Why is this an issue?

Making a base call when overriding a method is generally a good idea, but not in the case of GetHashCode and Equals for classes that directly extend Object. These methods are based on the object’s reference, meaning that no two objects that use those base methods can be equal or have the same hash.

Exceptions

This rule doesn’t report on guard conditions checking for reference equality. For example:

public override bool Equals(object obj)
{
  if (base.Equals(obj)) // Compliant, it's a guard condition.
  {
    return true;
  }
  ...
}

How to fix it

Code examples

Noncompliant code example

var m1 = new MyClass(2);
var m2 = new MyClass(2);

m1.Equals(m2) // False
m1.GetHashCode(); // 43942919
m2.GetHashCode(); // 59941935

class MyClass
{
    private readonly int x;
    public MyClass(int x) =>
        this.x = x;

    public override bool Equals(Object obj) =>
        base.Equals();

    public override int GetHashCode() =>
        x.GetHashCode() ^ base.GetHashCode(); // Noncompliant, base.GetHashCode returns a code based on the objects reference
}

Compliant solution

var m1 = new MyClass(2);
var m2 = new MyClass(2);

m1.Equals(m2) // True
m1.GetHashCode(); // 2
m2.GetHashCode(); // 2

class MyClass
{
    private readonly int x;
    public MyClass(int x) =>
        this.x = x;

    public override bool Equals(Object obj) =>
        this.x == ((MyClass)obj).x;

    public override int GetHashCode() =>
        x.GetHashCode()
}

Resources

Documentation