Span: Provide ToString

[ianhays]

Forked from https://github.com/dotnet/corefx/issues/14376

Work item here is to implement ToString on Span/ReadOnlySpan and have it print out a string similar to what a collection would print out (which is just the object tostring - type + address)

cc: @KrzysztofCwalina @jkotas @ahsonkhan to continue the discussion on ToString here.

[ianhays]

The crux of the issue is that there currently isn't any ToString available for either Span or ReadonlySpan. The question isn't so much whether the ToString should print something valuable as much as it is a question of whether it should be available at all.

new ReadOnlySpan<byte>().ToString() currently causes a compile error: error CS0029: Cannot implicitly convert type 'System.ReadOnlySpan<byte>' to 'System.ValueType'

It should instead compile and just print something basic e.g. System.ReadOnlySpan[{span.length}]

[jkotas]

The same problem exists for all ref types (unless they override ToString). Do you believe that fixing this problem just for Span is worth it?

[ianhays]

The same problem exists for all ref types (unless they override ToString).

Yes, the problem is present in all ref types.

What do you mean by override Tostring? From what parent does a ref type inherit a ToString to override? Or did you just mean implement ToString on the class directly?

Do you believe that fixing this problem just for Span is worth it?

I think it is worthwhile to ease adoption of the type, yes. In the world where almost everything can be ToStringed (including Memory and ReadOnlyMemory), it's better to adhere to that pattern even without the common parent.

[jkotas]

From what parent does a ref type inherit a ToString to override?

From System.ValueType. System.ValueType is parent of all structs.

Why do you think this will ease adoption of Span? I think ref types come with number of restrictions that are much more in peoples face than not being able to call ToString.

We have dummy overrides for GetHashCode and Equals on Span already, so I guess we can have implementation of ToString as well...

[ianhays]

From System.ValueType. System.ValueType is parent of all structs.

Interesting, I didn't think ValueType was a parent of a ref struct. So in that case, what is happening to the ToString that is being inherited from ValueType? We're not explicitly hiding it, so shouldn't the GetType implementation in ValueType be usable fromSpan already?

Further, why is it that a Span cannot be used as a ValueType? To do so results in a error CS0029: Cannot implicitly convert type 'System.ReadOnlySpan<byte>' to 'System.ValueType'. Does being a ref type dismiss that types ability to act polymorphically as its parent?

I think ref types come with number of restrictions that are much more in peoples face than not being able to call ToString.

True. My point of view is that it's better to lessen those restrictions rather than increase them, though it's very unlikely that ToString is going to be the blocking piece.

[jkotas]

Calling ValueTuple.ToString on a Span would require boxing. Boxing is not allowed on ref types. span.ToString() basically expands into:

ValueType o = (ValueType)span; // You will get the same error on this line
o.ToString();

You will get similar error when you try to do Console.WriteLine(span) for the same reason, even though you can do Console.WriteLine on almost everything else.

[ianhays]

Boxing is not allowed on ref types.

Ah gotcha, that was the bit I was missing. Thanks for the explanation.

[TahirAhmadov]

I would object to implementing ToString on Span types. For debugging purposes, we should use the appropriate attribute+backing method combination. For Span, there should be an extension method called AsString. We need to move away from ToString, because it's a giant anti-pattern, not sprinkle it everywhere.
Edit: Haha I see I'm too late, it's already been done. Still I'll leave this comment here for posterity. AsString would've been preferable because it communicates a conversion (and the necessary allocation). Oh well.