How can VS detect file encoding when we can't?

问题:

According to several answers here on SO, eg. How can I detect the encoding/codepage of a text file it is not possible to retrieve the encoding of a file.

How does VS do it then? If I "Save as", and select "Save with encoding" it shows the current encoding of my file.

回答1:

By default, the editor detects encoding by searching for byte order marks or charset tags. If neither is found in the current document, the code editor attempts to auto-detect UTF-8 encoding by scanning byte sequences. VS Help

If VS fails to convert any byte sequences to valid unicode symbols, it opens file in default system character set. No magic realy.