Value and nullability analysis

Different ways of analyzing values

ReSharper analyzes values in two general ways:

• By understanding the behavior of language constructs.

  In the example below, based on the fact that the parameter obj was explicitly checked for nullability, ReSharper reasonably supposes that the value of obj can be indeed 'null' and displays the corresponding warning:

  

• By relying on code annotation attributes ([CanBeNull], [NotNull], [ItemCanBeNull], [ItemNotNull])

  In this example, the method Bar is marked with the [CanBeNull] attribute, Using this information, ReSharper warns you that the return value of Bar can be null, and calling a method on it could lead to a 'null' dereference:

  

Value and nullability analysis modes

ReSharper can analyze values in different modes:

Optimistic

By default, value analysis mode is optimistic. In this mode, ReSharper only warns about possible 'null' dereference if the value was explicitly checked for nullability, or if the symbol is marked with the [CanBeNull] or [ItemCanBeNull] attribute. The optimistic mode is demonstrated in the examples above.

Pessimistic

In this mode, ReSharper warns about possible 'null' dereference in all contexts where the value is not checked for nullability explicitly, unless the symbol is marked with the [NotNull] or [ItemNotNull] attribute.

To change value analysis mode or disable the analysis, configure the Value analysis mode preference on the Code Inspection | Settings page of ReSharper options (Alt+R, O).

Further examples of value analysis

Here are some more examples of ReSharper's value and nullability analysis:

• If a nullability check has been already done with a simple LINQ query, ReSharper tells you that a further nullability check is redundant:

  

• The same happens if you mark a collection with the [ItemNotNull] attribute:

  

• The next example illustrates how value analysis works when the pessimistic mode is enabled. The value of the list was checked for nullability, but not list items. Therefore ReSharper warns here about possible 'null' dereference.