Inconsistent behavior in C# 8 nullable reference type handling with generics

Question

I have this code:

public T? Foo<T>()
    where T : class?
{
    return null;
}

It gives an error that is logical and expected:

A nullable type parameter must be known to be a value type or non-nullable reference type. Consider adding a 'class', 'struct', or type constraint.

Now I add one more constraint:

public T? Foo<T>()
    where T : class?, IDisposable // Could be any interface I guess
{
    return null;
}

Now interestingly enough error has just disappeared. Though it really seems to me we have conflicting constraints since interface is non-nullalbe while class? is.

Am I missing something here or is there a trouble with compiler?

Have you seen this? -> https://stackoverflow.com/questions/55975211/nullable-reference-types-how-to-specify-t-type-without-constraining-to-class — panoskarajohn, Jan 07 '20 at 15:22
Yeah, I tested with with `?`, and it compiles. Ironically enough it also compiles like that `where T : class, IDisposable?` — Ilya Chernomordik, Jan 07 '20 at 15:22
It seems the whole constraint is non-nullable if at least one constraint is non-nullable — Ilya Chernomordik, Jan 07 '20 at 15:25
@IlyaChernomordik sounds conclusive, since the constraints are AND-connected. — Holger, Jan 07 '20 at 15:31
Yes, for example, if you are making the constraints that T is both int and int? then T must always be non-null because that is the only way that it conforms to both requirements. — Robert McKee, Jan 07 '20 at 15:36

score 5 · Accepted Answer · answered Jan 07 '20 at 15:23

5

The generic type constraint where T : IDisposable means "T must be non-nullable and must implement IDisposable". Where you have multiple generic type constraints of differing nullabilities, the constraint overall is only nullable if all constraints are nullable.

So the fact that class? is nullable gets overridden by the fact that IDisposable is not.

You need where T : class?, IDisposable?.

answered Jan 07 '20 at 15:23

canton7

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Thanks for the answer, it seems it works this way, but it's kind of not obvious really and somewhat confusing. I have additionally tried to return nullable whith such a constraint, but that does not work: https://stackoverflow.com/questions/59631460/c-sharp-8-gives-a-warning-when-returning-a-nullable-generic-with-nullable-constr – Ilya Chernomordik Jan 07 '20 at 15:34
Agreed, I find it non-obvious and confusing as well. And easily forgotten! – canton7 Jan 07 '20 at 15:35

Inconsistent behavior in C# 8 nullable reference type handling with generics

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