Question

What’s up with this, anyway? I do a simple multiplication:

Int64 x = 11111111111;
Int64 y = 11111111111;
Int64 z = x * y;

And at the end of the multiplication, z shows a value of:

-5670418394979206991

This has clearly overflowed, but no exception is raised. I’d like one to be raised, but…

Note that this is on Windows Phone 7, but I don’t think this has any bearing on the issue. Or does it?

Answer 1

You can use a checked block as pointed out already by other answers:

checked
{
    long x = 11111111111;
    long y = 11111111111;
    long z = checked(x * y);
    // ...
}

Which results in an exception:

OverflowException: Arithmetic operation resulted in an overflow.

If you are just doing a single operation where you need checking you can use a checked expression instead:

long x = 11111111111;
long y = 11111111111;
long z = checked(x * y);

You can also set the /checked compiler option to have the default be checked.

To set this compiler option in the Visual Studio development environment:

1. Open the project’s Property Pages dialog box. For details, see Setting Visual C# Project Properties.
2. Click the Configuration Properties folder.
3. Click the Build property page.
4. Modify the Check for Arithmetic Overflow/Underflow property.

If you change the default to checked you can use unchecked blocks or expressions to get the unchecked behaviour again.