Is there a complete IEquatable implementation reference?
C#.NetEqualsGethashcodeIequatableC# Problem Overview
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
C# Solutions
Solution 1 - C#
IEquatable<T>
for a Value Type
Implementing 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>
{
}
Solution 2 - C#
I believe getting something as simple as checking objects for equality correct is a bit tricky with .NET's design.
For Struct
-
Implement
IEquatable<T>
. It improves performance noticeably. -
Since you're having your own
Equals
now, overrideGetHashCode
, and to be consistent with various equality checking overrideobject.Equals
as well. -
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 withEquals
methods.public struct Entity : IEquatable
{ 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)
-
Override
Equals
andGetHashCode
. -
Implementing
IEquatable<T>
isn't a must, but will be nice if you have one.public class Entity : IEquatable
{ 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..
-
Override
Equals
andGetHashCode
. -
Overload
==
and!=
to matchEquals
. Make sure it works for nulls. -
Implementing
IEquatable<T>
isn't a must, but will be nice if you have one.public class Entity : IEquatable
{ 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.
Solution 3 - C#
Upon reading MSDN, I'm pretty certain the best example of a proper implementation is in the IEquatable
public override bool Equals(Object obj)
{
if (obj == null) return base.Equals(obj);
if (! (obj is Person))
return false; // Instead of throw new InvalidOperationException
else
return Equals(obj as Person);
}
For those wondering about the deviation, it derives from the Object.Equals(Object) MSDN page:
> Implementations of Equals must not throw exceptions.
Solution 4 - C#
I found another reference, it's the .NET Anonymous Type implementation. For an anonymous type with an int and a double as properties I disassembled the following C# code:
public class f__AnonymousType0
{
// Fields
public int A { get; }
public double B { get; }
// Methods
public override bool Equals(object value)
{
var type = value as f__AnonymousType0;
return (((type != null)
&& EqualityComparer<int>.Default.Equals(this.A, type.A))
&& EqualityComparer<double>.Default.Equals(this.B, type.B));
}
public override int GetHashCode()
{
int num = -1134271262;
num = (-1521134295 * num) + EqualityComparer<int>.Default.GetHashCode(this.A);
return ((-1521134295 * num) + EqualityComparer<double>.Default.GetHashCode(this.B);
}
public override string ToString()
{
StringBuilder builder = new StringBuilder();
builder.Append("{ A = ");
builder.Append(this.A);
builder.Append(", B = ");
builder.Append(this.B);
builder.Append(" }");
return builder.ToString();
}
}
Solution 5 - C#
I only have to derive from this class
public abstract class DataClass : IEquatable<DataClass>
{
public override bool Equals(object obj)
{
var other = obj as DataClass;
return this.Equals(other);
}
public bool Equals(DataClass other)
{
return (!ReferenceEquals(null, other))
&& this.Execute((self2, other2) =>
other2.Execute((other3, self3) => self3.Equals(other3), self2)
, other);
}
public override int GetHashCode()
{
return this.Execute(obj => obj.GetHashCode());
}
public override string ToString()
{
return this.Execute(obj => obj.ToString());
}
private TOutput Execute<TOutput>(Func<object, TOutput> function)
{
return this.Execute((obj, other) => function(obj), new object());
}
protected abstract TOutput Execute<TParameter, TOutput>(
Func<object, TParameter, TOutput> function,
TParameter other);
}
And then implement the abstract method like this
public class Complex : DataClass
{
public double Real { get; set; }
public double Imaginary { get; set; }
protected override TOutput Execute<TParameter, TOutput>(
Func<object, TParameter, TOutput> function,
TParameter other)
{
return function(new
{
Real = this.Real,
Imaginary = this.Imaginary,
}, other);
}
}