Null is not false, part one

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2024-02-13 08:00:11

The way you typically represent a “missing” or “invalid” value in C# is to use the “null” value of the type. Every reference type has a “null” value; that is, the reference that does not actually refer to anything. And every “normal” value type has a corresponding “nullable” value type which has a null value.

The way these concepts are implemented is completely different. A reference is typically implemented behind the scenes as a 32 or 64 bit number. As we’ve discussed previously, that number should logically be treated as an “opaque” handle that only the garbage collector knows about, but in practice that number is the offset into the virtual memory space of the process that the referred-to object lives at, inside the managed heap. The number zero is reserved as the representation of null because the operating system reserves the first few pages of virtual memory as invalid, always. There is no chance that by some accident, the zero address is going to be a valid address in the heap.

By contrast, a nullable value type is simply an instance of the value type plus a Boolean that indicates whether the value is to be treated as a value, or as null. It’s just a syntactic sugar for passing around a flag. This is because value types need not have any “special” value that has no other meaning; a byte has 256 possible values and every one of them is valid, so a nullable byte has to have some additional storage.

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