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var
  IntVar: Integer;
begin
  IntVar := IntToStr(sl2[0]);					

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IntVar := IntVar +1;
// Alternative:
Inc(IntVar);					

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ShowMessage(IntToStr(IntVar));					

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if not FileExists('C:\datei.txt') then
sl:=TStringList.Create;
try
sl.Add(IntToStr(Neu));
sl.SaveToFile('C:\datei.txt');
finally
sl.free;
end;

sl2:=TStringList.Create;
try
sl2.LoadFromFile('C:\datei.txt');
Id := StrToInt(sl2[0]);
inc (Id);
ShowMessage(IntToStr(Id));
finally
sl2.free
end;

sl:=TStringList.Create;
try
sl.Add(IntToStr(Id));
sl.SaveToFile('C:\datei.txt');
finally
sl.free;
end;