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s := D:\Test\Test1\Test3\Daten.xls					

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ExtractFilename()					

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var
  FileName: String;
  PathAndFile: String;
begin
  {...}

  PathAndFile := 'IrgendeinText';
  FileName := ExtractFileName(PathAndFile);
  // => FileName = 'IrgendeinText'

  PathAndFile := '';
  FileName := ExtractFileName(PathAndFile);
  // => FileName = ''

  {...}
end;					

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procedure TForm1.Button1Click(Sender: TObject);
var
 s1:String;
begin
OpenDialog1.Execute;
s1:=ExtractFilename(openDialog1.FileName);
Edit1.text:=s1;
end;					

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procedure TForm1.Button1Click(Sender: TObject);
begin
  if OpenDialog1.Execute then
    Edit1.text := ExtractFilename(openDialog1.FileName);
end;