Get a handle to IFileDialog from a TFileDialogEvent to get the filename - delphi

Question

I am building a TFileSaveDialog descendent component. The descendent has a PushButton who's event is handled by:
function TFileDialogEvent.OnButtonClicked(const pfdc: IFileDialogCustomize;
dwIDCtl: DWORD): HResult; stdcall;
var
iImageEnIO: TImageEnIO;
iFilename: string;
iName: PChar;
pfd: IFileDialog;
begin
if dwIDCtl = dwVisualGroup8ID then
begin
iImageEnIO := TImageEnIO.Create(nil);
try
FileDialog.QueryInterface(
StringToGUID('{8016B7B3-3D49-4504-A0AA-2A37494E606F}'),
pfd);
// How to get correct valid handle to IFileDialog?
pfd.GetFileName(iName);
iFilename := string(iName);
if FileExists(iFilename) then
begin
The component also displays image information in various control labels correctly. The component sucessfully returns the selected filename and allows changing folders, but the getting the filename from pfd.GetFileName(iName) in the OnButtonClicked event is returning an invalid filename. I think the problem is caused by not getting the correct handle for pfd: IFileDialog.
UPDATE:
I solved this by defining
FileDialog: IFileDialog as a var at the component level then I called
function TFileDialogEvent.OnButtonClicked(const pfdc: IFileDialogCustomize;
dwIDCtl: DWORD): HResult; stdcall;
var
iImageEnIO: TImageEnIO;
iFilename: string;
pFolder: PWideChar;
iFolder: string;
iName: PChar;
pfd: IFileDialog;
hr: HRESULT;
aShellItem: IShellItem;
begin
if dwIDCtl = dwVisualGroup8ID then
begin
iImageEnIO := TImageEnIO.Create(nil);
try
FileDialog.QueryInterface(IFileDialog, pfd);
pfd.GetFileName(iName);
// Get the ShellItem
hr := SHCreateItemFromParsingName(iName, nil,
StringToGUID(SID_IShellItem), aShellItem);
// Get the folder
pfd.GetFolder(aShellItem);
// Get the folder displayname
aShellItem.GetDisplayName(SIGDN_FILESYSPATH, pFolder);
iFolder := string(pFolder);
if DirectoryExists( iFolder) then
iFilename := IncludeTrailingPathDelimiter( iFolder) + string(iName);
if FileExists(iFilename) then
begin
Thank-you all... Thank-you Rob... your post was helpful.

Answer 1

You're querying the object for an interface matching the GUID {8016B7B3-3D49-4504-A0AA-2A37494E606F}, and storing the result in an IFileDialog reference. The problem is that {8016B7B3-3D49-4504-A0AA-2A37494E606F} is the GUID for the IFileDialogCustomize interface, not IFileDialog. You attempt to call GetFileName, which is the sixth method of the IFileDialog interface, but since the variable actually holds an IFileDialogCustomize interface, control ends up being transferred to the sixth function of that interface instead. The compiler cannot catch the type mismatch for you, partly because you're constructing the GUID dynamically at run time (so it doesn't know the value at compile time), and partly because the second parameter to QueryInterface is untyped (so it can't know that the type of the variable is supposed to match the first parameter).
There's an easier way than computing the GUID at run time. Interface types are automatically useable as their associated GUIDs (when they have GUIDs). To request the IFileDialog interface, just pass that identifier directly to QueryInterface:
FileDialog.QueryInterface(IFileDialog, pfd);
You don't even have to call QueryInterface if you use the as operator:
pfd := FileDialog as IFileDialog;
When you call QueryInterface directly, you need to make sure you check the result for error codes. If you use the as operator, an unsupported interface will raise an exception. If you just want pass-fail without too much error checking, use the Supports function instead:
if Supports(FileDialog, IFileDialog, pfd) then ...

Answer 2

TSaveFileDialog has a public Dialog property of type IFileDialog, so you don't need to hunt for it manually, you already have direct access to it, eg:
function TFileDialogEvent.OnButtonClicked(const pfdc: IFileDialogCustomize; dwIDCtl: DWORD): HResult; stdcall;
var
iImageEnIO: TImageEnIO;
iFilename: string;
iName: PWideChar;
begin
if dwIDCtl <> dwVisualGroup8ID then
Result := E_NOTIMPL
else
begin
Result := S_OK;
if FAILED(Self.Dialog.GetFileName(iName)) then Exit;
try
iFilename := string(iName);
finally
CoTaskMemFree(iName);
end;
iImageEnIO := TImageEnIO.Create(nil);
try
if FileExists(iFilename) then
begin
...
end;
finally
iImageEnIO.Free;
end;
end;
end;