Span<T>'s Slice method do not throw if Length property is supplied as start parameter

[Gnbrkm41]

From MS Docs page on Span<T>.Slice(int) method: ArgumentOutOfRangeException is thrown if index(probably start?) is less than zero or greater than or equal to Length.

However, span.Slice(span.Length) does not seem to throw; The example code below executes with no problem.

"asd".ToCharArray().AsSpan().Slice(3)

It appears to be an incorrect document rather than a problem with the implementation, as the documentation for ReadOnlySpan<T>.Slice(int) says that ArgumentOutOfRangeException is thrown if start is greater than the number of items in the read-only span.

Is this behaviour expected? I also have found that XML documentations on these methods (Span.Slice, ReadOnlySpan.Slice) says so as well.

/// <exception cref="System.ArgumentOutOfRangeException">
/// Thrown when the specified <paramref name="start"/> or end index is not in range (&lt;0 or &gt;=Length).
/// </exception>

<0 or >=Length seems to imply that it should throw if I pass Length as the parameter.

[Gnbrkm41]

For me it seems reasonable for these methods to behave like what it does now, as similar method of string (string.Substring(int)) does return an empty string if given Length.

[tannergooding]

Looks like the check is only doing > length: https://github.com/dotnet/coreclr/blob/4dc930ea7b1aa429eb30f2ee0b460d5edf5a244f/src/System.Private.CoreLib/shared/System/ReadOnlySpan.Fast.cs#L270

Looks like it has always been this way, at least from tracing the sources back to the original PR implementing the portable versions: dotnet/corefx#13562.

[ahsonkhan]

It appears to be an incorrect document rather than a problem with the implementation

Yes, we need to update the documentation. Good catch :)

We should update the comments in source, but also update the docs (https://github.com/dotnet/dotnet-api-docs/), i.e. dotnet/dotnet-api-docs#1771

[Gnbrkm41]

BTW, is empty span returned from Slice(Length) expected to be equal to ReadOnlySpan<T>.Empty?

"asd".AsSpan().Slice(3) == ReadOnlySpan<char>.Empty

At the moment this code returns false, which if I understand correctly is because the internal pointer of the sliced span points to the next byte after the end of the array(?), whereas the static Empty property's one is null. (sort of, since it returns default, but the moral of the story is that the internal pointer's value are different, therefore two Spans are not the same)

in string.Substring's case it returns the cached string.Empty if length == 0, so I was expecting similar behaviour as well but apparently not.

One benefit I can see of this is that it makes it easier to check whether an instance of Span<T> is default(Span<T>) or an actual empty span (sliced span with length 0), but I'm not sure how big of an impact it would have.

[Gnbrkm41]

If the current behaviour is expected and preferred over ReadOnlySpan<T>.Empty, it would be better to document it as well.

[Gnbrkm41]

(the IsEmpty property is true for the former one)