動作確認

C++ builder XE4
RAD Studio 10.2 Tokyo Update 2 (追記: 2018/01/09)

ファイルのドラッグ&ドロップに対応する方法

---
Unit1.hの追加項目

Unit1.h

private:
  void __fastcall ReceiveDropFiles(TWMDropFiles Msg);
  BEGIN_MESSAGE_MAP
  MESSAGE_HANDLER(WM_DROPFILES, TWMDropFiles, ReceiveDropFiles)
  END_MESSAGE_MAP(TForm)

---
Unit1.cppの追加項目

Unit1.cpp

//---------------------------------------------------------------------------
void __fastcall TForm1::ReceiveDropFiles(TWMDropFiles Msg)
{
	int fileCount;
	wchar_t fileName[MAX_PATH + 10]; // 10: 余分に取る

	Application->BringToFront();

	memset(fileName, 0, sizeof(fileName));

	fileCount=(int)DragQueryFile((HDROP)Msg.Drop,  0xffffffff, NULL, MAX_PATH);

	for(int i=0; i < fileCount; i++){
		DragQueryFile((HDROP)Msg.Drop, i, fileName, MAX_PATH);
	}

	DragFinish((HDROP)Msg.Drop);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::FormCreate(TObject *Sender)
{
	DragAcceptFiles(Handle, True); //ドラッグ&ドロップを有効に
}

v0.2

ヘッダーファイルとcppファイルを全部掲載。
OnCreate()の関連付けは必要になる。

Unit1.h

//---------------------------------------------------------------------------

# ifndef Unit1H
# define Unit1H
//---------------------------------------------------------------------------
# include <System.Classes.hpp>
# include <Vcl.Controls.hpp>
# include <Vcl.StdCtrls.hpp>
# include <Vcl.Forms.hpp>
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published:	// IDE で管理されるコンポーネント
	void __fastcall FormCreate(TObject *Sender);
private:	// ユーザー宣言
	void __fastcall TForm1::processFile(String filepath);
	void __fastcall ReceiveDropFiles(TWMDropFiles Msg);
	BEGIN_MESSAGE_MAP
	MESSAGE_HANDLER(WM_DROPFILES, TWMDropFiles, ReceiveDropFiles)
	END_MESSAGE_MAP(TForm)
public:		// ユーザー宣言
	__fastcall TForm1(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
# endif

Unit1.cpp

//---------------------------------------------------------------------------

# include <vcl.h>
# pragma hdrstop

# include "Unit1.h"
//---------------------------------------------------------------------------
# pragma package(smart_init)
# pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
	: TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TForm1::ReceiveDropFiles(TWMDropFiles Msg)
{
	int fileCount;
	wchar_t fileName[MAX_PATH + 10]; // 10: 余分に取る

	Application->BringToFront();

	memset(fileName, 0, sizeof(fileName));

	fileCount=(int)DragQueryFile((HDROP)Msg.Drop,  0xffffffff, NULL, MAX_PATH);

	for(int i=0; i < fileCount; i++){
		DragQueryFile((HDROP)Msg.Drop, i, fileName, MAX_PATH);
	}

	DragFinish((HDROP)Msg.Drop);

	processFile(String(fileName));
	int nop=1;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::FormCreate(TObject *Sender)
{
	DragAcceptFiles(Handle, True); //ドラッグ&ドロップを有効に

}
//---------------------------------------------------------------------------

void __fastcall TForm1::processFile(String filepath)
{
	// ここにファイルの処理を書く
	ShowMessage(filepath + L" is dropped");
}