Windowsのごみ箱アイコンの変化を検知する

ごみ箱の ITEMIDLIST を取得

# include <ShlObj.h>

LPITEMIDLIST pidlBitBucket;
SHGetSpecialFolderLocation(nullptr, CSIDL_BITBUCKET, &pidlBitBucket);

SHGetSpecialFolderLocation function (shlobj_core.h) - Win32 apps | Microsoft Docs

シェルの変更通知に登録

# define WM_SHCHANGENOTIFY (WM_APP + 1)

SHChangeNotifyEntry changeNotifyEntry = { 0 };
changeNotifyEntry.fRecursive = FALSE;
changeNotifyEntry.pidl = pidlBitBucket;

ULONG ulID = SHChangeNotifyRegister(
    hWnd,
    (SHCNRF_ShellLevel | SHCNRF_NewDelivery),
    SHCNE_UPDATEIMAGE,
    WM_SHCHANGENOTIFY,
    1,
    &changeNotifyEntry);

SHChangeNotifyRegister function (shlobj_core.h) - Win32 apps | Microsoft Docs
SHChangeNotifyRegisterの第3引数(fEvents)にSHCNE_UPDATEIMAGEを渡すと、アイコンの変更が通知されます。

Window メッセージ を処理

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message) {
        case WM_SHCHANGENOTIFY:
            LPITEMIDLIST* ppidl;
            LONG lEvent;
            HANDLE hLock = SHChangeNotification_Lock((HANDLE)wParam, (DWORD)lParam, &ppidl, &lEvent);
        
            if (!hLock) {
                break;
            }

            if (lEvent == SHCNE_UPDATEIMAGE) {
                AnalyzeIcon();
            }

            SHChangeNotification_Unlock(hLock);
            break;

        ...
    }

    return 0;
}

SHChangeNotification_Lock function (shlobj_core.h) - Win32 apps | Microsoft Docs
SHChangeNotification_Unlock function (shlobj_core.h) - Win32 apps | Microsoft Docs

ごみ箱のアイコンを取得

# include <shellapi.h>

void AnalyzeIcon()
{
    SHFILEINFO shFileInfo = { 0 };

    if (SUCCEEDED(SHGetFileInfo(
        (LPCTSTR)pidlBitBucket,
        -1,
        &shFileInfo,
        sizeof(shFileInfo),
        (SHGFI_PIDL | SHGFI_ICON)))) {
        HICON hIcon = shFileInfo.hIcon;
        // アイコンを使う処理
        ...

        if (hIcon) {
            DestroyIcon(hIcon);
        }
    }
}

SHGetFileInfoA function (shellapi.h) - Win32 apps | Microsoft Docs
SHGetFileInfoの第1引数(pszPath)にITEMIDLISTのポインタ、第5引数にSHGFI_PIDLを渡すのがポイント。
取得したアイコンのハンドルは破棄が必要。

サンプル

# include <ShlObj.h>
# include <shellapi.h>
# define WM_SHCHANGENOTIFY (WM_APP + 1)

LPITEMIDLIST g_pidlBitBucket = nullptr;
ULONG g_ulID = 0;

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message) {
        case WM_CREATE:
            CoInitialize(nullptr);

            {
                SHChangeNotifyEntry changeNotifyEntry = { 0 };
                changeNotifyEntry.fRecursive = FALSE;
                changeNotifyEntry.pidl = g_pidlBitBucket;

                g_ulID = SHChangeNotifyRegister(
                    hWnd,
                    (SHCNRF_ShellLevel | SHCNRF_NewDelivery),
                    SHCNE_UPDATEIMAGE,
                    WM_SHCHANGENOTIFY,
                    1,
                    &changeNotifyEntry);

                if (!g_ulID) {
                    break;
                }
            }

            break;

        case WM_DESTROY:
            SHChangeNotifyDeregister(g_ulID);

            if (g_pidlBitBucket) {
                CoTaskMemFree(g_pidlBitBucket);
            }

            CoUninitialize();
            PostQuitMessage(0);
            break;

        case WM_SHCHANGENOTIFY:
        {
            LPITEMIDLIST* ppidl;
            LONG lEvent;
            HANDLE hLock = SHChangeNotification_Lock((HANDLE)wParam, (DWORD)lParam, &ppidl, &lEvent);

            if (!hLock) {
                break;
            }

            if (lEvent == SHCNE_UPDATEIMAGE) {
                AnalyzeIcon();
            }

            SHChangeNotification_Unlock(hLock);
        }

        break;

        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }

    return 0;
}

余談

.NETではshell32.dllを参照に追加するだけで、こんなコードが書けます。

using Shell32;
using System;

static class Program
{
    static void Main()
    {
        Shell shell = new Shell();
        // ShellSpecialFolderConstants::ssfBITBUCKET: 10
        Folder bitBucketFolder = shell.NameSpace(10);

        foreach (FolderItem2 item in bitBucketFolder.Items())
        {
            Console.WriteLine(item.Name);
        }
    }
}

Windows Shell - Win32 apps | Microsoft Docs

追記

IShellFolderは使用しないので、説明を削除しました。
(ごみ箱の中身を検証する時に使用します)