File size calculation, Int64, and differences between 32bit and 64bit

Question

I had problems with the following code:

var
  FileSize : Int64;
...
FileSize := Info.nFileSizeLow or (Info.nFileSizeHigh shl 32);

I expecte

Answer 1

Actually, this is pretty well documented in Delphi 7's help file, under "Integer types":

In general, arithmetic operations on integers return a value of type Integer--which, in its current implementation, is equivalent to the 32-bit Longint. Operations return a value of type Int64 only when performed on one or more Int64 operand. Hence the following code produces incorrect results.

The code example provided:

var
  I: Integer;
  J: Int64;
  ...
I := High(Integer);
J := I + 1;

To get an Int64 return value in this situation, cast I as Int64:

 ...
J := Int64(I) + 1;