Is there a different == null behavior for Span<T>?

[msedi]

Currently we are translating many of our functions to Span<T> and we found that there is a different behavior for Span<T> comparing against null.

Comparing regular structs to null always results in false, where in Span<T> it's somehow like using IsEmpty.

Is there a reason why this is done (enable a more transistion from arrays to Span<T>?). Obviously it's not possible to enable the same behavior for regular developers.

[alrz]

Span has an implicit conversion from T[] that's what null is passed to, therefore the behavior you are observing.

[msedi]

@alrz: Have you tested this? I was told that every struct comparison against null is automatically returning false (see here: #45030).

As you said there is an automatic conversion but does the compiler automatically infer a T[] from null?

I did some tests with this struct:

    public ref struct S<T>
    {
        public static bool operator ==(S<T> left, S<T> right) => left == right;
        public static bool operator !=(S<T> left, S<T> right) => !(left == right);

        public static implicit operator T[](S<T> s) => new T[0];
    }

and it shows that the == operator is not even called, also the operator T[] is not called when doing this:

         var s = new S<int>();
         Console.WriteLine(s == null);

[alrz]

You can see it in action here: https://sharplab.io/#v2:EYLgtghgzgLgpgJwDQxASwDYB8ACAmARgFgAoHAZgAJ9KBhSgb1MpeqpwBZKBZACgGUADhAB2AHjQiYAPkoQAlI2asVaAGaVeESgF4dlEQFcMGRUxIqVAX2UsbJK0A==

Notice the call to op_Implicit(!0[]), that's the conversion from array.

[msedi]

Thanks. I think I got it.