1. Equality Programming in C# Equality CSE 494R (proposed course for 459 Programming in C#) Prof. Roger Crawfis

2. Equality What does it mean for to variables to be equal: Numeric types – easy Strings – some caveats Employee records - ?

3. 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.

4. 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.

5. 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.

6. 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!

7. 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).

8. 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.

9. 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++.

10. 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 error x.Equals(y) => true y.Equals(x) => true Note: x is boxed!

11. 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).

12. 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!

13. The IEquatable interface Although the System.Object.Equals method can be applied to any type it will force a boxing of value types. The IEquatable 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 where T : IEquatable

14. Overriding Equality In general, do not change the default behavior (semantics). Implementing the default behavior for structs and the IComparable interface can avoid boxing and provide good performance improvements. Some immutable types may want different semantics: string, DateTime

15. 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.

16. 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.

17. 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.

18. 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.

19. 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 test = new Dictionary (); X t1 = new X(); test[t1] = 5; t1.x = 3; test[t1] = 4; Adds a new entry to the dictionary

20. 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 repeatable Equality is robust – no exceptions.

21. 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.

22. Example - Craps namespace OSU.Gambling.Craps { struct DiceRoll : IEquatable { 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

23. 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.

24. 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. }

25. 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); }

26. 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; }

27. 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); }

28. 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.

29. 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.

30. Example2 – Craps class public class DiceRoll : IEquatable { … public bool Equals(DiceRoll other) { if (other == null) return false; return die1 == other.die1 && die2 == other.die2 || die1 == other.die2 && die2 == other.die1; }

31. 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); } …

32. 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.

33. Equality Programming in C# Equality CSE 494R (proposed course for 459 Programming in C#) Prof. Roger Crawfis