CODEWITHSUNDEEP
c#genericsequality
n0rd
n0rd

Reputation: 12670

Generic equality implementation for derived classes

I want objects deriving certain class A to also somehow derive the implementation of the Equals(A other) that would do the following: if types of this and other are different, return false, otherwise return this.value == other.value.

My attempt looks like this:

public class A<T> : IEquatable<A<T>>
    where T: A<T>
{
    protected string Value { get; }
    public A(string value)
    {
        Value = value;
    }

    public bool Equals(A<T> other)
    {
        var concrete = other as T;
        if (concrete == null)
        {
            return false;
        }

        return concrete.Value == Value;
    }
}

public class B : A<B>
{
    public B(string value)
        : base(value)
    {

    }
}

public class C : A<C>
{
    public C(string value)
        : base(value)
    {

    }
}

class Program
{
    static void Main(string[] args)
    {
        var b1 = new B("val");
        var b2 = new B("val");
        var c = new C("val");

        Console.WriteLine(b1.Equals(b1));
        Console.WriteLine(b1.Equals(b2));
        Console.WriteLine(b2.Equals(b1));
        Console.WriteLine(b1.Equals(c));
        Console.WriteLine(b2.Equals(c));
        Console.WriteLine(c.Equals(b1));
        Console.WriteLine(c.Equals(b2));
    }
}

This works fine until we derive more:

public class D : C
{
    public D(string value)
        : base(value)
    {

    }
}

then it breaks:

        var d = new D("val");
        Console.WriteLine(d.Equals(c)); // prints "True"

and now I am stuck. How do I make it work? Both fixing the implementation to work with more than one level of inheritance and preventing more than one level of inheritance are acceptable.

I understand though that I just have to declare all first level descendants of A<T> as sealed, but that's the last resort unless it can be somehow enforced (so non-sealed descendants of A<T> would cause compilation error). Or maybe my approach is wrong completely?

Upvotes: 0

Views: 51

Answers (1)

Sweeper
Sweeper

Reputation: 273380

This is all because the as operator can convert subclasses to superclasses without problems.

What you want to do is to check the types and see whether they are equal:

if (this.GetType() == other.GetType()) {
    return false;
}

This question is kind of related, about the behaviours of GetType, typeof and is, which works similar to as.

Upvotes: 2

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