How RichEdit can accept file drag & dropping in BCB or Delphi program?

Question

I'm writing a program that could drag and drop text files onto the form to show and edit it by RichEdit.

I've used ChangeWindowMessageFilterEx to make sure that WM_DROPFILES and WM_COPYDATA can received by my Main Form:

  ChangeWindowMessageFilterEx(Handle, WM_DROPFILES, MSGFLT_ADD, NULL);
  ChangeWindowMessageFilterEx(Handle, WM_COPYDATA, MSGFLT_ADD, NULL);
  ChangeWindowMessageFilter(73 , MSGFLT_ADD);

and call DragAcceptFiles(Handle, true) in the form creation function.

Now the drag operation is valid on any places of the window but except the RichEdit, the cursor shows a deny icon when dragging on the RichEdit.

Dragging on any components, eg. text editors, panels, combo boxes and buttons, on the form can lead to receive the WM_DROPFILES message, but except RichEdit.

Actually, I'm sure that it is possible to drag files on the RichEdit because I have wrote the code last year, but I have lost the source code and forgot it. I'm trying to rebuild the same one now.

Here is the google drive download link to the executable file that I have finished last year. And here is the github url to the uncompleted source code that I'm writing currently.

Thank you for your watching.

Answer 1

I don't know why TRichEdit does not receive WM_DROPFILES when using a message map, but you could handle the WindowProc of the TRichEdit.

A possilble implementation could look like this:

• Drop a TRichEdit on your Form

• Modify header file

  private:    
  TWndMethod OldWindowProc;
  void __fastcall NewWindowProc(TMessage& Msg);
  

• Add implementation

  __fastcall TForm1::TForm1(TComponent* Owner)
  : TForm(Owner)
  {
      OldWindowProc = RichEdit1->WindowProc;
      RichEdit1->WindowProc = NewWindowProc;
      DragAcceptFiles(RichEdit1->Handle, true);
  }    
  
  void __fastcall TForm1::NewWindowProc(TMessage& Msg)
  {
      switch (Msg.Msg) {
          case WM_DROPFILES:
          {
              HDROP DropH = (HDROP)Msg.WParam;
              int droppedFileCount = DragQueryFile(DropH, 0xFFFFFFFF, NULL, 0);
              TStringList* Buffer = new TStringList();
              for (int i = 0; i < droppedFileCount; i++) {
                  int fileNameLength = DragQueryFile(DropH, i, NULL, 0);
                  String FileName;
                  FileName.SetLength(fileNameLength);
                  DragQueryFile(DropH, i, FileName.w_str(), fileNameLength + 1);
                  Buffer->LoadFromFile(FileName);
                  RichEdit1->Lines->AddStrings(Buffer);
                  RichEdit1->Lines->Add("");
              }
              delete Buffer;
              DragFinish(DropH);
              Msg.Result = 0;
              break;
          }
          case CM_RECREATEWND:
              DragAcceptFiles(RichEdit1->Handle, true);
              break;
      default:;
      }
      OldWindowProc(Msg);
  }