CodeQL documentation

Dubious type test of ‘this’

ID: cs/type-test-of-this
Kind: problem
Severity: warning
Precision: high
   - testability
   - maintainability
   - language-features
Query suites:
   - csharp-security-and-quality.qls

Click to see the query in the CodeQL repository

If a type Derived inherits (either directly or indirectly) from a type Base , then it has a dependency on Base by virtue of this inheritance relationship - that is, it cannot be used without Base also being present. If, in addition to making Derived inherit from Base, you also write code that depends on Derived within Base, you cause Base to depend on Derived as well, resulting in a dependency cycle between the two types. Dependency cycles are a well-known design smell, in that they make code both difficult to read and difficult to test.

In the situation just described, the dependency cycle has been introduced by writing code that checks whether or not this is an instance of a derived type. This is a very unwise thing to do - a type should never know about its specific descendants, even though it may of course choose to impose some constraints on them as a group (such as the need for every derived type to implement a method with a specific signature).


The appropriate solution to this problem is to redesign the base and derived types so that there is no longer a need for the base type to depend on the types that derive from it.


In this example BaseClass attempts to check instances of itself to see which subclass they are and then performs different actions depending on the result. This is very bad practice.

class DubiousTypeTestOfThis
    class BaseClass
        public int add(int x)
            if (this is FiveAdder)
                return x + 5;

            if (this is TenAdder)
                return x + 10;

            return 0;

    class FiveAdder : BaseClass
        // ...

    class TenAdder : BaseClass
        // ...