No overflow exception for int in C#?

Question

I had this weird experience with problem number 10 on Project Euler (great site by the way). The assignment was to calculate the sum of all the prime numbers below two million.

I used an int for the sum, and my algorith produced an answer, but when i pasted it to verify the answer, it was wrong.

It turned out that the result was too big to fit in an int, but wouldn't this cause an overflow error or something? Instead, it just returned a value far off from the real answer.

When I changed the type to long, everything was hunky dory.

Answer 1

C# integer operations don’t throw exceptions upon overflow by default. You can achieve that via the project settings, or by making the calculation checked:

int result = checked(largeInt + otherLargeInt); 

Now the operation will throw.

The opposite is unchecked, which makes any operation explicitly unchecked. Obviously, this only makes sense when you’ve got checked operations enabled in the project settings.

Answer 2

In C# an OverflowException is not thrown (in VB the exception is thrown per default).

To get the excpetion you have to embed your code in a checked context:

byte value = 241; checked {     try      {         sbyte newValue = (sbyte) value;         Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.",              value.GetType().Name, value,              newValue.GetType().Name, newValue);     }     catch (OverflowException)      {         Console.WriteLine("Exception: {0} > {1}.", value, SByte.MaxValue);     } }        

MSDN explains in more detail:

For the arithmetic, casting, or conversion operation to throw an OverflowException, the operation must occur in a checked context. By default, arithmetic operations and overflows in Visual Basic are checked; in C#, they are not. If the operation occurs in an unchecked context, the result is truncated by discarding any high-order bits that do not fit into the destination type.