== vs. Object.Equals(object) in .NET

Question

So, when I was a comparative novice to the novice I am right now, I used to think that these two things were syntactic sugar for each other, i.e. that using one over the other was simply a personal preference. Over time, I'm come to find that these two are not the same thing, even in a default implementation (see this and this). To further confuse the matter, each can be overridden/overloaded separately to have completely different meanings.

Is this a good thing, what are the differences, and when/why should you use one over the other?

Answer 1

string x = "hello"; string y = String.Copy(x); string z = "hello"; 

To test if x points to the same object as y:

(object)x == (object)y  // false x.ReferenceEquals(y)    // false x.ReferenceEquals(z)    // true (because x and z are both constants they                         //       will point to the same location in memory) 

To test if x has the same string value as y:

x == y        // true x == z        // true x.Equals(y)   // true y == "hello"  // true 

Note that this is different to Java. In Java the == operator is not overloaded so a common mistake in Java is:

y == "hello"  // false (y is not the same object as "hello") 

For string comparison in Java you need to always use .equals()

y.equals("hello")  // true