NullableIsType.cs

using System;

int? x = 3;
if (x is int) 
    Console.WriteLine(x);
x = null;
if (x is null) 
    Console.WriteLine("x is null");

​NullableIsType.output-Explained.md

int? is a nullable type. Docs

Nullable types extend their base types to allow null values. For example, int? i = null is allowed, even though int i = null is not.

x is null is pattern matching. Docs

Pattern matching checks whether value matches a condition, and at the same time transforms it into new value(s). For example, a is int i checks that a is int, and if true produces a new value i equal to a, but typed as int.

​NullableIsType.output.txt

3
x is null

​​.sharplab.json

{
    "version": 1,
    "target": "Run",
    "mode": "Release",
    "branch": "core-x64"
}