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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

Overload operator equals on overriding ValueType.Equals

TypeName

OverloadOperatorEqualsOnOverridingValueTypeEquals

CheckId

CA2231

Category

Microsoft.Usage

Breaking Change

Non Breaking

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

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);

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

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

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));
        }

    }

}

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