Use annotations to refine code inspection in C#

Code inspection and many other JetBrains Fleet features largely rely on knowing behavior of language constructs to detect issues, suggest possible improvements, and help you in other ways.

However, this kind of analysis cannot detect everything. For example, if a method is designed as a formatting method, no structural analysis will find a possible issue if a method invocation does not include necessary arguments.

In this and a lot of other cases, the JetBrains Fleet's JetBrains.Annotations is of a great help. By using attributes declared in this framework you can make JetBrains Fleet analyze code the way you need it. For example, you can annotate a method with the [StringFormatMethod] to indicate custom methods that work the same as System.String.Format():

[StringFormatMethod("message")]
void ShowError(string message, params object[] args) { /* do something */ }

void Foo(string failureDetails) {
  ShowError("Failed: {0}"); // Warning: Non-existing argument in format string
}
    

This being the simplest example, there are other helpful attributes with more complex use cases. You can find the full list of these attributes in the reference.

In most cases, code annotation attributes enable specific code inspections, for example:

StringFormatMethodAttribute helps detect misuses of string formatting methods.
CanBeNullAttribute and NotNullAttribute are associated with the 'Possible 'null' assignment to entity marked with 'Value cannot be null' attribute' inspection. For more information, refer to Value and nullability analysis in C#.
CannotApplyEqualityOperatorAttribute is bound to the 'Compare with '==' types marked by 'CannotApplyEqualityOperatorAttribute'' inspection
ContractAnnotationAttribute can be used to define contracts for your functions and turn on the corresponding inspections. For example, you can use [ContractAnnotation("input:null => false")] to notify the consumers of the function bool Foo(object input) that always returns false when parameter s is null.

Also, code annotations enable more quick-fixes, code completion suggestions, and code generation features on annotated items. The annotations are also required to create and use Source templates in C#.

JetBrains Fleet allows you to annotate code symbols in two ways:

You can annotate symbols in your source code as shown in the example above. In this case, you need to reference the JetBrains.Annotations namespace in your project. For more information, refer to Annotations in C# source code.
Even if you do not have access to sources, you can annotate symbols in compiled library code. For more information, refer to External annotations for C#.

Last modified: 11 February 2024