Overloading operator== versus Equals()

Question

I\'m working on a C# project on which, until now, I\'ve used immutable objects and factories to ensure that objects of type Foo can always be compared for equal

Answer 1

There's a big difference between overloading == and overriding Equals.

When you have the expression

if (x == y) {

The method that will be used to compare variables x and y is decided at compile time. This is operator overloading. The type used when declaring x and y is used to define which method is used to compare them. The actual type within x and y (i.e., a subclass or interface implementation) is irrelevant. Consider the following.

object x = "hello";
object y = 'h' + "ello"; // ensure it's a different reference

if (x == y) { // evaluates to FALSE

and the following

string x = "hello";
string y = 'h' + "ello"; // ensure it's a different reference

if (x == y) { // evaluates to TRUE

This demonstrates that the type used to declare the variables x and y is used to determine which method is used to evaluate ==.

By comparison, Equals is determined at runtime based on the actual type within the variable x. Equals is a virtual method on Object that other types can, and do, override. Therefore the following two examples both evaluate to true.

object x = "hello";
object y = 'h' + "ello"; // ensure it's a different reference

if (x.Equals(y)) { // evaluates to TRUE

and the following

string x = "hello";
string y = 'h' + "ello"; // ensure it's a different reference

if (x.Equals(y)) { // also evaluates to TRUE