The shell gives you the IDataObject; all you have to do is drag it around. (This is the first of a five-part series.)

Start with the scratch program, and add the function GetUIObjectOfFile from an earlier article. Also, change the calls to CoInitialize and CoUninitialize to OleInitialize and OleUninitialize, respectively, since we're now going to be using full-on OLE and not just COM.

In order to initiate a drag/drop operation, we need a drop source:

class CDropSource : public IDropSource
{
public:
  // *** IUnknown ***
  STDMETHODIMP QueryInterface(REFIID riid, void **ppv);
  STDMETHODIMP_(ULONG) AddRef();
  STDMETHODIMP_(ULONG) Release();

  // *** IDropSource ***
  STDMETHODIMP QueryContinueDrag(BOOL fEscapePressed, DWORD grfKeyState);
  STDMETHODIMP GiveFeedback(DWORD dwEffect);

  CDropSource() : m_cRef(1) { }
private:
  ULONG m_cRef;
};

HRESULT CDropSource::QueryInterface(REFIID riid, void **ppv)
{
  IUnknown *punk = NULL;
  if (riid == IID_IUnknown) {
    punk = static_cast<IUnknown*>(this);
  } else if (riid == IID_IDropSource) {
    punk = static_cast<IDropSource*>(this);
  }

  *ppv = punk;
  if (punk) {
    punk->AddRef();
    return S_OK;
  } else {
    return E_NOINTERFACE;
  }
}

ULONG CDropSource::AddRef()
{
  return ++m_cRef;
}

ULONG CDropSource::Release()
{
  ULONG cRef = --m_cRef;
  if (cRef == 0) delete this;
  return cRef;
}

HRESULT CDropSource::QueryContinueDrag(
          BOOL fEscapePressed, DWORD grfKeyState)
{
  if (fEscapePressed) return DRAGDROP_S_CANCEL;

  // [Update: missing paren repaired, 7 Dec]
  if (!(grfKeyState & (MK_LBUTTON | MK_RBUTTON)))
    return DRAGDROP_S_DROP;

  return S_OK;
}

HRESULT CDropSource::GiveFeedback(DWORD dwEffect)
{
  return DRAGDROP_S_USEDEFAULTCURSORS;
}

As you can see, this drop source is extraordinarily boring. Even the interesting methods are uninteresting.

The IDropSource::QueryContinueDrag method is pretty much boilerplate. If the Escape key was pressed, then cancel the drag/drop operation. If the mouse buttons are released, then complete the operation. Otherwise, continue the operation.

The IDropSource::GiveFeedback method is even less interesting. It merely returns DRAGDROP_S_USEDEFAULTCURSORS to indicate that it wants default drag feedback.

Believe it or not, we now have everything we need to drag a file.

void OnLButtonDown(HWND hwnd, BOOL fDoubleClick,
                   int x, int y, UINT keyFlags)
{
  IDataObject *pdto;
  // In a real program of course
  // you wouldn't use a hard-coded path.
  // [comment added 11am because apparently some
  // people thought this wasn't self-evident.]
  if (SUCCEEDED(GetUIObjectOfFile(hwnd,
                    L"C:\\Windows\\clock.avi",
		    IID_IDataObject, (void**)&pdto))) {
    IDropSource *pds = new CDropSource();
    if (pds) {
      DWORD dwEffect;
      DoDragDrop(pdto, pds, DROPEFFECT_COPY | DROPEFFECT_LINK,
                 &dwEffect);
      pds->Release();
    }
    pdto->Release();
  }
}

    HANDLE_MSG(hwnd, WM_LBUTTONDOWN, OnLButtonDown);

To drag an object, you need two things, a data object and a drop source. We created our drop source above, and the data object comes from the shell. All that's left to do is start the drag/drop operation by calling the DoDragDrop function.

Notice that we specify that the permitted operations are DROPEFFECT_COPY and DROPEFFECT_LINK. We specifically disallow DROPEFFECT_MOVE because this program doesn't present a folder-like window; the user has no expectation that the drag/drop will result in a Move operation.

Next time, adding Move support, just to see how it works.