1. 16: Equals
Equals
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

2. Objectives Describe equality testing
16: Equals
Objectives
Describe equality testing
Show how to override Object Equals
error checking
casting
chaining to base class
comparing data
Present static helper method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

3. Equality methods in Object class
16: Equals
Equality methods in Object class
Object class provides two methods to test equality
instance version is primary method
static version is convenient helper
class Object {
public virtual bool Equals(object o)
public static bool Equals(object a, object b) ...
... }
equality
methods
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

4. Object Equals Object class instance Equals compares references
does not compare object data
performs identity test
Student a = new Student("Ann", 32, 8000);
Student b = new Student("Ann", 32, 8000);
if (a.Equals(b))
...
false, different objects
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

5. Override Can override instance Equals virtual method
typical motivation is when distinct objects might be equal
class Student {
public override bool Equals(object o)
...
} ... }
override Equals
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

6. Override details Several steps to override instance Equals
test for null
verify compared objects have same type
chain to base class
downcast parameter
compare data
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

7. Test for null Should test parameter against null return false if null
16: Equals
Test for null
Should test parameter against null
return false if null
not an error, do not throw exception
class Student {
public override bool Equals(object o)
if (o == null)
return false;
...
} ... }
null?
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

8. Type testing Should verify objects have same type
16: Equals
Type testing
Should verify objects have same type
typical to compare Type objects to require exactly same type
not common to use is operator
class Student {
public override bool Equals(object o)
...
if (this.GetType() != o.GetType())
return false;
} ... }
not same type?
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

9. Downcast and compare Must downcast parameter to access data
16: Equals
Downcast and compare
Must downcast parameter to access data
can choose to compare some or all data
class Student {
int id;
public override bool Equals(object o)
...
Student s = (Student)o;
return this.id == s.id;
} ... }
downcast
compare data
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

10. Inheritance Equals typically chains to base version if provided
allows base version to compare base data
return false if base parts not equal
class Person {
string name;
int age;
public override bool Equals(object o) { ... } ... }
base provides
Equals method
class Student : Person {
public override bool Equals(object o)
...
if (!base.Equals(o)) return false;
} ... }
derived version
chains to base
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

11. Do not chain to Object instance Equals
Do not chain to Object version of Equals
Object version performs identity test
returns true only if comparing the same objects
base class
is Object
class Person {
public override bool Equals(object o)
...
} ... }
Equals should
not chain to base
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

12. Complete implementation
16: Equals
Complete implementation
Complete code for reference
class Student : Person {
public override bool Equals(object o)
if (o == null) return false;
if (this.GetType() != o.GetType()) return false;
if (!base.Equals(o)) return false;
Student s = (Student)o;
return this.id == s.id;
} ... }
null?
not same type?
compare base
downcast
compare data
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

13. Static Equals Object provides convenient static version of Equals
main service is handling of null references
returns false if first argument is null
static Equals
public static bool Equals(object a, object b) {
if (a == b)
return true;
if (a == null || b == null)
return false;
return a.Equals(b); }
true if same object
or both are null
delegate to
instance Equals
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

14. GetHashCode and Equals
Equal objects must give same hash code
if type overrides Equals it must also override GetHashCode
Allows efficient containment test in hash tables
can hash tested object and search only that bucket
Student a = new Student("Ann", 32, 8000);
Student b = new Student("Ann", 32, 8000);
if (a.Equals(b))
...
if (a.GetHashCode() == b.GetHashCode())
if objects
are equal
hash codes must
be the same
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

15. Summary Object class provides framework for equality testing
16: Equals
Summary
Object class provides framework for equality testing
instance Equals
static Equals
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003