c# - What is a difference between "foo is null" and "foo == null"

Question

Is there any difference between: foo is null and foo == null?

Answer 1

Short version: For well-behaved types, there is no difference between foo is null and foo == null.

Long version:

When you write foo == null and an appropriate overload of operator == exists, then that's what is called. Otherwise, reference equality is used for reference types and value equality is used for value types.

When you write foo is null for a reference type, this is compiled as if you wrote object.Equals(null, foo) (notice the switched order, it makes a difference). In effect, this performs reference equality comparison between foo and null. For a value type, foo is null does not compile.

This means that if you write a class with operator == that says that some instance of foo is equal to null, then foo == null will give different result than foo is null.

An example showing this:

using System;

public class Foo
{
    public static void Main()
    {
        var foo = new Foo();
        Console.WriteLine(foo == null);
        Console.WriteLine(foo is null);
    }

    public static bool operator ==(Foo foo1, Foo foo2) => true;
    // operator != has to exist to appease the compiler
    public static bool operator !=(Foo foo1, Foo foo2) => false;
}

This code outputs:

True
False

When you overload operator ==, you should make it behave in a reasonable way, which, among other things, means you should not say that foo == null is true for non-null foo. As a side effect of this, under normal circumstances, foo == null and foo is null will have the same value.