Today's Little Program applies an arbitrary filter to the Browse for Folder dialog: We will filter out drives smaller than 512GB. Hey, remember, I said that Little Programs do not need motivation.

Today's smart pointer class library is... (rolls dice)... WRL!

#define STRICT
#define STRICT_TYPED_ITEMIDS
#include <windows.h>
#include <shlobj.h>
#include <propkey.h>
#include <propvarutil.h>
#include <wrl/implements.h>
using namespace Microsoft::WRL;

struct ComVariant : public VARIANT
{
    ComVariant() { VariantInit(this); }
    ~ComVariant() { VariantClear(this); }
};

WRL does not come with a "smart VARIANT" class analogous to ATL's CComVariant, so we provide our own very simple one.

class CFunnyFilter :
  public RuntimeClass<
                RuntimeClassFlags<RuntimeClassType::ClassicCom>,
                IFolderFilter>
{
public:
  // *** IFolderFilter ***
  IFACEMETHODIMP ShouldShow(
        IShellFolder* psf,
        PCIDLIST_ABSOLUTE pidlFolder,
        PCUITEMID_CHILD pidlItem)
  {
    if (!IsMyComputerFolder(psf)) return S_OK;

    ComPtr<IShellFolder2> spsf2;
    if (FAILED(ComPtr<IUnknown>(psf).As(&spsf2))) {
      return S_OK;
    }

    ComVariant svt;
    if (FAILED(spsf2->GetDetailsEx(pidlItem,
                                   &PKEY_Capacity, &svt))) {
      return S_OK;
    }

    if (VariantToUInt64WithDefault(svt, 0) >=
                             512ULL*1024ULL*1024ULL*1024ULL) {
      return S_OK;
    }

    return S_FALSE;
  }

  IFACEMETHODIMP GetEnumFlags(
        IShellFolder* psf,
        PCIDLIST_ABSOLUTE pidlFolder,
        HWND *phwnd,
        DWORD *pgrfFlags) { return S_OK; }
};

Our custom Should­Show method first checks if we are showing children of My Computer. If not, then we allow the item to pass through the filter.

Next, we convert the folder to IShell­Folder2. If we can't, then we allow the item to pass through the filter. (Arbitrary choice.)

Next, we ask for the capacity of the item. If we can't (no media in drive, or it's not a drive in the first place), then we allow the item to pass through the filter. (Arbitrary choice.)

Next, we look at the capacity, and if it's at least 512GB, then we allow the item to pass through the filter.

Otherwise, we have a drive smaller than 512GB, so we filter it out.

That's it! Let's install this filter in the callback function:

int CALLBACK BrowseCallbackProc(HWND hwnd,
    UINT uMsg, LPARAM lParam, LPARAM lpData)
{
  switch (uMsg) {
  case BFFM_IUNKNOWN:
    if (lParam) {
      IUnknown *punk = reinterpret_cast<IUnknown*>(lParam);
      ComPtr<IFolderFilterSite> spFilterSite;
      if (SUCCEEDED(ComPtr<IUnknown>(punk).As(&spFilterSite)))
        spFilterSite->SetFilter(Make<CFunnyFilter>().Get());
      }
    }
    break;
  }
  return 0;
}

When we get the BFFM_IUNKNOWN message, we convert the IUnknown (cast to LPARAM) to a IFolder­Filter­Site and tell it to apply our custom filter.

Okay, let's plug it in and see if smoke comes out.

int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,
                   LPSTR lpCmdLine, int nShowCmd)
{
  CCoInitialize init;
  BROWSEINFO bi = { };
  TCHAR szDisplayName[MAX_PATH];
  bi.pszDisplayName = szDisplayName;
  bi.lpfn = BrowseCallbackProc;
  bi.ulFlags = BIF_NEWDIALOGSTYLE;
  PIDLIST_ABSOLUTE pidl = SHBrowseForFolder(&bi);
  CoTaskMemFree(pidl);
  return 0;
}

The tricky part here is that we have to pass the BIF_NEW­DIALOG­STYLE flag because it is the new Browse for Folder dialog that sends the BFFM_IUNKNOWN message.