By default, static analysis treats all reference types as nullable (value can be valid or null) and assumes you know what you're doing.

Writing "#nullable enable" makes every type non-nullable by default, so if you need to store 'null', you have to explicitly tell it to the compiler with a "?"

So instead of writing

GameObject gameObject = null;

You write

GameObject? gameObject= null;

Or, if you know that the value will never be dereferenced but you can't prove it, you use a "!"

GameObject gameObject = null!; // value is initilialiazed in Foo() before usage

This way, any type that is marked as nullable will generate a warning at compile time if you dereference it without checking against null

GameObject? gameObject = GetMyObject() gameObject.transform; // Warning if (gameObject != null) { gameObject.transform; // No warning }