Delphi DragDrop Component in Threads - multithreading

Question

i use this component for processing drag and drop files
http://melander.dk/delphi/dragdrop
unit DragThread;
interface
uses
Classes,DragDrop, DropTarget,DragDropFile,Dialogs,SysUtils;
type
TDragThread = class(TThread)
private
{ Private declarations }
ArraysLength : Integer;
DragComponent : TDropFileTarget;
DragArray,HashsArray : Array of string;
Procedure FDArray;
//Procedure FDHArray;
protected
procedure Execute; override;
Public
Constructor Create(Com: TDropFileTarget);
Destructor Destroy; Override;
end;
implementation
{ TDragThread }
Constructor TDragThread.Create(Com: TDropFileTarget);
begin
inherited Create(True);
DragComponent := Com;
end;
Destructor TDragThread.Destroy;
begin
//DragComponent.Free;
end;
Procedure TDragThread.FDArray;
var
A : Integer;
begin
SetLength(DragArray,DragComponent.Files.Count);
SetLength(HashsArray,DragComponent.Files.Count);
ShowMessage(IntToStr(DragComponent.Files.Count)); // just working in the first time !!
for A := 0 to DragComponent.Files.Count -1 do begin
DragArray[A] := DragComponent.Files[A];
//ShowMessage(DragComponent.Files[A]);
end;
ArraysLength := DragComponent.Files.Count-1;
//ShowMessage(DragComponent.Files[0]);
end;
procedure TDragThread.Execute;
begin
{ Place thread code here }
FDArray;
end;
end.
the strange thing that the Drop process working just one time then the DragComponent.Files.Count gives 0 for ever .!!
that's how i call it
procedure TForm1.DropFileDrop(Sender: TObject; ShiftState: TShiftState;
APoint: TPoint; var Effect: Integer);
var
DropThread : TDragThread;
begin
DropThread := TDragThread.Create(DropFile);
DropThread.Resume;
end;
i want to know why this happened and thanks in advance :) .

Answer 1

Don't operate VCL components from other threads.
There's no guarantee that the component's drop-event information will continue to be valid once the drop event has completed.
Copy all the information you need out of the component when you construct the thread (i.e., fully populate DragArray) and then use that cached data when executing the thread. Don't store a reference in DragComponent or you might be tempted to use it from the thread's Execute method, which you really shouldn't do.