`If WinActive` together with `GetKeyState` - autohotkey

Question

Can anybody explain, why I get "Error" message box?
I think, the code is self-explanatible. I was "surprised" when I discovered it doesn't work for some reason.
(When you press F1 in opened Notepad window, you should see "It works!" message. Instead, I get an "Error" message).
I've tried different ways to fix it, i.e. percents, variables assignments, parentheses, but currently it still doesn't work.
#SingleInstance, Force
SetTitleMatchMode, 2
f1::
+f1::
GetKeyState, shift_state, Shift
msgbox, %shift_state%
if (WinActive("Notepad") and shift_state = D)
msgbox, It works! By the way, the Shift key is pressed.
else if (WinActive("Notepad") and shift_state = U)
msgbox, It works! The Shift key is not pressed.
else
msgbox, error
return

Answer 1

In expressional mode literal strings must be enclosed in double quotes to distinguish them from variables.
https://autohotkey.com/docs/Variables.htm#Expressions
expressional mode:
#SingleInstance, Force
SetTitleMatchMode, 2
f1::
+f1::
GetKeyState, shift_state, Shift
msgbox, %shift_state%
if (WinActive("Notepad") and shift_state = "D")
msgbox, It works! By the way, the Shift key is pressed.
else if (WinActive("Notepad") and shift_state = "U")
msgbox, It works! The Shift key is not pressed.
else
msgbox, error
return
traditional mode:
#SingleInstance, Force
SetTitleMatchMode, 2
f1::
+f1::
GetKeyState, shift_state, Shift
msgbox, %shift_state%
IfWinActive Notepad
{
If shift_state = D
msgbox, It works! By the way, the Shift key is pressed.
else If shift_state = U
msgbox, It works! The Shift key is not pressed.
}
else
msgbox, error
return