Override equals on overloading operator equals

Cause

A public type implements the equality operator, but does not override System.Object.Equals.

Rule Description

The equality operator is intended to be a syntactically convenient way of accessing the functionality of the Equals method. If you implement the equality operator, its logic must be identical to that of Equals.

The C# compiler issues a warning if your code violates this rule.

How to Fix Violations

To fix a violation of this rule, you should either remove the implementation of the equality operator, or override Equals and have the two methods return the same values. If the equality operator does not introduce inconsistent behavior, you can fix the violation by providing an implementation of Equals that calls the Equals method in the base class.

When to Exclude Warnings

It is safe to exclude a warning from this rule if the equality operator returns the same value as the inherited implementation of Equals. The Example section includes a type that could safely exclude a warning from this rule.

Example

The following example shows a type with inconsistent definitions of equality. BadPoint changes the meaning of equality by providing a custom implementation of the equality operator, but does not override Equals so that it behaves identically.

using System;

namespace UsageLibrary
{   
    public class BadPoint
    {
        private int x,y, id;
        private static int NextId;
        
        static BadPoint()
        {
            NextId = -1;
        }
        public BadPoint(int x, int y)
        {
            this.x = x;
            this.y = y;
            id = ++(BadPoint.NextId); 
        }
        
        public override string ToString()
        {
            return String.Format("([{0}] {1},{2})",id,x,y);
        }
        
        public int X {get {return x;}}
        
        public int Y {get {return x;}}
        public int Id {get {return id;}}
        
        public override int GetHashCode()
        {
            return id;
        }
        // Violates rule: OverrideEqualsOnOverridingOperatorEquals.
        
        // BadPoint redefines the equality operator to ignore the id value.
        // This is different from how the inherited implementation of 
        // System.Object.Equals behaves for value types. 
        // It is not safe to exclude the violation for this type. 
        public static bool operator== (BadPoint p1, BadPoint p2)
        {
            return ((p1.x == p2.x) && (p1.y == p2.y));
        }
        // The C# compiler and rule OperatorsShouldHaveSymmetricalOverloads require this.
        public static bool operator!= (BadPoint p1, BadPoint p2)
        {
            return !(p1 == p2);
        }
    }
}

The following code tests the behavior of BadPoint.

using System;

namespace UsageLibrary
{   
    public class TestBadPoint
    {
        public static void Main()
        {
            BadPoint a = new BadPoint(1,1);
            BadPoint b = new BadPoint(2,2);
            BadPoint a1 = a;
            BadPoint bcopy = new BadPoint(2,2);
            
            Console.WriteLine("a =  {0} and b = {1} are equal? {2}", a, b, a.Equals(b)? "Yes":"No");
            Console.WriteLine("a == b ? {0}", a == b ? "Yes":"No");
            Console.WriteLine("a1 and a are equal? {0}", a1.Equals(a)? "Yes":"No");
            Console.WriteLine("a1 == a ? {0}", a1 == a ? "Yes":"No");
            
            // This test demonstrates the inconsistent behavior of == and Object.Equals.
            Console.WriteLine("b and bcopy are equal ? {0}", bcopy.Equals(b)? "Yes":"No");
            Console.WriteLine("b == bcopy ? {0}", b == bcopy ? "Yes":"No");
        }
    }
}

This example produces the following output.

Output

a =  ([0] 1,1) and b = ([1] 2,2) are equal? No
a == b ? No
a1 and a are equal? Yes
a1 == a ? Yes
b and bcopy are equal ? No
b == bcopy ? Yes

The following example shows a type that technically violates this rule, but does not behave in an inconsistent manner.

using System;

namespace UsageLibrary
{
    public struct GoodPoint
    {
        private int x,y;
        
        public GoodPoint(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: OverrideEqualsOnOverridingOperatorEquals,
        // but does not change the meaning of equality;
        //  the violation can be excluded.
        
        public static bool operator== (GoodPoint px, GoodPoint py)
        {
            return px.Equals(py);
        }
        
        // The C# compiler and rule OperatorsShouldHaveSymmetricalOverloads require this.
        public static bool operator!= (GoodPoint px, GoodPoint py)
        {
            return !(px.Equals(py));
        }
    }
}

The following code tests the behavior of GoodPoint.

using System;

namespace UsageLibrary
{ 
    public class TestGoodPoint
    {
        public static void Main()
        {
            GoodPoint a = new GoodPoint(1,1);
            GoodPoint b = new GoodPoint(2,2);
            GoodPoint a1 = a;
            GoodPoint bcopy = new GoodPoint(2,2);
            
            Console.WriteLine("a =  {0} and b = {1} are equal? {2}", a, b, a.Equals(b)? "Yes":"No");
            Console.WriteLine("a == b ? {0}", a == b ? "Yes":"No");
            Console.WriteLine("a1 and a are equal? {0}", a1.Equals(a)? "Yes":"No");
            Console.WriteLine("a1 == a ? {0}", a1 == a ? "Yes":"No");
            
            // This test demonstrates the consistent behavior of == and Object.Equals.
            Console.WriteLine("b and bcopy are equal ? {0}", bcopy.Equals(b)? "Yes":"No");
            Console.WriteLine("b == bcopy ? {0}", b == bcopy ? "Yes":"No");
        }
    }
}

This example produces the following output.

Output

a =  (1,1) and b = (2,2) are equal? No
a == b ? No
a1 and a are equal? Yes
a1 == a ? Yes
b and bcopy are equal ? Yes
b == bcopy ? Yes

The following example shows a class (reference type) that violates this rule.

The following example fixes the above violation by overriding Object.Equals.

The following example shows a structure (value type) that violates this rule.

The following example fixes this violation by overriding ValueType.Equals.

Related Rules

Do not overload operator equals on reference types

Operator overloads have named alternates

Operators should have symmetrical overloads

Override GetHashCode on overriding Equals

Overload operator equals on overriding value type equals