I've found info about the use of a 'proxy' to approach the behaviour in Named indexed property in C#?

C#:

public interface IMyProp1
{
  int this[int idx] { get; }
}
public interface IMyProp2
{
  int this[int idx] { get; }
}


public interface IMyThing: IMyProp1, IMyProp2
{
 IMyProp1 Field1;
 IMyProp2 Field2;
}

In C++/CLI, I've found some info here: https://msdn.microsoft.com/en-us/library/2f1ec0b1.aspx But it's not specific about interface I've written the following (trial and error with the help of the VS2015 compiler)

public ref class MyThing: IMyThing
{
  virtual property int default[int]
  {
    int get(int idx) { return fld1[idx]; }
  }
  virtual property IMyProp1 Field1 { IMyProp1 get() { return this; } }
  virtual property IMyProp2 Field2 { IMyProp2 get() { return this; } }
private:
  array<int>^ fld1;
  array<int>^ fld2;

}

But I don't see how I can implement 2 different behaviours, as

virtual property int default[int]

is unique. Even if there are 2 'different' interfaces (I concede it's the same signature), I can't figure out a way to specify 2 different implementations:

virtual property int IMyProp1::default[int] { int get(int idx) { return fld1[idx]; }
virtual property int IMyProp2::default[int] { int get(int idx) { return fld2[idx]; }

I've found this information about explicit interface implementation in C++

interface class MyInterface1 { void f(); };
interface class MyInterface2 { void f(); };
ref class MyClass : MyInterface1, MyInterface2
{
  public:
  virtual void f1() = MyInterface1::f
  {
    Console::WriteLine("MyClass::f1 == MyInterface1::f");
  }

  virtual void f2() = MyInterface2::f
  {
    Console::WriteLine("MyClass::f2 == MyInterface2::f");
  }
};

But can't figure out a way to mix that with the indexed property signature.

score 1 · Accepted Answer · answered May 16 '17 at 19:10

1

For explicit implementation of properties, you specify the explicit override on each accessor method.

Try something like:

virtual property int Indexer1[int]
{
    int get(int idx) = IMyProp1::default[int]::get { return fld1[idx]; }
}

answered May 16 '17 at 19:10

Ben Voigt

277,958
43
419
720

That was it! I hadn't found this info from MSDN, they only document that the syntax changed from managed c++, but they didn't mention that you had to override each accessor. Thanks Ben! – Laurent May 17 '17 at 06:27

Implement an interface with a default property, in cli/c++

1 Answers1