File Explorer Add-ons

// Thumbnail provider interface implementation
class ThumbnailProvider : public IThumbnailProvider
{
public:
    // Generate thumbnail from file
    HRESULT GetThumbnail(
        UINT cx,              // Requested width
        HBITMAP *phbmp,       // Output bitmap
        WTS_ALPHATYPE *pdwAlpha  // Alpha channel type
    );
};

HKEY_CLASSES_ROOT\.md
  └─ shellex
      └─ {8895b1c6-b41f-4c1c-a562-0d564250836f}
          = "{CLSID of MarkdownPreviewHandler}"

src/modules/previewpane/
├─ MonacoPreviewHandler/
│   ├─ MonacoPreviewHandler.cpp     # C++ wrapper
│   └─ MonacoPreviewHandlerControl/  # .NET implementation
├─ MarkdownPreviewHandler/
│   ├─ MarkdownPreviewHandler.cpp
│   └─ MarkdownPreviewHandlerControl/
├─ SvgPreviewHandler/
├─ PdfPreviewHandler/
├─ GcodePreviewHandler/
└─ common/                      # Shared utilities

// Preview handler isolation
// Handlers run in prevhost.exe surrogate process
// Protects File Explorer from crashes
// Each handler in separate AppDomain/.NET process

// C++ Preview Handler Interface
class PreviewHandler : 
    public IPreviewHandler,
    public IInitializeWithStream,
    public IOleWindow
{
public:
    // Load file content
    HRESULT Initialize(IStream* pstream, DWORD grfMode);
    
    // Create preview window
    HRESULT SetWindow(HWND hwnd, const RECT* prc);
    
    // Render preview
    HRESULT DoPreview();
    
    // Cleanup
    HRESULT Unload();
};