Pattern matching equal null vs is null

Question

From Microsoft new-features-in-c-7-0:

public void PrintStars(object o)
{
    if (o is null) return;     // constant pattern "null"
    if (!(o is int i)) return; // type pattern "int i"
    WriteLine(new string('*', i));
}

Whats the diferrence of o == null and o is null?

Answer 1

The o is null is translated to object.Equals(null, o) (you can see it here).

The object.Equals code is written as:

public static bool Equals(Object objA, Object objB)
{
    if (objA == objB)
    {
        return true;
    }
    if (objA == null || objB == null)
    {
        return false;
    }
    return objA.Equals(objB);
}

so in the end there will be a o == null (the first if). Note that System.Object doesn't define the operator==, so the one used is the one for reference types that is reference equality.

Theorically, by watching the called code, one could think that o == null (with o a System.Object) should be faster than o is null (less operations)... But who knows? :-)

The end result is that, through two different routes, o is null and o == null (with o a System.Object) return the same result.

By looking we can even see that o == null is the same as object.ReferenceEquals(o, null) (with o a System.Object) :-).

the interesting question should be, why doesn't the C# compiler translates the x is null to object.ReferenceEquals(x, null)?. Note that, thanks to how the boxing of nullable types is done, it would work even for:

int? a = null;
if (a is null) { /* */ }

changes to the compiler made this response invalid... If you click on the "here" link you can see it

Answer 2

Since the answer of @xanatos is outdated (but that is only mentioned at the very end) I'm writing a new one, because I wanted to know this as well and researched it.

In short: if you don't overload the == operator, then o == null and o is null are the same.
If you do overload the == operator, then o == null will call that, but o is null won't.

o is null always does the same as ReferenceEquals(o, null), i.e. it only checks if the value is null, it doesn't call any operators or Equals methods.

Longer answer: here is a SharpLab sample that showcases the various ways to check for null.

If you view the result in IL form you see that:

• is null and ReferenceEquals result in the same code
• o == null will call the overloaded operator==
• object.Eqauls(o, null) calls that method
• if you comment the operator== in class C you will see that o == null now produces the same code as o is null