CA1046: Do not overload operator equals on reference types

 

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The latest version of this topic can be found at CA1046: Do not overload operator equals on reference types.

TypeName|DoNotOverloadOperatorEqualsOnReferenceTypes|
|CheckId|CA1046|
|Category|Microsoft.Design|
|Breaking Change|Breaking|

A public or nested public reference type overloads the equality operator.

For reference types, the default implementation of the equality operator is almost always correct. By default, two references are equal only if they point to the same object.

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

It is safe to suppress a warning from this rule when the reference type behaves like a built-in value type. If it is meaningful to do addition or subtraction on instances of the type, it is probably correct to implement the equality operator and suppress the violation.

The following example demonstrates the default behavior when comparing two references.

using System;

namespace DesignLibrary
{
   public class MyReferenceType
   {
      private int a, b;
      public MyReferenceType (int a, int b)
      {
         this.a = a;
         this.b = b;
      }

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

The following application compares some references.

using System;

namespace DesignLibrary
{
    public class ReferenceTypeEquality
    {
       public static void Main()
       {
          MyReferenceType a = new MyReferenceType(2,2);
          MyReferenceType b = new MyReferenceType(2,2);
          MyReferenceType c = a;
         
          Console.WriteLine("a = new {0} and b = new {1} are equal? {2}", a,b, a.Equals(b)? "Yes":"No");
          Console.WriteLine("c and a are equal? {0}", c.Equals(a)? "Yes":"No");
          Console.WriteLine("b and a are == ? {0}", b == a ? "Yes":"No");
          Console.WriteLine("c and a are == ? {0}", c == a ? "Yes":"No");     
       }
    }
}

This example produces the following output.

a = new (2,2) and b = new (2,2) are equal? No
c and a are equal? Yes
b and a are == ? No
c and a are == ? Yes

CA1013: Overload operator equals on overloading add and subtract

Object.Equals
Equality Operators

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