Overload operator equals on overriding ValueType.Equals

TypeName	OverloadOperatorEqualsOnOverridingValueTypeEquals
CheckId	CA2231
Category	Microsoft.Usage
Breaking Change	Non Breaking

Cause

A value type overrides Object.Equals but does not implement the equality operator.

Rule Description

In most programming languages there is no default implementation of the equality operator (==) for value types. If your programming language supports operator overloads, you should consider implementing the equality operator. Its behavior should be identical to that of Equals.

You cannot use the default equality operator in an overloaded implementation of the equality operator. Doing so will cause a stack overflow. To implement the equality operator, use the Object.Equals method in your implementation. For example:

If (Object.ReferenceEquals(left, Nothing)) Then
    Return Object.ReferenceEquals(right, Nothing)
Else
    Return left.Equals(right)
End If

if (Object.ReferenceEquals(left, null)) 
    return Object.ReferenceEquals(right, null);
return left.Equals(right);

How to Fix Violations

To fix a violation of this rule, implement the equality operator.

When to Suppress Warnings

It is safe to suppress a warning from this rule; however, we recommend that you provide the equality operator if possible.

Example

The following example defines a type that violates this rule.

using System;

namespace UsageLibrary
{
    public struct PointWithoutHash
    {
        private int x,y;

        public PointWithoutHash(int x, int y)
        {
            this.x = x;
            this.y = y;
        }

        public override string ToString()
        {
            return String.Format("({0},{1})",x,y);
        }

        public int X {get {return x;}}

        public int Y {get {return x;}}

        // Violates rule: OverrideGetHashCodeOnOverridingEquals. 
        // Violates rule: OverrideOperatorEqualsOnOverridingValueTypeEquals. 
        public override bool Equals (object obj)
        {
            if (obj.GetType() != typeof(PointWithoutHash))
                return false;

            PointWithoutHash p = (PointWithoutHash)obj;   
            return ((this.x == p.x) && (this.y == p.y));
        }

    }

}

Related Rules

Do not overload operator equals on reference types

Operator overloads have named alternates

Operators should have symmetrical overloads

Override equals on overloading operator equals

Override GetHashCode on overriding Equals

See Also

Reference

Object.Equals