S4201 — Null checks should not be combined with "is" operator checks

Why is this an issue?

There’s no need to null test in conjunction with an is test. null is not an instance of anything, so a null check is redundant.

Noncompliant code example

if (x != null && x is MyClass) { ... }  // Noncompliant

if (x == null || !(x is MyClass)) { ... } // Noncompliant

Compliant solution

if (x is MyClass) { ... }

if (!(x is MyClass)) { ... }