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c#overflow

Why this specific operation overflow (cf. CS0220) in C#?

I try to construct a big Int64 with nibble information stored in bytes.

The following lines of code work as expected:

Console.WriteLine("{0:X12}", (Int64)(0x0d * 0x100000000));
Console.WriteLine("{0:X12}", (Int64)(0x0d * 0x1000000));
Console.WriteLine("{0:X12}", (Int64)(0x0d * 0x100000));

Why does the following line lead to a compile error CS0220 "The operation overflows at compile time in checked mode" and the others do not?

Console.WriteLine("{0:X12}", (Int64)(0x0d * 0x10000000));

The result would be:

FFFFFFFFD0000000

instead of:

0000D0000000

Can anyone explain this? I will now convert with shift operators, but still curious why this approach does not work!

Update: The error also occurs when using (Int64)(0x0d << 28).

Solution

  • You need to mark the constant values specifically as longs (Int64s) or possibly ulongs (UInt64s), otherwise by default they will be interpreter as ints (i.e. Int32s), clearly causing overflows. Casting after the multiplication operation won't do you any good in this case.

    Haven't tested, but this code should work:

    Console.WriteLine("{0:X12}", 0x0dL * 0x100000000L);
Console.WriteLine("{0:X12}", 0x0dL * 0x1000000L);
Console.WriteLine("{0:X12}", 0x0dL * 0x100000L);