Signed integer overflow behaviour

Question

When adding two positive Int32 values with a theoretical result greater than Int32.MaxValue can I count on the overflown value always being negative?

I mean to do this as a way to check for overflow without using a checked context and exception handling (like proposed here: http://sandbox.mc.edu/~bennet/cs110/tc/orules.html ), but is this behaviour garantueed?

From what I've read so far is signed integer overflow in C# defined behaviour (Is C#/.NET signed integer overflow behavior defined?) (in contrast to C/C++) and Int32 is two-complement so I would be thankful for someone with better understanding of this subject than me to verify this.

Update
Quote from link 1:

The rules for detecting overflow in a two's complement sum are simple:

If the sum of two positive numbers yields a negative result, the sum has overflowed.

If the sum of two negative numbers yields a positive result, the sum has overflowed.

Otherwise, the sum has not overflowed.

Dmitrii Bychenko · Accepted Answer · 2016-07-21 14:56:49Z

4

Rule #2 from

http://sandbox.mc.edu/~bennet/cs110/tc/orules.html

is incorrect

If the sum of two negative numbers yields a positive result, the sum has overflowed.

Counter example:

  int a = int.MinValue;
  int b = int.MinValue;

  unchecked {
    // 0
    Console.Write(a + b);
  }

However, the rule can be simply amended

If the sum of two negative numbers yields a non-negative result, the sum has overflowed.

As for Rule #1

If the sum of two positive numbers yields a negative result, the sum has overflowed.

it's correct one

edited Jul 21, 2016 at 14:56

answered Jul 21, 2016 at 14:31

Dmitrii Bychenko

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Comments

user743382 · Accepted Answer · 2016-07-21 14:58:57Z

1

No, you cannot.

You already talk about checked context, where you know that overflow causes an exception to be thrown. However, your seems to assume that the lack of a checked keyword indicates you're in unchecked context. That isn't necessarily the case. The default context when neither checked nor unchecked is specified is configurable, can be different in multiple projects that share the same source files, and can even be different between different configurations of the same project.

If you want integer overflow to wrap, be explicit, use the unchecked keyword.

answered Jul 21, 2016 at 14:58

user743382

2 Comments

ArgusMagnus Over a year ago

I know. While it might be worth pointing out to others, I think the way I worded the question implies that I mean to use this in a unchecked context. Otherwise I wouldn't get an overflown value but an OverflowException.

user743382 Over a year ago

@ArgusMagnus The way you worded the question implies to me that you mean to use this in the default implicit unchecked context, which is risky. The point of my answer is to make it an explicit unchecked context instead.

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