New rule proposal: Do not use nullable variables in arithmetic

[b-jeltes-tjip]

Nullable values in arithmetic result in a null value when any of the variables are null. A developer might expect a null int? to be considered as 0.

Example

Consider this code example, where the developer adds nullable integers a and b together, and store it in c. The developer expects a null value to be considered as "0".

Non-compliant code

int? a = null;
int? b = 3;

var c = a + b;

Developer's expectation: c == 3
Actual outcome: c == null

Compliant code

Compliant code would substitute the value within the arithmetic operation:

var c = (a ?? 0) + (b ?? 0);

Or use non-nullable variables like this:

int aSubstituted = a ?? 0;
int bSubstituted = b ?? 0;

var c = aSubstituted + bSubstituted;

Conclusion

The developer should be reminded to not use nullable variables in arithmetic, or to provide a non-null value as a substitute to prevent unexpected outcomes.