No overflow exception for int in C#?

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开发者_StackOverflow 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.

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.

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.

It's because, by default C# do not throw any exception for integer overflow as well as underflow. There are couple of things you can do here.

Option 1

You have to enable the exception to be thrown by go to Project => properties => Build tab => Advanced => check for arithmetic overflow underflow.(make sure you tick the option)

Make sure you tick the option

Option 2

Use a checked block and throw an overflow exception to handle the situation. A sample code snippet would be

        try
        {
            checked
            {
                int y = 1000000000;
                short x = (short)y;
            }
        }
        catch (OverflowException ex)
        {
            MessageBox.Show("Overflow");
        }
        catch (Exception ex)
        {
            MessageBox.Show("Error");
        }

I have already added a comment, but maybe it would be interesting for some of you:

msdn tells us:

Integer arithmetic overflow either throws an OverflowException or discards the most significant bits of the result

but

Decimal arithmetic overflow always throws an OverflowException.

also

When integer overflow occurs, what happens depends on the execution context, which can be checked or unchecked. In a checked context, an OverflowException is thrown. In an unchecked context, the most significant bits of the result are discarded and execution continues. Thus, C# gives you the choice of handling or ignoring overflow.

By default, C# does not check for arithmetic overflow on integers. You can change this with the /checked compiler option or by enabling "Check for arithmetic overflow/underflow" in Visual Studio (project properties - Build - Advanced).

You can use checked and unchecked keywords to override the default on a case-by-case basis. If you rely on checking taking place in a piece of code, explicitly enabling it using checked would be a good idea.

int j = checked(i * 2);

checked
{
    int j = i * 2;
    // Do more stuff
}

Note that floating point operations never throw an OverflowException, and decimal operations always throw an OverflowException. See also C# operators.

I have a mathematical operation that is truncated without generating error, even if the destination is a large variable:

Int count1 = (1000*10000) x 1024; //an exception is not raised

OR

Even if the target variable is large it is still truncated and throws no exception and I don't understand why count2 is treated as an int32 and not an int64

Int64 count2 = (1000*10000) x 1024;

Note: 10000 is a variable read from a configuration file

You can set checked directly in csproj by adding:

  <PropertyGroup>
    <CheckForOverflowUnderflow>true</CheckForOverflowUnderflow>
  </PropertyGroup>

It's the equivalent of ticking that Check for arithmetic overflow/underflow checkbox in the designer.