#59 – Using Unchecked Keyword to Avoid Overflow Exceptions

If the default project setting is to check for arithmetic overflow/underflow, you can temporarily avoid checking using the unchecked keyword.  This reverts to the default behavior of wrapping the result value.

Assume that we’ve changed the project setting so that we do check for arithmetic overflow.  We can then avoid an exception by using the unchecked keyword on an expression:

 int n1 = int.MaxValue;   // 2147483647 (0x7FFFFFFF)
 int n2 = unchecked(n1 + 1);  // Wraps: -2147483648
 int n3 = n1 + 1;             // Throws OverflowException--default project setting

We can also use the unchecked keyword on an entire statement block:

 int n1 = int.MaxValue;   // 2147483647 (0x7FFFFFFF)
     int n2 = n1 + 1;  // Wraps: -2147483648
     int n4 = n1 * 2;  // Wraps: -2

You can also use unchecked to avoid overflow checking on constant expressions at compile-time.  This results in no compile-time error and no overflow exception at run-time.

 int n1 = int.MaxValue + 1;  // Compile-time error: overflow
 int n2 = unchecked(int.MaxValue + 1);

#58 – Generate Exceptions on Integer Overflow Using Checked Operator

By default, when an overflow occurs as a result of an arithmetic operation on an integer type, the result wraps.

 int n1 = int.MaxValue;      // 2147483647 (0x7FFFFFFF)
 n1 = n1 + 1;                // Now -2147483648 (wrapped)

You can instead cause an exception to be thrown whenever an integer overflow condition occurs. To do this, use the checked keyword on the expression:

 int n1 = int.MaxValue;      // 2147483647 (0x7FFFFFFF)
 n1 = checked(n1 + 1);       // Throws OverflowException

You can also use the checked keyword on the statement block that contains the expression:

     int n1 = int.MaxValue;   // 2147483647 (0x7FFFFFFF)
     n1 = n1 + 1;             // Throws OverflowException

You can also make a change to your project so that the default behavior is to throw the exception, even without the checked keyword.  Go to Project Properties, click on Build tab, then click on the Advanced button at the bottom of the window.  This brings up the Advanced Build Settings dialog.