Programming in C#Equality

CSE 494R(proposed course for 459 Programming in C#)

Prof. Roger Crawfis

Equality

What does it mean for to variables to be equal:Numeric types – easyStrings – some caveatsEmployee records - ?

Equality

Two basic types of equality: Value equality

Two variables are equal if they have the same value (mean the same thing).

Referential equality Two variables are equal if they refer to the same

instance (pointer or storage equality).

Difference: Are two 2008 Lamborghini Gallardo’s equal?

Theoretically, it depends on the context.

Default Equality

As expected, all value types have value-based equality.double a = 1.2;

double b = 1.2;

bool areEqual = a == b; // true.Strings also have value equality.

string name = “Crawfis”;

string instructor = “Crawfis”;

bool areEqual = a == b; // true.

Default Equality

What is the default equality for classes?In C++ there is none.In C# there is. It is referential equality.

class foo {…};

foo A = new foo();

foo B = new foo();

bool areEqual = A == B; // false.

object C = A; // C refers to the same instance as A.

bool areEqual = C == A; // true.

Default Equality

Subtleties:int[] a = {0,1,2,3};

int[] b = {0,1,2,3};

int[] a = {0,1,2,3};

int[] b = a;

int[] a = null;

int[] b = null;

a == b => false

a == b => true

a == b => true!

Default Equality

The default equality for structs is simply the pairwise comparison between each of its fields.

That is, two structs are equal if all of their fields are equal.Value-based field are compared by value.Reference fields are compared by reference

(by default).

Testing for Equality

There are actually five protocols or methods that you can use to test for equality.

They may provide different results!!!Witness one says they are the same.Witness two says they are different.

Testing for Equality

Operators for equality (== and !=).Recall that all operators are defined as

static methods for a type. If there are different types on each side of

the conditional, the compiler decides which type to use.

All types have these operators defined.Unlike C++.

Testing for Equality

The Object.Equals method exists for all types.

It is virtual and hence can be overridden.Determines at run-time which type’s

Equals method is called (Polymorphic).int x = 5;

object y = 5;x == y => compile time errorx.Equals(y) => truey.Equals(x) => true

Note: x is boxed!

Testing for Equality

Why have both of these? If a variable is null, calling Equals on it will

result in a run-time exception.There is overhead associated with virtual

function calls (and all function calls).The == operator can be statically inlined.Sometimes we want different behavior:

We both own the same car (equals) but mine is not yours (not equals).

Testing for Equality

There are also two static methods in the object class.static bool object.Equals(object o1, object o2); Simply calls o1.Equals if o1 != null.

static bool object.ReferenceEquals(object o1, object o2);

Since Equals can be overridden, this provides a forced reference equality check.

Note: object.ReferenceEquals(5,5) = false!

The IEquatable<T> interface

Although the System.Object.Equals method can be applied to any type it will force a boxing of value types.

The IEquatable<T> interface allows value types which implement the interface to be called using a.Equals(b) without the boxing.

Used as a generic constraint on user classes:class Test<T> where T : IEquatable<T>

Overriding Equality

In general, do not change the default behavior (semantics).

Implementing the default behavior for structs and the IComparable<T> interface can avoid boxing and provide good performance improvements.

Some immutable types may want different semantics: string, DateTime

Overriding Equality

But what if I have Employee records and I want to check if two instances are equal?

Do not build this into the Employee class.Use plug-in comparison classes to

dynamically specify the behavior you want.This is what is used in the collection classes.

Equality Semantics

If you override the semantics, then you must also override the hash code algorithm.

There are several rules that you should make sure you follow for each of ==, Equals, and GetHashCode.

If you override one of these you should probably override them all.

Overriding GetHashCode

Rules for GetHashCode:Consistency – it must return the same value if

called repeated on the same object.Even if you change the object!!!!Base it on an immutable value of the object.

Equality – it must return the same value on two objects for which Equals returns true.

Robust – it should not throw exceptions.Efficient – GetHashCode should generate a

random distribution among all inputs.

Overriding GetHashCode

For reference types, each instance has a unique hash-code based on the storage location.

Can add it to a Collection (Dictionary), change the contents, and still get it back out using the hash code.

If you override the hash code based on content rather than storage location, then the instance may need to be removed from the Dictionary, changed and added back.

Overriding GetHashCode

Structs and value types are different, since they are never changed in-place, they are copied out, modified and copied back in.

Hence this problem exists regardless of whether you override GetHashCode.

struct X { public int x; }

Dictionary<X, int> test = new Dictionary<X, int>();

X t1 = new X();

test[t1] = 5;

t1.x = 3;

test[t1] = 4;

Adds a new entry to the dictionary

Overriding Equals

Rules for overriding equals:An object should equal itself (reflexive).An object can not equal null

Since it is an instance method call.Unless it is a Nullable type.

Equality is commutative and transitive.a == b => b == a; b == c => a ==c;

Equality operations are repeatableEquality is robust – no exceptions.

Overriding == and !=

You should always override == for value types for efficiency.

You should never (or rarely) override it for reference types.

Rules for overriding ==a != b should be equal to !(a == b). It should have the same semantics as

Equals.Hence the previous rules apply.

Example - Craps

namespace OSU.Gambling.Craps

{

    struct DiceRoll : IEquatable<DiceRoll>

    {

        internal DiceRoll(int die1, int die2)

        {

            this.die1 = die1;

            this.die2 = die2;

        }

        private int die1, die2;

       

I’ve never played this

game.

Requires two dice.

I really want to control the

creation, so make this constructor

internal or private.

Alas structs always have a public default constructor

Example - Craps

        public int Die1 { get { return die1+1; } }

        public int Die2 { get { return die2+1; } }

Add one to allow for a default value of zero.

Error checking should be done to ensure that die1 and die2 lie between 0 and 5.

Example - Craps

        public bool Equals(DiceRoll other)

        {

            return die1 == other.die1 && die2 == other.die2

            || die1 == other.die2 && die2 == other.die1;

        }

        public override bool Equals(object other)

        {

            if (!(other is DiceRoll)) return false;

            return Equals((DiceRoll)other); // unbox the struct and compare.

        }

Example - Craps

        public static bool operator ==(DiceRoll die1, DiceRoll die2)

        {

            return die1.Equals(die2);

        }

        public static bool operator !=(DiceRoll die1, DiceRoll die2)

        {

            return !die1.Equals(die2);

        }

Example - Craps

        public override int GetHashCode()

        {

            return die1*11 + die2;

        }

        public override int GetHashCode()        {            if (die1 > die2)                return 11 * die1 + die2;            else                return 11 * die2 + die1;        }

Example - Craps

        public static DiceRoll RollDice()

        {

            DiceRoll roll = new DiceRoll();

            roll.die1 = random.Next(0,5);

            roll.die2 = random.Next(0,5);

            return roll;

        }

        private static readonly Random random;

        static DiceRoll()

        {

            random = new Random(System.DateTime.Now.Millisecond);

        }

    }

Design Principle

The previous code illustrated a good design principle you should follow:

Ensure that zero is a valid state for all value types.

Dice only have values from 1 to 6, so to make 0 a valid state we add one on the output.

Avoiding all of this

The previous example can be made trivial and avoid all of this by simply requiring that die1 always be greater than die2 in the implementation.Control the creation.

Works theoretically, but you may want the die to look more random for presentation purposes.

Example2 – Craps class

    public class DiceRoll : IEquatable<DiceRoll>

    {

…

        public bool Equals(DiceRoll other)

        {

            if (other == null)

                return false;

            return die1 == other.die1 && die2 == other.die2

            || die1 == other.die2 && die2 == other.die1;

        }

Example2 – Craps class

        public override bool Equals(object other)

        {

            if (other == null)

                return false;

            if (object.ReferenceEquals(this, other))

                return true;

            if (this.GetType() != other.GetType())

                return false;

            return Equals(other as DiceRoll);

        }

…

Example2 – Craps class

…

        public static DiceRoll RollDice()

        {

            return new DiceRoll(random.Next(0,5), random.Next(0,5));

        }

The only way to create a DiceRoll now.

Programming in C#Equality

CSE 494R(proposed course for 459 Programming in C#)

Prof. Roger Crawfis