C# Generic Interfaces casting issue

Question

Should this be possible using c# 4 and VS 2010? I am doing some processing on class(es) which implements generic interface(s), and after processing would like to convert the objects to a simpler interface so I can extract certain properties defined by common Interfaces.

interface IMyInterface
{
    public Id { get; set; }
}

interface IFile<T1, T2> where T1 : IMyInterface where T2 : IMyInterface
{
    Int64 prop1 { get; set; }
    T1 t1 { get; set; }
    T2 t2 { get; set; }
}

ClassA : IMyInterface
{
    ... Implement some properties plus interface
    public Id { get; set; }
}

ClassB : IMyInterface
{
    ... Implement some properties plus interface
    public Id { get; set; }
}

For example this class has ClassX and ClassY which I want to be certain types for processing/saving, but after that I only want to extract common properties like an ID which is common amongst all classes that Implement this generic Interface (other properties are not common in t1, t1)

ClassSomething : IFile<ClassA, ClassB>
{
    ... Implement properties plus interface 
    public ClassX t1 
    { get {}   set {} }
    public ClassY t2 
    { get {}   set {} }
}



IList<IFile<TItem1, TItem2>> list = new List<IFile<TItem1, TItem2>>() 
    ClassA ca = new ClassA();
    ... Fill in the interface defined properties
    ... Fill the list with objects of ClassSomething

foreach (IFile<TItem1, TItem2> x in list)
{
    // This fails
    IFile<IMyInterface, IMyInterface> interfaceItem = 
        (IFile<IMyInterface, IMyInterface>)x;
}

The cast of x above (t1 and t2 properties specifically) to the simpler IMyInterface interface fails.

There are quite a few generic interface questions but I didn't see (or recognize?) any solutions.

You may need to look at c# 4.0 covariance and contravariance — ca9163d9
– ca9163d9, Commented Jan 26, 2012 at 19:56
Wow, fast and good looking answers. I'll need to take a bit and digest the answers. :) — lko
– lko, Commented Jan 26, 2012 at 20:30
Great answers, they are all helpful, I'll pick an answer based on the popular vote. (I <3 SO) — lko
– lko, Commented Oct 19, 2012 at 7:45

Anton Tykhyy · Accepted Answer · 2012-01-26 20:03:26Z

5

The solution you are looking for is called variance (covariance and contravariance). However your IMyInterface cannot be made either covariant or contravariant in T1 and T2, because it has both public getters and public setters accepting T1 and T2:

interface IAnimal {}
class Dog : IAnimal { public void Bark () ; }
class Cat : IAnimal { public void Meow () ; }

var dogs = new FileImpl<Dog, Dog> () ;
dogs.t1  = new Dog () ;

var file = (IFile<IAnimal, IAnimal>) dogs ; // if this were OK...
file.t1  = new Cat () ;                     // this would have to work
dogs.t1.Bark () ;                           // oops, t1 is a cat now

answered Jan 26, 2012 at 20:03

Anton Tykhyy

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Comments

Dr. Wily's Apprentice · Accepted Answer · 2012-01-26 20:20:16Z

2

Just to expand on Anton Tykhyy's answer,

Yes, you could achieve this with C# 4, provided that you are willing/able to make the following change to your IFile interface:

interface IFile<out T1, out T2> where T1 : IMyInterface where T2 : IMyInterface
{
    Int64 prop1 { get; set; }
    T1 t1 { get; }
    T2 t2 { get; }
}

I've added the out keyword to the generic parameters, and I've removed the set; from the t1 and t2 properties.

answered Jan 26, 2012 at 20:20

Dr. Wily's Apprentice

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1 Comment

Sergey Vyacheslavovich Brunov Over a year ago

Agree with you, I would also apply covariance if it was possible.

sll · Accepted Answer · 2012-01-26 19:55:24Z

1

Why just not access x.T1 and x.T2 and then cast both to IMyInterface?

foreach (IFile<TItem1, TItem2> x in list)
{
   var t1 = x.T1 as IMyInterface;
   var t2 = x.T2 as IMyInterface;
}

answered Jan 26, 2012 at 19:55

sll

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Comments

CodingGorilla · Accepted Answer · 2012-01-26 19:57:53Z

1

I think you're confusing the generic constraits with inheritance. The one has nothing to do with the other. The constraints on the generic interface are compiler instructions that tell the compiler that the generic arguments must meet a specific requirment. Logically, to us, that means that ClassSomething definitely has an implementation of IMyInterface. But that's us, the compiler doesn't translate those constraints to any kind of inheritence mapping, so it still only knows that it's an instance of ClassSomething which implements IFile<ClassA, ClassB>. So it won't let you directly cast that to IFile<IMyInterface, IMyInterface>.

answered Jan 26, 2012 at 19:57

CodingGorilla

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