== Why is this an issue?

One of the possible ways of performing https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast[type-testing] is via the https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/is[is operator]: `food is Pizza`.

The `is` operator is often used before a direct https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast#cast-expression[cast] to the target type, as a more flexible and powerful alternative to the https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast#as-operator[as operator], especially when used to perform https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast#type-testing-with-pattern-matching[pattern matching].

[source,csharp]
----
if (food is Pizza pizza)
----

There's no valid reason to test `this` with `is`. The only plausible explanation for such a test is that you're executing code in a parent class conditionally based on the kind of child class `this` is.

[source,csharp]
----
public class Food
{
  public void DoSomething() 
  {
    if (this is Pizza) // Noncompliant 
    {
      // Code specific to Pizza...
    }
  }
}
----

However, code that's specific to a child class should be _in_ that child class, not in the parent.

== How to fix it

One way is to take advantage of the https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/tutorials/oop[object-orientation] of C# and use https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/object-oriented/polymorphism[polymorphism].

* Make the method https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/virtual[virtual], if it is not already. That will allow derive classes to perform https://en.wikipedia.org/wiki/Method_overriding[method overriding].
* Move the code at the right level of the type hierarchy.
* Use https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/base[base] to call the method on the base class that has been overridden.

When simple method polymorphism is not enough, for example because multiple sections of the parent method should be reused, the https://en.wikipedia.org/wiki/Template_method_pattern[Template method pattern] might help.

=== Code examples

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
public class Food
{
  public void DoSomething() 
  {
    // Code shared by all Food...
    if (this is Pizza) // Noncompliant 
    {
      // Code specific to Pizza...
    }
  }
}
----

==== Compliant solution

[source,csharp,diff-id=1,diff-type=compliant]
----
public class Food
{
  public virtual void DoSomething() 
  {
    // Code shared by all Food...
  }
}

public class Pizza : Food
{
  public override void DoSomething()
  {
    base.DoSomething();
    // Code specific to Pizza...
  }  
}
----

== Resources

=== Documentation

* https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast[Type-testing operators and cast expressions - is, as, typeof and casts]
* https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/pattern-matching[Pattern matching overview]
* https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/tutorials/oop[Object-Oriented programming (C#)]
* https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/object-oriented/polymorphism[Polymorphism]
* https://en.wikipedia.org/wiki/Template_method_pattern[Template method pattern]
* https://en.wikipedia.org/wiki/Method_overriding[Method overriding]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Offload the code that's conditional on this "is" test to the appropriate subclass and remove the test.


'''
== Comments And Links
(visible only on this page)

include::../comments-and-links.adoc[]
endif::env-github,rspecator-view[]
