When a class implements the IEquatable<T> interface, it enters a contract that, in effect, states "I know how to compare two
instances of type T or any type derived from T for equality.". However if that class is derived, it is very unlikely that the base class will know how
to make a meaningful comparison. Therefore that implicit contract is now broken.
Alternatively IEqualityComparer<T> provides a safer interface and is used by collections or Equals could be made
virtual.
This rule raises an issue when an unsealed, public or protected class implements IEquatable<T> and the
Equals is neither virtual nor abstract.
Noncompliant code example
using System;
namespace MyLibrary
{
public class Base : IEquatable<Base> // Noncompliant
{
public bool Equals(Base other)
{
if (other == null) { return false; }
// do comparison of base properties
return true;
}
public override bool Equals(object other) => Equals(other as Base);
}
class A : Base
{
public bool Equals(A other)
{
if (other == null) { return false; }
// do comparison of A properties
return base.Equals(other);
}
public override bool Equals(object other) => Equals(other as A);
}
class B : Base
{
public bool Equals(B other)
{
if (other == null) { return false; }
// do comparison of B properties
return base.Equals(other);
}
public override bool Equals(object other) => Equals(other as B);
}
internal class Program
{
static void Main(string[] args)
{
A a = new A();
B b = new B();
Console.WriteLine(a.Equals(b)); // This calls the WRONG equals. This causes Base.Equals(Base)
// to be called which only compares the properties in Base and ignores the fact that
// a and b are different types. In the working example A.Equals(Object) would have been
// called and Equals would return false because it correctly recognizes that a and b are
// different types. If a and b have the same base properties they will be returned as equal.
}
}
}
Compliant solution
using System;
namespace MyLibrary
{
public sealed class Foo : IEquatable<Foo>
{
public bool Equals(Foo other)
{
// Your code here
}
}
}
