Why is this an issue?

When overloading some arithmetic 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 Equals(Object) and GetHashCode().
• When providing operator +, -, *, / or % you should also provide operator ==, respecting the previous guideline.

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

How to fix it

Make sure to implement all related operators.

Code examples

Noncompliant code example

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

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

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

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

    public override int GetHashCode()
    {
        return HashCode.Combine(right, left);
    }

    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);
    }
}

Resources

Documentation

• Microsoft Learn - Operator overloading
• Microsoft Learn - Equality operators
• Microsoft Learn - Arithmetic operators (C# reference)