mov     [hwnd],eax    

Since you are using macros to create WindowProc, you must also return from the function using the ret macro (in small letters). RET generates a simple assembler command, not a macro instruction to exit WindowProc.

push    ebx esi edi    

Better save these registers on entry and restore them on exit, in WindowProc.

Code:

push    ebx esi edi    

MаtQuasar wrote:

Better save these registers on entry and restore them on exit, in WindowProc.

Code:

push    ebx esi edi    

I was actually wondering this since it pushes those registers to the stack in the template file with the "uses" macroinstruction, but from what i understand and again, i am very new to this. The only reason to push those registers is if i end up using them again somewhere else in my program, and it's really just there to save them incase i forget i'm using them, is that correct?

proc  GetError  uses rax rbx rsi rdi
locals
   errMes  db  128 dup(0)
   errBuf  db  128 dup(0)
endl
        invoke  GetLastError
        push    rax
        invoke  FormatMessage,FORMAT_MESSAGE_FROM_SYSTEM,0,rax,0,addr errMes,128,0
        pop     rax
       cinvoke  wsprintf,addr errBuf,<'WinError:  0x%08X',10,'%s',0>,rax,addr errMes
        invoke  MessageBox,0,addr errBuf,0,0
        ret
endp

;//---- Now after any Win32API function, we call it like this: --------

        invoke  CreateFile,......   ; any API in the code
       stdcall  GetError            ;<------- check the result!
    

Why use the WinDbg kernel debugger in user mode? It's like shooting sparrows with a cannon. There's a universal one https://x64dbg.com/ , designed specifically for user applications in the ring(3). It's a powerful and convenient tool that supports many modern solutions—external plugins, scripts for automating the debugging process, etc. After calling each Win32API in your code, it automatically inserts its own GetLastError() in the background to display the error code in the register window.

You can write your own error checking routine — there's the GetLastError() fn for that (it simply returns the error code) and FormatMessage(), which converts the error code into a human-readable text equivalent. Here's an example of such a routine (use only in the debug version of the program):

Code:

proc  GetError  uses rax rbx rsi rdi
locals
   errMes  db  128 dup(0)
   errBuf  db  128 dup(0)
endl
        invoke  GetLastError
        push    rax
        invoke  FormatMessage,FORMAT_MESSAGE_FROM_SYSTEM,0,rax,0,addr errMes,128,0
        pop     rax
       cinvoke  wsprintf,addr errBuf,<'WinError:  0x%08X',10,'%s',0>,rax,addr errMes
        invoke  MessageBox,0,addr errBuf,0,0
        ret
endp

;//---- Now after any Win32API function, we call it like this: --------

        invoke  CreateFile,......   ; any API in the code
       stdcall  GetError            ;<------- check the result!
    

The only issue is 'RET' instead of 'ret'.
TEMPLATE.ASM shipped with FASM has 'ret' and not 'RET'.
'uses ebx esi edi' is needed only if your 'WindowProc' uses (no pun!) these registers.