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Code inspection: '??' condition is known to be null or not null

Last modified: 08 April 2024

If you want to assign a value, pass an argument, or return from a method based on the nullability of an identifier, the clearest syntax you can use in these cases is the ?? (null-coalescing) operator.

A null-coalescing expression works as follows. The left-hand operand is evaluated first, and if it is null, the right-hand operand is evaluated and the result becomes the result of the whole expression.

However, redundant null-coalescing expressions produce dead code and impede readability. As it follows from the logic of the ?? operator, using null as the right-hand operand does not make sense. Below, ReSharper suggests removing the right-hand operand null together with the ?? operator, because if the newCategory is null, then null will be assigned to the Category anyway:

Another situation when the null-coalescing operator is redundant is when the left-hand operand can never be null. In this case, the right-hand operand never gets reached and ReSharper suggests removing the unreachable code: