The Dangers of Magic Values

November 17, 2018

A regression recently appeared in a legacy codebase we’re maintaining: empty values were being saved to the database, which was causing downstream problems. There were in-code checks to prevent that from happening, but the checks weren’t being triggered – empty-value saves were erroneously being allowed to proceed.

After some investigation, we discovered that it was caused by a special magic value used to represent NULL, coupled with a recently-added rounding mechanism. Since 0 is a valid value for the quantity in question, the original developers of the code chose -9999.9999 to indicate a non-value. When the rounding mechanism was introduced, it failed to account for the precision of the magic value, so false negatives of the form “isNull = (-9999.99 != -9999.9999)” occurred.

This highlights the risk of overloading a scalar value to represent both value and state. Using a strongly-typed class object in place of the simple scalar would be a better approach. Better yet are .NET’s nullable types (e.g. “decimal? temperature”, “int? sensorId”). Nullable types have many added benefits, including built-in sematic clarity with APIs like “HasValue()” and support for NULL-comparisons.

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