Exact rules for variance validity

Exact rules for variance validity

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I thought it might be interesting for you all to get a precise description of how exactly it is that we determine when it is legal to put "in" and "out" on a type parameter declaration in C# 4. I'm doing this here because (1) it's of general interest, and (2) our attempt to make a more human-readable version of this algorithm in the draft C# 4.0 specification accidentally introduced some subtle errors. We're working on correcting those errors for the final release of the specification; until then, these definitions are the definitions I actually worked from to do the implementation, so they're accurate.

These definitions are pretty much stolen straight from the CLI spec section on variance; I am indebted to its authors for their careful and precise definitions.

The first things we need to define are three kinds of "validity" for types. I want to define "valid covariantly", "valid contravariantly" and "valid invariantly", but only as applied to types. We'll talk about what makes an interface declaration a valid one later; we need these definitions first.

Before we get into a precise definition, I want to talk about what "valid covariantly" means logically. The idea that we are attempting to capture here boils down to "the type we're talking about is not contravariant". Whether it is covariant, we don't really care. All we care about it is that if it is valid covariantly, then it is not contravariant. Similarly, by "valid contravariantly", we mean "not covariant".

This leads us to our first brain-hurting definition: A type is "valid invariantly" if it is both valid covariantly and valid contravariantly. That sounds a bit crazy -- how can it be valid invariantly if it is both covariant and contravariant? But "valid covariantly" does not mean "is covariant", it means "it's guaranteed to not be contravariant". So if a type is valid covariantly and valid contravariantly, then it is guaranteed to be neither contravariant nor covariant, and therefore must be invariant.

Anyway. We want to take any specific type and classify it as valid covariantly, valid contravariantly, or both, a.k.a., valid invariantly. (We don't need to worry about coming up with a term for the "or none of the above" case because in practice that case is never relevant.)

Nested types present a problem; we might have an interface I<T> inside a class C<U>. For simplicity's sake, let's assume that the type C<U>.I<T> is analyzed as if it were C.I<U, T>; logically that's the same thing and it will keep the notation simpler below. Similarly, we could have a "generic enum", C<U>.E; suspend disbelief, pretend that C.E<U> is legal and move on. Remember, for the purposes of working out problems in type algebra, where the generic type arguments go lexically is irrelevant; all that matters is their number and order.

Nullable types are considered to be the generic struct type Nullable<T>.

A type is valid covariantly if it is:

1) a pointer type, or a non-generic class, struct, enum, delegate or interface type.

This should make sense. Remember, what we're getting at is "not contravariant". Pointers and non-generic types are by definition not generic and therefore not variant either. These are the easy cases.

2) An array type T[] where T is valid covariantly.

Because we have array covariance in C#, but not array contravariance, we can make arrays valid covariantly. Again, remember, "valid covariantly" means "not contravariant".

3) A generic type parameter type, if it was not declared as being contravariant.

Generic type parameter types are of course types as far as the compiler is concerned. If you're inside the declaration of a generic interface then you can use its type parameters as types. In that context, such types are valid covariantly if they were not declared as contravariant (in).

4) A constructed class, struct, enum, interface or delegate type X<T1, ... Tk> might be valid covariantly. To determine if it is, we examine each type argument differently, depending on whether the corresponding type parameter was declared as covariant (out), contravariant (in), or invariant (neither). (Of course the generic type parameters of classes and structs will never be declared 'out' or 'in'; they will always be invariant.) If the ith type parameter was declared as covariant, then Ti must be valid covariantly. If it was declared as contravariant, then Ti must be valid contravariantly. If it was declared as invariant, then Ti must be valid invariantly.

As one would expect, covariant validity preserves the direction of validity; covariant parameters must be valid covariantly, contravariant parameters must be valid contravariantly.

OK, I hope that was relatively painless. The rules for contravariant validity are similar; as one would expect from the "backwards" nature of contravariance, the directions are reversed in the complicated case:

A type is valid contravariantly if it is:

1) a pointer type or non-generic class, struct, enum, delegate or interface type.

2) An array type T[] where T is valid contravariantly.

Arrays are covariant in their element type. Covariance preserves the direction of variance. Therefore, to be valid contravariantly, an array (covariant in its element type) must be contravariantly valid in its element type.

3) A generic type parameter type, if it was not declared as being covariant.

Remember, "valid contravariantly" means "not covariant". It does not mean "contravariant".

4) A constructed class, struct, enum, interface or delegate type X<T1, ... Tk> might be valid contravariantly. If the ith type parameter was declared as contravariant, then Ti must be valid covariantly. If it was declared as covariant, then Ti must be valid contravariantly. If it was declared as invariant, then Ti must be valid invariantly. (Again, of course all the type parameters contributed from classes and structs will be declared invariant.)

As one might expect, contravariant validity reverses the direction of validity.

Now perhaps you see why we wanted to rewrite these definitions into something more human-readable for the spec. And perhaps you also see why we accidentally introduced errors in doing so; bending your brain around all this logic is not always easy.

OK, now that we've got that, we can make a definition of what it means for an interface to be valid. An interface must meet the following conditions:

* The return types of all non-void interface methods must be valid covariantly.

* Every formal parameter type of all interface methods must be valid contravariantly. (Invariantly if it is an out or ref parameter.)

* For all generic methods on an interface, every constraint on the generic method type parameters must be valid contravariantly.

* All its base interface types must be valid covariantly.

* The type of a property or indexer must be valid covariantly if it has a "getter" and valid contravariantly if it has a "setter".

* Any formal parameter types of an indexer must be valid contravariantly.

* The delegate types of all its events must be valid contravariantly.

The first two are pretty straightforward; return types "go out" so they have to be valid covariantly, formal parameter types "go in", so they have to be valid contravariantly. But what's up with the third one? What do constraints on generic method type parameters have to do with interface validity?

Well, let's suspend the third rule and see what goes wrong.

interface I<out T>
    void M<U>() where U : T;
    // illegal; this has to be valid contravariantly but it is a covariant type parameter constraint.
    // Let it ride for now and demonstrate an error.

class C<T> : I<T> { public void M<U>() {} }
// the constraint is inherited implicitly and not re-stated.

I<Giraffe> igiraffe = new C<Giraffe>(); // C<T> implements I<T>
I<Animal> ianimal = igiraffe; // interface is covariant in T
ianimal.M<Turtle>(); // satisifies the constraint that U must be an Animal.

Uh oh. ianimal is really an instance of C<Giraffe>. The M<U> method on C<Giraffe> inherits a requirement that U inherit from Giraffe. Turtle does not inherit from Giraffe. Therefore we've just violated the constraint on M<U>. The only places where we can catch this is in the declaration; every other step is perfectly legal. Therefore, a constraint cannot be covariant. But if we make it contravariant (or invariant) then it all works out. For example, let's make a contravariant type parameter constraint:

interface I2<in T> // contravariant this time
    void M<U>() where U : T;

class C2<T> : I2<T> { public void M<U>() {} }
I2<Animal> i2animal = new C2<Animal>(); // C2<T> implements I2<T>
I2<Mammal> i2mammal = i2animal; // interface is contravariant in T
i2mammal.M<Giraffe>(); // satisifies the constraint that U must be an Animal.

And now everything is fine; the compile-time constraint checker verifies that Giraffe is Mammal; at runtime it must be Animal, and so the compiler has verified that by verifying that it is Mammal. 

The rules for delegate declarations are a straightforward simplification of the rules for interface declarations. To be a valid delegate declaration, the return type must be valid covariantly (or void), the formal parameter types must be valid contravariantly (or invariantly if they are out/ref), and any type parameter constraints must be valid contravariantly.

  • >Enums and pointers are non-generic so they are not variant.

    If we want to be absolutely precise, then enums can be generic (if nested in generic classes or structs), but of course they can not be variant.

  • > An array type T[] where T is valid covariantly.

    Does this rule also work for multidimensional arrays?

    Yes! -- Eric

  • As far as the confusingness of the spec as you laid it out, I think you could do an awful lot to make it less confusing just by inventing some silly new words and using them instead of "valid covariantly" and "valid contravariantly".

    I suggest "copossible" and "contrapossible".

    I think your suggestion is inpossible. -- Eric

  • Just out of interest, would it be fair to suggest that if the CLI supported "out" parameters natively (instead of using "ref" with an advisory attribute) we'd be able to use them for variance in some situations? For example:

       public interface IFoo<out T>
           void PassValueOut(out T value);

    I realise it's *not* possible at the moment - just checking my logic as much as anything else.

    If you could enforce in the CLR that out parameters are write-only variables then yes, you could make this work. But as long as out parameters are read-write then no, this cannot work:

    class C : IFoo<Bird>
      public void PassValueOut(out Bird b)
        b = new Chicken();
        b.LayEggs(); // reading from b is the problem. A giraffe just laid an egg.

    class X
      static Animal q;
      public static void Blah() { q = new Giraffe(); }
      public static void Main()
        IFoo<Animal> f = new C(); // Covariant conversion
        f.PassValueOut(out q);

    -- Eric


  • > would it be fair to suggest that if the CLI supported "out" parameters natively (instead of using "ref" with an advisory attribute) we'd be able to use them for variance in some situations?

    No, it would not be typesafe.

    interface Foo<out T>
      void PassValueOut(out T value);

    class A : Foo<string>
     public void PassValueOut(out string value) 
       value = "";
       X = 1;
       string x = value; // Bang!
     static object X;
     static void Main()
       Foo<object> a = new A();
       a.PassValueOut(out X);

  • I was just thinking the same thing Jon posted. The unfortunate fact that the CLR treats out as ref is quite limiting here. However, I'm not sure what should happen when I do:

    object result;
    Foo<int> foo = ...;
    foo.PassValueOut(out result);

    The boxing here is quite difficult since we are storing an int in an object, but the method storing it has no idea that it's an object. Remember that for eeky multithreading scenarios, the result should be set at the appropriate times, and not only when the method returns.

  • @nikov: Eek, good call. The difference between "return" and "out" being the way that the method itself can reference the variable.

    Time to go to bed, I think.

  • @configurator: The answer is that it should fail to compile (and indeed does). "out" is like ref - the argument type has to *exactly* match the declared parameter type. This is specified in section (C# 3).

  • I understand the need for "contravariantly valid" , but it is kinda hard to understand that "contravariantly valid" actually means "covariantly invalid". However wrote that spec had lots of fun :)

    Anyhow, does this boils down to Thou shall not use "where U : T" in out interfaces, or am I missing a point here?

    @nikov, if we define an out parameter as native write-only (in effect assign-only) than

     string x = value;

    would be a compile error, so it would be possible to use out parameters for variance.

  • Eric, can you comment on why C# or what the matter CLR dont derive (co/contravarience) on their own?

    This way people wouldn't "forget" to place the right in/out

  • Once again you're mixing up how C# implements out with the concept of an out parameter in general. There's no fundamental reason an "out" can't be another return (i.e. the method has its own variable which is the correct type, and assigns it to the address that was passed in at the end), except for the fact that it doesn't.

    Yes, that would invalidate the case of assigning to X outside the method, but there is an argument that this case _should_ not be allowed.

  • What is the reason for the special terminology? Since "valid covariantly" means "not contravariant", why isn't "not contravariant" the term that's used? That has an intuitively clear meaning, while "valid covariantly" seems strange and vague.

    I'm okay with considering the terms as abstract entities with no contextual meaning (which I did for reading this article), but the terms do look as if they have some particular motivation, which makes me wonder what it is.

    Well, we don't want to say "not contravariant" because that's then vague and confusing. What, exactly, is "not contravariant"? As I discussed last time, "contravariant" is an adjective which applies to a projection on types. But "covariantly valid" is not an adjective which refers to a projection on types, its an adjective which refers to a type.

    Basically, by "covariantly valid" we're trying to capture the notion of "if this type is used in a position where it could conceivably be modified via a conversion justified by variance, do we know that it can never be modified in a direction which would not be typesafe?" That's a bit of a mouthful! 

    When we wrote the spec we tried to define "safe for covariance", "unsafe for covariance", and so on, instead of the awkward "covariantly valid", but we made some small errors doing so. Hopefully we'll manage to suss out a reasonable wording that still works. -- Eric

  • "I think your suggestion is inpossible. -- Eric"

    I thought of that just after I posted - but note that in your entire discussion you only mention the concept of "valid invariantly" to define it (and explain that the definition is weird and counterintuitive) and then once each in #4 of the two other definitions. If you just replaced the sentence that says "If it was declared as invariant, then Ti must be valid invariantly. " with "If it was declared as invariant, then Ti must be both copossible AND contrapossible" then you wouldn't need to define "inpossible" at all.

    Unfortunately. ;)

  • Or I should have said - Infortunately.

  • @Joren

    > What is the reason for the special terminology? Since "valid covariantly" means "not contravariant", why isn't "not contravariant" the term that's used?

    Assume T is a covariant type parameter of an interface and type System.Collections.Generic.List<T> appears somewhere within this interface. Then type List<T> is not covariant and is not contravariant (because classes cannot be variant). But at the same time it is neither valid covariantly nor valid contravariantly, because when T varies, for instance, from string to object, List<T> changes from List<string> to completely unrelated type List<object>! There is no reference conversion from List<string> to List<object>. So, List<T> can appear neither in input positions nor in output positions in this interface.

    Now you see that "not contravariant" is not interchangeable with "valid covariantly".

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