چگونه می توان چندین سطر را با Shift در DBGrid انتخاب کرد؟

چگونه می توان چندین سطر را با Shift در DBGrid انتخاب کرد؟
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property BM1: TBookmark read FBM1 Write SetBM1;

property BM2: TBookmark read FBM2 Write SetBM2;

procedure Markieren(Sender: TObject);
function Shiftgedr: Boolean;


procedure TForm1.Markieren(Sender: TObject);
var
Richtung: string;
TempBM: TBookmark;
begin
with (Sender as TDBGRID).DataSource.Dataset do
begin
if (BOF and EOF) then
Exit;
DisableControls;
try
try
GotoBookmark(BM1);
case DBGrid1.DataSource.DataSet.CompareBookmarks(BM1, BM2) of
-1: Richtung := 'Unten';
1: Richtung := 'Oben';
0: Richtung := 'Gleich';
end;
TempBM := DBGrid1.DataSource.DataSet.GetBookmark;
while DBGrid1.DataSource.DataSet.CompareBookmarks(BM2, TempBM) <> 0 do
begin
DBGrid1.SelectedRows.CurrentRowSelected := True;
if Richtung = 'Unten' then
Next
else
Prior;
TempBM := DBGrid1.DataSource.DataSet.GetBookmark;
end;
finally
FreeBookmark(tempbm);
end;
finally
EnableControls;
end;
end;
end;

function TForm1.Shiftgedr: Boolean;
var
State: TKeyboardState;
begin
GetKeyboardState(State);
Result := ((State[VK_SHIFT] and 128) <> 0);
end;

begin
if not Shiftgedr then
Merke1 := nil;

if Merke1 = nil then
Merke1 := DBGrid1.DataSource.DataSet.GetBookmark
else
Merke2 := DBGrid1.DataSource.DataSet.GetBookmark;

if (Merke1 <> nil) and (Merke2 <> nil) then
begin
if Shiftgedr then
begin
Markieren(Sender);
end;
end;
end;