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CA2231: Overload operator equals on overriding ValueType.Equals


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The latest version of this topic can be found at CA2231: Overload operator equals on overriding ValueType.Equals.

|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, 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));


CA1046: Do not overload operator equals on reference types

CA2225: Operator overloads have named alternates

CA2226: Operators should have symmetrical overloads

CA2224: Override equals on overloading operator equals

CA2218: Override GetHashCode on overriding Equals