C# inline checked statement does not work

Question

I have two test methods. The first one works fine. The second one does not throw an exception, but it should. Why doesn't the second one throw a exception?

[TestMethod]
[ExpectedException(typeof(OverflowException))]
public void LongToInt_OverflowWithCheckedBlock()
{
    checked
    {
        int maxValue = 2147483647;
        long longValue = (maxValue + 1);
        int intValue = (int)longValue;
    }
}

[TestMethod]
[ExpectedException(typeof(OverflowException))]
public void LongToInt_OverflowWithCheckedStatement()
{

    int maxValue = 2147483647;
    long longValue = (maxValue + 1);
    int intValue = checked((int)longValue);     // No Exception is thrown, why?
}

Answer 1

The reason the first one throws and the second one doesn't is because your comparison is a bit off.

• In the 1st method you check everything
• In the 2nd method you only check the cast from long to int.

If you compare them like below they are equal and neither will throw:

private static void MethodA()
{
    int maxValue = 2147483647;
    long longValue = ( maxValue + 1 );
    checked
    {
        int intValue = ( int ) longValue;
    }
}

private static void MethodB()
{
    int maxValue = 2147483647;
    long longValue = ( maxValue + 1 );
    int intValue = checked( ( int ) longValue);
}

The reason for this is because this line: int intValue = checked(( int ) longValue); isn't the one that's throwing, its this line:

long longValue = ( maxValue + 1 );

If I put the checked there they will both throw:

long longValue = checked( maxValue + 1 );

Why it throws here:

It throws because ( maxValue + 1 ) is adding 1 to an int called maxValue (exception occurs here) and after that assigning it to a long, if you cast the int to a long before doing the adding 1 it will not throw:

long longValue = ( (long) maxValue + 1 );

Answer 2

As MSDN document said,

The checked keyword is used to explicitly enable overflow checking for integral-type arithmetic operations and conversions.

The first test method is already checking if there is an overflowing value within the declared block.

The second test method is only checking the overflowing value this line of code only.

int intValue = checked((int)longValue); //Checked expression

Since the overflowing is happen before the checked expression, the CLR already converted the value. And the value of that is -2147483648 which is legal since int range is

What is the difference?

Test Method 1 : Checked block

Test Method 2: Checked expression