windows - Why is delayed expansion in a batch file not working in this case?

Question

This code

@echo off
setlocal EnableDelayedExpansion

set myvar=first
set first=second

echo myvar:!myvar!
set myvar=!myvar!
echo myvar:!myvar!

gives

myvar:first
myvar:first

on Windows Vista SP2.

The output I had expected is

myvar:first
myvar:second

Why the difference and how to obtain desired effect?

Answer 1

The problem is that set myvar=!myvar! expands to set myvar=first,
you set it with the same content, and then you ask echo myvar:!myvar! to show the content of myvar.

I will try to add some more explanations, even if Aacini and shf301 already answered the question.

Both showed the double expansion with the !%var%! construct, and Aacini explained why it can work, and why the reversed version %!var!% can't work.

IMHO there are four different expansions.
Delayed Expansion:
As Aacini explained the delayed expansion is safe against any special characters in the content (it can handle ALL characters from 0x01 to 0xFF).

Percent Expansion:
The percent expansion can't handle or removes some characters (even with escaping).
It can be useful for simple content, as it can expand variables after an endlocal barrier.

setlocal
set "myVar=simple content"

(
endlocal
set result=%myVar%
)

FOR-Loop-Parameters expansion:
It is safe, if the delayed expansion is disabled, else the delayed expansion phase is executed after the expansion of the %%a variables.
It can be useful, as it can expand variables after an endlocal barrier

setlocal EnableDelayedExpansion
set "var=complex content &<>!"
for /F "delims=" %%A in ("!var!") DO (
  endlocal
  set "result=%%A"
)

SET Expansion:
set var expands also a variable, and it is always safe and works independent of the delayed expansion mode.

Aacini just explained how the call %%%var%%% construct work, I only want to give some additional remarks.
call is stackable, you can use many of them and each restarts the parser.

set "var=%%var%%#"
call call call call call echo %var%

results to %var%######

But call have many disadvantages/side effects!
Each call double all carets ^
You can say: "Hey I've tested it and I can't see any doubling"

call call call call echo ^^

result ^

Nevertheless it's true, but it's mostly hidden, as each restart also have a special character phase where carets escapes the next character, but you can see the doubling effect with

call call call call echo "^^"^^

result "^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^"^

Even if a call expansion restarts the parser, you can never use the delayed expansion in any phase (only in the first one).

call stops working if it detects unescaped special characters.

echo you ^& me
call echo you & me
call echo you ^& me
call echo you ^^& me
call echo you ^^^& me

Only the first results to the output you & me, all the others fails.

Another problem is that call is extremly slow, a call set var=content is ~50times slower than set var=content, the cause is that call try to start an external program.

@echo off
setlocal
(
   echo echo *** External batch, parameters are '%%*'
) > set.bat
set "var="
call set var=hello
set var

I hope it was interesting a bit ...
And if you want to go more in depth you can read CALL me, or better avoid call
and How does the Windows Command Interpreter (CMD.EXE) parse scripts?