In C#, null is just one possible value of the regular type Foo. The compiler cannot tell whether you have started returning null where you previously only returned non-null values, nor can it tell whether you've handled those null values correctly at the consumption site.

In F#, None is a subtype of the Option<Foo> union type, which is different from the plain Foo type and from the other subtype Some Foo. If you change a function that used to return a plain Foo into one returning Option<Foo>, every place in your code where that function is consumed will throw a compile error until you edit it to unwrap and handle the distinct Some Foo and None subtypes.