PathAddExtensionW

// SHLWAPI.dll  (Unicode / -W)
#include <windows.h>

BOOL PathAddExtensionW(
    LPWSTR pszPath,
    LPCWSTR pszExt   // optional
);

<DllImport("SHLWAPI.dll", CharSet:=CharSet.Unicode, ExactSpelling:=True)>
Public Shared Function PathAddExtensionW(
    <MarshalAs(UnmanagedType.LPWStr)> pszPath As System.Text.StringBuilder,   ' LPWSTR in/out
    <MarshalAs(UnmanagedType.LPWStr)> pszExt As String   ' LPCWSTR optional
) As Boolean
End Function

' pszPath : LPWSTR in/out
' pszExt : LPCWSTR optional
Declare PtrSafe Function PathAddExtensionW Lib "shlwapi" ( _
    ByVal pszPath As LongPtr, _
    ByVal pszExt As LongPtr) As Long
' Unicode(W): 文字列は ByVal As LongPtr とし StrPtr(unicodeStr) を渡す
' VBA7前提(PtrSafe)。32bit Office では LongPtr→Long。Integer=16bit / Long=32bit / LongLong=64bit。

import ctypes
from ctypes import wintypes

PathAddExtensionW = ctypes.windll.shlwapi.PathAddExtensionW
PathAddExtensionW.restype = wintypes.BOOL
PathAddExtensionW.argtypes = [
    wintypes.LPWSTR,  # pszPath : LPWSTR in/out
    wintypes.LPCWSTR,  # pszExt : LPCWSTR optional
]

require 'fiddle'
require 'fiddle/import'

lib = Fiddle.dlopen('SHLWAPI.dll')
PathAddExtensionW = Fiddle::Function.new(
  lib['PathAddExtensionW'],
  [
    Fiddle::TYPE_VOIDP,  # pszPath : LPWSTR in/out
    Fiddle::TYPE_VOIDP,  # pszExt : LPCWSTR optional
  ],
  Fiddle::TYPE_INT)
# Wide strings: pass str.encode("UTF-16LE") + "\x00\x00"

#[link(name = "shlwapi")]
extern "system" {
    fn PathAddExtensionW(
        pszPath: *mut u16,  // LPWSTR in/out
        pszExt: *const u16  // LPCWSTR optional
    ) -> i32;
}
// crates: windows-sys provides ready-made bindings for this API.

$sig = @"
[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("SHLWAPI.dll", CharSet = CharSet.Unicode)]
public static extern bool PathAddExtensionW([MarshalAs(UnmanagedType.LPWStr)] System.Text.StringBuilder pszPath, [MarshalAs(UnmanagedType.LPWStr)] string pszExt);
"@
$api = Add-Type -MemberDefinition $sig -Name 'SHLWAPI_PathAddExtensionW' -Namespace Win32 -PassThru
# $api::PathAddExtensionW(pszPath, pszExt)

#uselib "SHLWAPI.dll"
#func global PathAddExtensionW "PathAddExtensionW" wptr, wptr
; PathAddExtensionW varptr(pszPath), pszExt   ; 戻り値は stat
; pszPath : LPWSTR in/out -> "wptr"
; pszExt : LPCWSTR optional -> "wptr"
; ※HSP3.7は #func のため戻り値はシステム変数 stat に格納されます。

#uselib "SHLWAPI.dll"
#cfunc global PathAddExtensionW "PathAddExtensionW" var, wstr
; res = PathAddExtensionW(pszPath, pszExt)
; pszPath : LPWSTR in/out -> "var"
; pszExt : LPCWSTR optional -> "wstr"
; ※出力/バッファ引数は var 方式(変数を直接渡す)。varptr 方式にも切替可。

; BOOL PathAddExtensionW(LPWSTR pszPath, LPCWSTR pszExt)
#uselib "SHLWAPI.dll"
#cfunc global PathAddExtensionW "PathAddExtensionW" var, wstr
; res = PathAddExtensionW(pszPath, pszExt)
; pszPath : LPWSTR in/out -> "var"
; pszExt : LPCWSTR optional -> "wstr"
; ※出力/バッファ引数は var 方式(変数を直接渡す)。varptr 方式にも切替可。

import (
	"golang.org/x/sys/windows"
	"unsafe"
)

var (
	shlwapi = windows.NewLazySystemDLL("SHLWAPI.dll")
	procPathAddExtensionW = shlwapi.NewProc("PathAddExtensionW")
)

// pszPath (LPWSTR in/out), pszExt (LPCWSTR optional)
r1, _, err := procPathAddExtensionW.Call(
	uintptr(pszPath),
	uintptr(unsafe.Pointer(windows.StringToUTF16Ptr(pszExt))),
)
_ = err  // syscall.Errno (valid when the call sets last-error)
_ = r1   // BOOL

function PathAddExtensionW(
  pszPath: PWideChar;   // LPWSTR in/out
  pszExt: PWideChar   // LPCWSTR optional
): BOOL; stdcall;
  external 'SHLWAPI.dll' name 'PathAddExtensionW';

result := DllCall("SHLWAPI\PathAddExtensionW"
    , "Ptr", pszPath   ; LPWSTR in/out
    , "WStr", pszExt   ; LPCWSTR optional
    , "Int")   ; return: BOOL

●PathAddExtensionW(pszPath, pszExt) = DLL("SHLWAPI.dll", "bool PathAddExtensionW(char*, char*)")
# 呼び出し: PathAddExtensionW(pszPath, pszExt)
# pszPath : LPWSTR in/out -> "char*"
# pszExt : LPCWSTR optional -> "char*"
# なでしこ1は32bit・ANSI(Shift_JIS)。文字列=char*(ANSI)、ポインタ/ハンドル=void*(4byte)。
# ※-W(Unicode)関数。なでしこ1はANSIのため -A 版の利用を推奨。