Why is this an issue?

When implementing operator overloads, it is very important to make sure that all related operators and methods are consistent in their implementation.

The following guidelines should be followed:

• When providing operator == you should also provide operator != and vice-versa.
• When providing operator == you should also provide Equals(Object) and GetHashCode().
• When providing operator +, -, *, / or % you should also provide operator ==, respecting previous guidelines.

This rule raises an issue when any of these guidelines are not followed on publicly-visible type (public, protected or protected internal).

Noncompliant code example

using System;

namespace MyLibrary
{
  public class Foo // Noncompliant
  {
    private int left;
    private int right;

    public Foo(int l, int r)
    {
      this.left = l;
      this.right = r;
    }

    public static Foo operator +(Foo a, Foo b)
    {
      return new Foo(a.left + b.left, a.right + b.right);
    }

    public static Foo operator -(Foo a, Foo b)
    {
      return new Foo(a.left - b.left, a.right - b.right);
    }
  }
}

Compliant solution

using System;

namespace MyLibrary
{
  public class Foo
  {
    private int left;
    private int right;

    public Foo(int l, int r)
    {
      this.left = l;
      this.right = r;
    }

    public static Foo operator +(Foo a, Foo b)
    {
      return new Foo(a.left + b.left, a.right + b.right);
    }

    public static Foo operator -(Foo a, Foo b)
    {
      return new Foo(a.left - b.left, a.right - b.right);
    }

    public static bool operator ==(Foo a, Foo b)
    {
      return (a.left == b.left && a.right == b.right);
    }

    public static bool operator !=(Foo a, Foo b)
    {
      return !(a == b);
    }

    public override bool Equals(Object obj)
    {
      Foo a = obj as Foo;
      if (a == null)
        return false;
      return this == a;
    }

    public override int GetHashCode()
    {
       return (this.left * 10) + this.right;
    }
  }
}