Is there a complete IEquatable implementation reference?

Question

Many of my questions here on SO concerns IEquatable implementation. I found it being extremely difficult to implement correctly, because there are many hidden bugs in the naïve implementation, and the articles I found about it are quite incomplete. I want to find or write a definitive reference which must include:

• How to implement IEquatable correctly
• How to override Equals correctly
• How to override GetHashCode correctly
• How to implement the ToString method correctly
• How to implement the operator == correctly
• How to implement the operator != correctly

Such a complete reference already exists?

PS: Even MSDN reference seems flawed to me

Answer 1

Implementing IEquatable<T> for a Value Type

Implementing IEquatable<T> for a value type is a little bit different than for a reference type. Let's assume we have the Implement-Your-Own-Value-Type archetype, a Complex number struct.

public struct Complex
{
    public double RealPart { get; set; }
    public double ImaginaryPart { get; set; }
}

Our first step would be to implement IEquatable<T> and override Object.Equals and Object.GetHashCode:

public bool Equals(Complex other)
{
    // Complex is a value type, thus we don't have to check for null
    // if (other == null) return false;

    return (this.RealPart == other.RealPart)
        && (this.ImaginaryPart == other.ImaginaryPart);
}

public override bool Equals(object other)
{
    // other could be a reference type, the is operator will return false if null
    if (other is Complex)
        return this.Equals((Complex)other);
    else
        return false;
}

public override int GetHashCode()
{
    return this.RealPart.GetHashCode() ^ this.ImaginaryPart.GetHashCode();
}

With very little effort we have a correct implementation, excepting the operators. Adding the operators is also a trivial process:

public static bool operator ==(Complex term1, Complex term2)
{
    return term1.Equals(term2);
}

public static bool operator !=(Complex term1, Complex term2)
{
    return !term1.Equals(term2);
}

An astute reader would notice that we should probably implement IEquatable<double> since Complex numbers could be interchangeable with the underlying value type.

public bool Equals(double otherReal)
{
    return (this.RealPart == otherReal) && (this.ImaginaryPart == 0.0);
}

public override bool Equals(object other)
{
    // other could be a reference type, thus we check for null
    if (other == null) return base.Equals(other);

    if (other is Complex)
    {
        return this.Equals((Complex)other);
    }
    else if (other is double)
    {
        return this.Equals((double)other);
    }
    else
    {
        return false;
    }
}

We need four operators if we add IEquatable<double>, because you can have Complex == double or double == Complex (and the same for operator !=):

public static bool operator ==(Complex term1, double term2)
{
    return term1.Equals(term2);
}

public static bool operator ==(double term1, Complex term2)
{
    return term2.Equals(term1);
}

public static bool operator !=(Complex term1, double term2)
{
    return !term1.Equals(term2);
}

public static bool operator !=(double term1, Complex term2)
{
    return !term2.Equals(term1);
}

So there you have it, with minimal effort we have a correct and useful implementation IEquatable<T> for a value type:

public struct Complex : IEquatable<Complex>, IEquatable<double>
{
}

Answer 2

I believe getting something as simple as checking objects for equality correct is a bit tricky with .NET's design.

For Struct

1) Implement IEquatable<T>. It improves performance noticeably.

2) Since you're having your own Equals now, override GetHashCode, and to be consistent with various equality checking override object.Equals as well.

3) Overloading == and != operators need not be religiously done since the compiler will warn if you unintentionally equate a struct with another with a == or !=, but its good to do so to be consistent with Equals methods.

public struct Entity : IEquatable<Entity>
{
    public bool Equals(Entity other)
    {
        throw new NotImplementedException("Your equality check here...");
    }

    public override bool Equals(object obj)
    {
        if (obj == null || !(obj is Entity))
            return false;

        return Equals((Entity)obj);
    }

    public static bool operator ==(Entity e1, Entity e2)
    {
        return e1.Equals(e2);
    }

    public static bool operator !=(Entity e1, Entity e2)
    {
        return !(e1 == e2);
    }

    public override int GetHashCode()
    {
        throw new NotImplementedException("Your lightweight hashing algorithm, consistent with Equals method, here...");
    }
}

For Class

From MS:

Most reference types should not overload the equality operator, even if they override Equals.

To me == feels like value equality, more like a syntactic sugar for Equals method. Writing a == b is much more intuitive than writing a.Equals(b). Rarely we'll need to check reference equality. In abstract levels dealing with logical representations of physical objects this is not something we would need to check. I think having different semantics for == and Equals can actually be confusing. I believe it should have been == for value equality and Equals for reference (or a better name like IsSameAs) equality in the first place. I would love to not take MS guideline seriously here, not just because it isn't natural to me, but also because overloading == doesn't do any major harm. That's unlike not overriding non-generic Equals or GetHashCode which can bite back, because framework doesn't use == anywhere but only if we ourself use it. The only real benefit I gain from not overloading == and != will be the consistency with design of the entire framework over which I have no control of. And that's indeed a big thing, so sadly I will stick to it.

With reference semantics (mutable objects)

1) Override Equals and GetHashCode.

2) Implementing IEquatable<T> isn't a must, but will be nice if you have one.

public class Entity : IEquatable<Entity>
{
    public bool Equals(Entity other)
    {
        if (ReferenceEquals(this, other))
            return true;

        if (ReferenceEquals(null, other))
            return false;

        //if your below implementation will involve objects of derived classes, then do a 
        //GetType == other.GetType comparison
        throw new NotImplementedException("Your equality check here...");
    }

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

    public override int GetHashCode()
    {
        throw new NotImplementedException("Your lightweight hashing algorithm, consistent with Equals method, here...");
    }
}

With value semantics (immutable objects)

This is the tricky part. Can get easily messed up if not taken care..

1) Override Equals and GetHashCode.

2) Overload == and != to match Equals. Make sure it works for nulls.

2) Implementing IEquatable<T> isn't a must, but will be nice if you have one.

public class Entity : IEquatable<Entity>
{
    public bool Equals(Entity other)
    {
        if (ReferenceEquals(this, other))
            return true;

        if (ReferenceEquals(null, other))
            return false;

        //if your below implementation will involve objects of derived classes, then do a 
        //GetType == other.GetType comparison
        throw new NotImplementedException("Your equality check here...");
    }

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

    public static bool operator ==(Entity e1, Entity e2)
    {
        if (ReferenceEquals(e1, null))
            return ReferenceEquals(e2, null);

        return e1.Equals(e2);
    }

    public static bool operator !=(Entity e1, Entity e2)
    {
        return !(e1 == e2);
    }

    public override int GetHashCode()
    {
        throw new NotImplementedException("Your lightweight hashing algorithm, consistent with Equals method, here...");
    }
}

Take special care to see how it should fare if your class can be inherited, in such cases you will have to determine if a base class object can be equal to a derived class object. Ideally, if no objects of derived class is used for equality checking, then a base class instance can be equal to a derived class instance and in such cases, there is no need to check Type equality in generic Equals of base class.

In general take care not to duplicate code. I could have made a generic abstract base class (IEqualizable<T> or so) as a template to allow re-use easier, but sadly in C# that stops me from deriving from additional classes.