How to use new .NET 9 string.Join(char, ReadOnlySpan<string?>) with ReadOnlySpan<string>?

[kindermannhubert]

Hi,

what is the expected way to use APIs like

string.Join(char, ReadOnlySpan<string?>)

when I have ReadOnlySpan<string> in hand (which I expect would be much more often than ReadOnlySpan<string?>)?

The problem I'm having is the following example:

void Test(ReadOnlySpan<string> span)
{
    var joined = string.Join(';', span);
    ...
}

generates warning

Argument of type 'ReadOnlySpan<string>' cannot be used for parameter 'value' of type 'ReadOnlySpan<string?>' in 'string string.Join(char separator, params ReadOnlySpan<string?> value)' due to differences in the nullability of reference types.

I understand the problem but I'm wondering if there is some simple way to overcome this issue (I don't consider suppressing the warning here as a good solution). If not, I'm afraid the usability of such an API is severely harmed.

[huoyaoyuan]

The warning disappears in <LangVersion>preview</LangVersion>, the new first-class span feature should allow nullability covariance for spans.

Meanwhile, you can use ReadOnlySpan<string?>.CastUp to workaround this.

[kindermannhubert]

Great, thank you!