Unchecking Overflows

I recently ran into a new (for me) compiler error from the following line of code:

UInt16 section_length = (UInt16)(SizeOfSection - 3);

where SizeOfSection is a UInt32.  The error messages were as follows:

1. The operation overflows at compile time in checked mode
2. Constant value '4294967293' cannot be converted to a 'ushort' (use 'unchecked' syntax to override)

Strangely, even though these were marked as compilation errors, the code compiled and ran correctly.

Anyway, the fix was (as suggested) to use the unchecked syntax:

UInt16 section_length = unchecked((UInt16)(SizeOfSection - 3));

The bizarre thing about this, though, is that other code that casts UInt32 to UInt16 didn’t produce this error, so I would be interested to know what triggered it in this case…


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