Substraction of two ulong giving wrong output

Question

DateTime epochStart = new DateTime(1970, 01, 01, 0, 0, 0, 0, DateTimeKind.Utc);
TimeSpan currentTs = DateTime.UtcNow - epochStart;
ulong serverTotalSeconds = Convert.ToUInt64(currentTs.TotalSeconds);
ulong requestTotalSeconds = Convert.ToUInt64(requestTimeStamp);
ulong r = (serverTotalSeconds - requestTotalSeconds);
if (r > requestMaxAgeInSeconds)
{
     return true;
}

The above c# code sometimes giving the wrong subtracted value.

For example, for following values

serverTotalSeconds = 1615184795
requestTotalSeconds = 1615184796

"r" is returning value 18446744073709551615

I am not able to understand the cause of the issue. Can somebody point out what exactly is wrong here?

Why not use `DateTimeOffset.UtcNow.ToUnixTimeSeconds` to get the Unix epoch timestamp? — ProgrammingLlama, Mar 08 '21 at 06:38
[Does this answer your question?](https://stackoverflow.com/questions/45270715/c-sharp-ulong-overflow-underflow-somehow-allowed) — ProgrammingLlama, Mar 08 '21 at 06:39
What value do you expect `r` to hold as a result of that calculation? — ProgrammingLlama, Mar 08 '21 at 06:40
Using an `unsigned long` and then going negative leads to this behaviour. It would be the same, if you take `(ulong)0 - (ulong)1`. If a result can be negative you have to use a signed type (e.g. `long`). — Oliver, Mar 08 '21 at 06:43

score 2 · Accepted Answer · answered Mar 08 '21 at 06:46

The answer is expected. the difference is -1. which can be represented as in 8 bytes as 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111 in binary and this is equivalent to 18446744073709551615 in decimal.

If you use long in place of unsigned long. it will give you the expected result if you are expecting the different to be negative also.

Substraction of two ulong giving wrong output

1 Answers1