How can I improve this code: Inheritance and IEquatable<>

This is an example about what I´m trying to do:

public class Foo : IEquatable<Foo>
{
    public bool Equals(Foo other)
    {
        Type type1 = this.GetType();
        Type type2 = other.GetType();

        if (type1 != type2)
            return false;

        if (type1 == typeof(A))
        {
            A a = (A)thi开发者_如何学Cs;
            A b = (A)other;

            return a.Equals(b);
        }
        else if (type1 == typeof(B))
        {
            B c = (B)this;
            B d = (B)other;

            return c.Equals(d);
        }
        else
        {
            throw new Exception("Something is wrong");
        }
    }
}

public class A : Foo, IEquatable<A>
{
    public int Number1 { get; set; }
    public int Number2 { get; set; }

    public bool Equals(A other)
    {
        return this.Number1 == other.Number1 && this.Number2 == other.Number2;
    }
}

public class B : Foo, IEquatable<B>
{
    public int Number1 { get; set; }
    public int Number2 { get; set; }
    public int Number3 { get; set; }

    public bool Equals(B other)
    {
        return this.Number1 == other.Number1 && this.Number2 == other.Number2 && this.Number3 == other.Number3;
    }
}

But as you can see above, I'd have to use many conditionals 'if' to identify the real type. The problem is I have to use the base class. For example:

A a = new A();
Foo foo = a;

foo.Equals(another);

As a direct answer your question, you appear to implement IEquatable<Foo> by always deferring to the (concrete) sub-class's IEquatable<self> implementation. This would look something like:

(Bad code, for demonstration only)

// You need to specify what you want when this method is called on a 
// vanilla Foo object. I assume here that Foo is abstract. If not, please
// specify desired behaviour.
public bool Equals(Foo other)
{
    if (other == null || other.GetType() != GetType())
        return false;

    // You can cache this MethodInfo..
    var equalsMethod = typeof(IEquatable<>).MakeGenericType(GetType())
                                           .GetMethod("Equals");

    return (bool)equalsMethod.Invoke(this, new object[] { other });
}

But it really isn't clear why you need the equality comparisons to always go "through" the base-class's IEquatable<self> implementation.

The framework already has the virtual Equals method that will result in dispatching equality-calls to the appropriate method. In addition, EqualityComparar<T>.Default (which is used by most collection-types for making equality checks) already has the smarts to choose IEquatable<self>.Equals(self) or object.Equals(object)as appropriate.

Trying to create an implementation of equality in the base-class that just forwards the request is adding no value to anything, as far as I can see.

Without further explanation on why you need the base-class IEquatable<> implementation, I recommend just implementing equality properly on each type. For example:

public class A : Foo, IEquatable<A>
{
    public int Number1 { get; set; }
    public int Number2 { get; set; }

    public bool Equals(A other)
    {
        return other != null 
            && Number1 == other.Number1
            && Number2 == other.Number2;
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as A);
    }

    public override int GetHashCode()
    {
        return Number1 ^ Number2;
    }
}

Try this piece of code:

public class Foo : IEquatable<Foo>
{
    public virtual bool Equals(Foo other)
    {
        return true;
    }
}

public class A : Foo,IEquatable<A>
{
    public int Number1 { get; set; }
    public int Number2 { get; set; }

    public override bool Equals(Foo other)
    {
        if (other.GetType() == typeof(A))
        {
            return Equals((A)other);                
        }
        throw new InvalidOperationException("Object is not of type A");
    }
    public bool Equals(A other)
    {
        return this.Number1 == other.Number1 && this.Number2 == other.Number2;
    }
}

public class B : Foo,IEquatable<B>
{
    public int Number1 { get; set; }
    public int Number2 { get; set; }
    public int Number3 { get; set; }

    public override bool Equals(Foo other)
    {
        if (other.GetType() == typeof(B))
        {
            return Equals((B)other);

        }
        throw new InvalidOperationException("Object is not of type B");
    }
    public bool Equals(B other)
    {
        return this.Number1 == other.Number1 && this.Number2 == other.Number2 && this.Number3 == other.Number3;
    }
}

Note : You can use Assert functionality to do typechecking.

One option is to move the Number1 and Number2 properties to the base class, and only compare the member added to the subclass in the subclasses' equality methods.

class Foo
{
    // move the common properties to the base class
    public int Number1 { get; set; }
    public int Number2 { get; set; }

    public override bool Equals(object obj)
    {
        Foo objfoo = obj as Foo;
        return 
            objfoo != null
            // require objects being compared to be of
            // the same derived type (optionally)
            && this.GetType() == obj.GetType()
            && objfoo.Number1 == this.Number1
            && objfoo.Number2 == this.Number2;
    }
    public override int GetHashCode()
    {
        // xor the hash codes of the elements used to evaluate
        // equality
        return Number1.GetHashCode() ^ Number2.GetHashCode();
    }
}

class A : Foo, IEquatable<A>
{
    // A has no properties Foo does not.  Simply implement
    // IEquatable<A>

    public bool Equals(A other)
    {
        return this.Equals(other);
    }

    // can optionally override Equals(object) and GetHashCode()
    // to call base methods here
}

class B : Foo, IEquatable<B>
{
    // Add property Number3 to B
    public int Number3 { get; set; }
    public bool Equals(B other)
    {
        // base.Equals(other) evaluates Number1 and Number2
        return base.Equals(other)
            && this.Number3 == other.Number3;
    }
    public override int GetHashCode()
    {
        // include Number3 in the hashcode, since it is used
        // to evaluate equality
        return base.GetHashCode() ^ Number3.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        return this.Equals(obj as B);
    }
}

I think that derived classes should not be handled in base classes. Usually, "Foo" will know nothing about A and B.

It's still possible to make the base IEquatable implementation virtual, allowing A and B to override it and perform their specific equality checks, even if both equality checking and checked instance are available only as "Foo" or "Object".

That would treat .Equals(Foo obj) like a more specific form of Object.Equals(Object obj).