Why doesn't “set -P” work after a pipe?

Question

C:\\>type c:\\output.txt
abcd
C:\\>type c:\\output.txt | set /p V1=

C:\\>set
... A bunch of junk, NOT seeing \"V1\"

What happened? According

Answer 1

To add a brief example of how to confirm the other answers, you can immediately check the contents of the variable on the other side of the pipe using another set command IE:

ECHO HELLO | (set /P hi= && set hi)

Will output

hi=HELLO

But after execution the variables are destroyed : (, so a further 'set hi' will reveal:

Environment variable hi not defined