When should I use == and when should I use Equals?

The Equals method is just a virtual one defined in System.Object, and overridden
by whichever classes choose to do so. The == operator is an operator which can be overloaded
by classes, but which usually has identity behaviour.

For reference types where == has not been overloaded, it compares whether two references
refer to the same object - which is exactly what the implementation of Equals does in

Value types do not provide an overload for == by default.
However, most of the value types provided by the framework provide their
own overload. The default implementation of Equals for a value
type is provided by ValueType, and uses reflection
to make the comparison, which makes it significantly slower than a
type-specific implementation normally would be. This implementation also
calls Equals on pairs of references within the two values
being compared.

However, the main difference between the two types of comparison in
normal use (where you're unlikely to be defining your own value types
very often) is polymorphism. Operators are overloaded, not overridden,
which means that unless the compiler knows to call the more specific
version, it'll just call the identity version. To illustrate that, here's
an example:

using System;

public class Test
 static void Main()
        // Create two equal but distinct strings
        string a = new string(new char[] {'h', 'e', 'l', 'l', 'o'});
        string b = new string(new char[] {'h', 'e', 'l', 'l', 'o'});
        Console.WriteLine (a==b);
        Console.WriteLine (a.Equals(b));
        // Now let's see what happens with the same tests but
        // with variables of type object
        object c = a;
        object d = b;
        Console.WriteLine (c==d);
        Console.WriteLine (c.Equals(d));

The results are:


The third line is False because the compiler can only call the non-overloaded version of ==
as it doesn't know that the contents of c and d are both string references. As they
are references to different strings, the identity operator returns false.

So, when should you use which operator? My rule of thumb is that for almost all reference types, use Equals
when you want to test equality rather than reference identity. The exception is for strings - comparing strings with
== does make things an awful lot simpler and more readable but you need to remember that both
sides of the operator must be expressions of type string in order to get the comparison to work properly.

For value types, I'd normally use == for easier-to-read code. Things would get tricky if a value type
provided an overload for == which acted differently to Equals, but I'd consider such a type
very badly designed to start with.

[Author: Jon Skeet]