Question

I’m currently reading through C# 4.0 In A Nutshell, and in chapter 2 it states that if an int object has a value of int.MinValue (-2147483648) and the value is decreased, as the value cannot go any lower, it goes to the highest value, int.MaxValue (2147483647):

int a = int.MinValue;
Console.WriteLine(a); //-2147483648
a = a-1;
Console.WriteLine(a); //-2147483648
Console.WriteLine (a == int.MaxValue);  // True

The book then goes on to mention how you can protect from this happening using checked.

However, I’m wondering why this is possible? Surely this could cause a lot of issues if its not protected with checked?

Is it not a bit of a flaw in the design? Why would anyone want a minimum value to become a maximum value?

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Furthermore, I’ve noticed VB.NET will throw an ‘Arithmetic overflow’ instead of letting this happen:

Dim a As Integer
a = Integer.MaxValue
a = a + 1
Response.Write(a) 'Arithmetic overflow exception here

Answer 1

Sometimes it is desired behaviour. Other times, it simply isn’t necessary to deal with and check for overflows, as they slow down program execution. Take a game drawing unbound (FPS). At 1000FPS, a small 1ms extra check could have a significant affect on framerate. Other times, it may be expected execution (I’ve only seen one use of this to date) – it flows on from what happens in C#’s ancestor – C++ and C.

Also, please note that the default behaviour (checked or unchecked) varies between configurations and environments:

If neither checked nor unchecked is used, a constant expression uses
the default overflow checking at compile time, which is checked.
Otherwise, if the expression is non-constant, the run-time overflow
checking depends on other factors such as compiler options and
environment configuration.

Since the code you posted is non-constant (and also due to various environment variables), no checking for overflows is performed.