C++20&Win API ドロップされたOLEデータオブジェクトに含まれるShellItemを列挙する

ドロップされたOLEデータオブジェクトに含まれるShellItem（ファイル、フォルダ、特殊フォルダ等）を列挙するサンプルコードです。

#define STRICT
#include <Windows.h>
#include <ShlObj.h>
#include <ShObjIdl.h>

#include <bit>
#include <format>
#include <string>
#include <vector>

auto MY_WINDOW_CLASS_NAME{ L"Project1" };
auto MY_WINDOW_WINDOW_NAME{ L"Project1" };

LRESULT CALLBACK MyWndProc(HWND hWnd, UINT uiMsg, WPARAM wParam, LPARAM lParam);

int WINAPI wWinMain(HINSTANCE, HINSTANCE, LPWSTR, int nCmdShow)
{
    if (HRESULT hr = OleInitialize(nullptr); FAILED(hr))
        return hr;

    WNDCLASSEX wce{
        sizeof(wce),
        0,
        MyWndProc,
        0,
        0,
        nullptr,
        LoadIcon(nullptr, IDI_APPLICATION),
        LoadCursor(nullptr, IDC_ARROW),
        reinterpret_cast<HBRUSH>(COLOR_3DFACE + 1),
        nullptr,
        MY_WINDOW_CLASS_NAME,
        LoadIcon(nullptr, IDI_APPLICATION) };
    if (!RegisterClassEx(&wce))
        return 0;

    auto hWnd{ CreateWindowEx(
        WS_EX_OVERLAPPEDWINDOW,
        MY_WINDOW_CLASS_NAME,
        MY_WINDOW_WINDOW_NAME,
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        CW_USEDEFAULT, CW_USEDEFAULT,
        nullptr,
        nullptr,
        nullptr,
        nullptr) };
    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);

    MSG msg;
    while (GetMessage(&msg, nullptr, 0, 0) > 0)
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    OleUninitialize();

    return static_cast<int>(msg.wParam);
}

template<DWORD INIT = 256, DWORD EXPANDING = 256>
std::wstring GetClipboardFormatNameW(CLIPFORMAT cf)
{
    std::wstring buffer;
    for (DWORD size = INIT;; size += EXPANDING)
    {
        buffer.resize(size);
        auto copied{ GetClipboardFormatNameW(cf, buffer.data(), size) };
        if (copied != buffer.size() - 1)
        {
            buffer.resize(copied);
            buffer.shrink_to_fit();
            return std::move(buffer);
        }
    }
}

std::wstring GetShellItemDisplayName(IShellItem* pShellItem, SIGDN sigdnName)
{
    LPWSTR pszName;
    if (SUCCEEDED(pShellItem->GetDisplayName(sigdnName, &pszName)))
    {
        std::wstring s{ pszName };
        ::CoTaskMemFree(pszName);
        return std::move(s);
    }
    return {};
}

// https://docs.microsoft.com/ja-jp/windows/desktop/api/oleidl/nn-oleidl-idroptarget
class CMyDropTarget : public IDropTarget
{
private:
    ULONG m_ref;
    HWND m_hwndEdit;

    CMyDropTarget() {}

public:
    constexpr CMyDropTarget(HWND hwndEdit)
        : m_ref(1), m_hwndEdit(hwndEdit)
    {
    }

    HRESULT QueryInterface(REFIID riid, void** ppvObject)
    {
        if (IsEqualIID(riid, __uuidof(IUnknown)))
        {
            *ppvObject = static_cast<IUnknown*>(this);
        }
        else if (IsEqualIID(riid, __uuidof(IDropTarget)))
        {
            *ppvObject = static_cast<IDropTarget*>(this);
        }
        else
        {
            return E_NOINTERFACE;
        }
        AddRef();
        return S_OK;
    }

    ULONG AddRef()
    {
        return InterlockedIncrement(&m_ref);
    }

    ULONG Release(void)
    {
        if (InterlockedDecrement(&m_ref) == 0)
        {
            delete this;
            return 0;
        }
        return m_ref;
    }

    HRESULT STDMETHODCALLTYPE DragEnter(
        IDataObject* pDataObj,
        DWORD grfKeyState,
        POINTL pt,
        DWORD* pdwEffect)
    {
        *pdwEffect = DROPEFFECT_LINK;
        return S_OK;
    }

    HRESULT DragOver(
        DWORD grfKeyState,
        POINTL pt,
        DWORD* pdwEffect)
    {
        return S_OK;
    }

    HRESULT DragLeave()
    {
        return S_OK;
    }

    HRESULT Drop(
        IDataObject* pDataObj,
        DWORD grfKeyState,
        POINTL pt,
        DWORD* pdwEffect)
    {
        std::vector<CLIPFORMAT> cfs;

        FORMATETC fmtetc{ static_cast<FORMATETC>(RegisterClipboardFormat(CFSTR_SHELLIDLIST)) };
        if (!pDataObj->QueryGetData(&fmtetc))
            return S_OK;

        IShellItemArray* pShellItems{ nullptr };
        if (SUCCEEDED(SHCreateShellItemArrayFromDataObject(pDataObj,
            IID_IShellItemArray, (void**)&pShellItems)))
        {
            DWORD c;
            if (SUCCEEDED(pShellItems->GetCount(&c)))
            {
                std::wstring ss;
                ss.append(std::format(
                    L"SIGDN_NORMALDISPLAY, "
                    "SIGDN_PARENTRELATIVEPARSING,"
                    "SIGDN_DESKTOPABSOLUTEPARSING, "
                    "SIGDN_PARENTRELATIVEEDITING, "
                    "SIGDN_DESKTOPABSOLUTEEDITING, "
                    "SIGDN_FILESYSPATH, "
                    "SIGDN_URL, "
                    "SIGDN_PARENTRELATIVEFORADDRESSBAR, "
                    "SIGDN_PARENTRELATIVE, "
                    "SIGDN_PARENTRELATIVEFORUI\r\n"));

                for (UINT i = 0; i < c; i++)
                {
                    IShellItem* pShellItem{ nullptr };
                    if (SUCCEEDED(pShellItems->GetItemAt(i, &pShellItem)))
                    {
                        ss.append(std::format(L"{}, {}, {}, {}, {}, {}, {}, {}, {}, {}\r\n",
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_NORMALDISPLAY),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_PARENTRELATIVEPARSING),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_DESKTOPABSOLUTEPARSING),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_PARENTRELATIVEEDITING),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_DESKTOPABSOLUTEEDITING),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_FILESYSPATH),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_URL),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_PARENTRELATIVEFORADDRESSBAR),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_PARENTRELATIVE),
                            GetShellItemDisplayName(pShellItem, SIGDN::SIGDN_PARENTRELATIVEFORUI)));
                    }
                    pShellItem->Release();
                }
                SetWindowText(m_hwndEdit, ss.c_str());
            }
            pShellItems->Release();
        }

        *pdwEffect = DROPEFFECT_LINK;
        return S_OK;
    }

    constexpr HWND GetEditWindowHandle() const
    {
        return m_hwndEdit;
    }
};

CMyDropTarget* pDropTarget{ nullptr };

LRESULT CALLBACK MyWndProc(HWND hWnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uiMsg)
    {
    case WM_CREATE:
        {
            // データ表示用のテキストボックス作成
            auto hwndEdit1{ CreateWindowEx(
                WS_EX_CLIENTEDGE,
                L"edit",
                L"ファイルやURLをドロップしてください。",
                WS_CHILD | WS_VISIBLE | WS_HSCROLL | WS_VSCROLL | ES_MULTILINE,
                0, 0, 0, 0,
                hWnd,
                nullptr,
                nullptr,
                nullptr) };
            SendMessage(hwndEdit1, WM_SETFONT,
                std::bit_cast<WPARAM>(GetStockObject(DEFAULT_GUI_FONT)),
                MAKELPARAM(FALSE, 0));
            // ドロップ対象として登録
            pDropTarget = new CMyDropTarget(hwndEdit1);
            RegisterDragDrop(hWnd, pDropTarget);
            pDropTarget->Release();
            return 0;
        }
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    case WM_QUIT:
        RevokeDragDrop(hWnd);
        return 0;
    case WM_SIZE:
        MoveWindow(
            pDropTarget->GetEditWindowHandle(),
            0, 0,
            LOWORD(lParam), HIWORD(lParam),
            TRUE);
        return 0;
    }

    return DefWindowProc(hWnd, uiMsg, wParam, lParam);
}