unit uDragFromShell;
{ 单元名称：uDragFromShell
单元说明：让TwinControl控件能接收外来的文件拖放
其它声明}
interface

uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, ShellAPI;

type

TShellDragEvent = procedure(Sender: Tobject; Filename: string) of object;
TDragFromShell = class(Tcomponent)
private
FOwnerHandle: Thandle;
FOldAownerWindowProc: Pointer;
FOnShellDragDrop: TShellDragEvent;
procedure AownerWindowProc(var Message: TMessage);
procedure SetOnShellDragDrop(const Value: TShellDragEvent);
protected
procedure DoShellDragDrop(filename: string);
public
constructor Create(Aowner: Tcomponent); override;
destructor Destroy; override;
published
property OnShellDragDrop: TShellDragEvent read FOnShellDragDrop write SetOnShellDragDrop;
end;

procedure Register;

implementation

procedure Register;
begin
RegisterComponents('Arn:o)', [TDragFromShell]);
end;

{ TDragFromShell }

procedure TDragFromShell.AownerWindowProc(var Message: TMessage);
var
count, index, hdrop: integer;
Pfilename: pchar;
begin
if Message.MSG <> WM_DropFILES then
Message.Result := CallWindowProc(FOldAownerWindowProc, FOwnerHandle, Message.Msg, Message.WPARAM, Message.LPARAM) else
begin
Hdrop := message.WParam;
Getmem(pfilename, Max_path);
Count := DragQueryfile(hdrop, maxdword, Pfilename, max_path - 1);
for index := 0 to count - 1 do
begin
DragQueryfile(Hdrop, index, Pfilename, maxbyte);
DoShellDragDrop(StrPas(Pfilename));
end;
Freemem(Pfilename);
Dragfinish(Hdrop);
end;
end;

constructor TDragFromShell.Create(Aowner: Tcomponent);
begin
FOldAownerWindowProc := nil;
if not (Aowner is Twincontrol) then
begin
raise Exception.Create('The DragFromShell''s Owner must be a TWinControl');
end;
inherited;
FOwnerHandle := Twincontrol(Aowner).handle;
DragAcceptFiles(FOwnerHandle, true);
FOldAownerWindowProc := Pointer(GetWindowLong(FOwnerHandle, GWL_WNDPROC));
SetWindowLong(FOwnerHandle, GWL_WNDPROC, integer(MakeObjectInstance(AownerWindowProc)));
end;

destructor TDragFromShell.Destroy;
begin
DragAcceptFiles(FOwnerHandle, false);
SetWindowLong(FOwnerHandle, GWL_WNDPROC, Integer(FOldAownerWindowProc));
inherited;
end;

procedure TDragFromShell.DoShellDragDrop(filename: string);
begin
if Assigned(fonshellDragDrop) then
fonshellDragDrop(Self, Filename);
end;

procedure TDragFromShell.SetOnShellDragDrop(const Value: TShellDragEvent);
begin
FOnShellDragDrop := Value;
end;

end.

示例：这里给出了使用uDragFromShell的关键代码。

interface
uses uDragFromShell;

type
TDlgDragDrop = class(TForm)
...
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
private
DragFromShell:TDragFromShell;
procedure DropFiles(Sender: Tobject; Filename: string);
end;

implementation

procedure TDlgDragDrop.FormCreate(Sender: TObject);
begin
DragFromShell:=TDragFromShell.Create(self);
DragFromShell.OnShellDragDrop:=DropFiles;
end;

procedure TDlgDragDrop.FormDestroy(Sender: TObject);
begin
DragFromShell.Free;
end;

procedure TDlgDragDrop.DropFiles(Sender: Tobject; Filename: string);
begin
SendMessage(application.MainForm.Handle, FCP_FILEOPEN, 0, Integer(Filename));
//这里是拖放文件的具体处理代码
end;