Value and nullability analysis

JetBrains Rider performs value analysis to help you find possible 'null' dereferences as well as redundant boolean comparisons and null checks.

Different ways of analyzing values

JetBrains Rider can analyze values in several 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, JetBrains Rider 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]) when Nullable reference types are disabled.
In the following example, the method Bar is marked with the [CanBeNull] attribute, Using this information, JetBrains Rider warns you that the return value of Bar can be null, and calling a method on it could lead to a 'null' dereference:
If necessary, you can make code analysis even more precise with contract annotations. They help you define dependencies between arguments of a function and its return value.
In C# 8.0 and later, JetBrains Rider can reuse results of the compiler analysis if Nullable reference types are enabled in the project.

Value and nullability analysis modes

JetBrains Rider can analyze values in different modes:

Optimistic: By default, value analysis mode is optimistic. In this mode, JetBrains Rider 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, JetBrains Rider 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 Editor | Inspection Settings page of JetBrains Rider settings Control+Alt+S.

Nullable reference types

If nullable reference types (NRT) are enabled, JetBrains Rider reuses the results of C# compiler analysis.

NRT improve overall precision of the nullability analysis, but there are cases when NRT contracts can be violated, for example, when values are coming from code without #nullable context. In such cases you may get false positive warnings for nullability checks (for example, Expression is always 'true' or 'false') . In such cases, you can choose to ignore nullable API contracts and report a problem only when previous operations with the value in your code guarantee that it can or cannot be null.

You can change this behavior right from the Alt+Enter menu:

JetBrains Rider: Nullable reference types

... or using the Nullable reference types' warnings mode option on the Editor | Inspection Settings page of JetBrains Rider settings .

When JetBrains Rider ignores nullable API contracts, nullability analysis relies on the program control flow to report redundant null checks. For example:

var myString = ApiMethod();

if (myString is null)
    throw new ApplicationException("the string is null");

// warning 'Expression is always true'
// 'myString' cannot be null because it's already checked for null in our code
if (myString != null)
    Console.WriteLine(myString);

For more information about NRT and how JetBrains Rider supports them, watch this webinar recording:

Further examples of value analysis

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

If a nullability check has been already done with a simple LINQ query, JetBrains Rider 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 JetBrains Rider warns here about possible 'null' dereference.

Last modified: 01 August 2022