Why does en-dash (–) trigger illegal XML character error (C#/SSMS)?

Question

This is not a question on how to overcome the \"XML parsing: ... illegal xml character\" error, but about why it is happening? I know tha

Answer 1

Can you modify the XML encoding declaration? If so;

declare @xml XML = N'
  
  
';

select @xml

(No column name)

Speculative Edit

Both of these fail with illegal xml character:

set @xml = ' – '
set @xml = ' – '

because they pass a non-unicode varchar to the XML parser; the string contains Unicode so must be treated as such, i.e. as an nvarchar (utf-16) (otherwise the 3 bytes comprising the – are misinterpreted as multiple characters and one or more is not in the acceptable range for XML)

This does pass a nvarchar string to the parser, but fails with unable to switch the encoding:

set @xml = N' – '

This is because an nvarchar (utf-16) string is passed to the XML parser but the XML document states its utf-8 and the – is not equivalent in the two encodings

This works as everything is utf-16

set @xml = N' – '