What is the default value of the nullable type “int?” (including question mark)?

Question

The default value for int? — and for any nullable type that uses the “type?” declaration — is null.

Why this is the case:

int? is syntactic sugar for the type Nullable<T> (where T is int), a struct. (reference)
The Nullable<T> type has a bool HasValue member, which when false, makes the Nullable<T> instance “act like” a null value. In particular, the Nullable<T>.Equals method is overridden to return true when a Nullable<T> with HasValue == false is compared with an actual null value.
From the C# Language Specification 11.3.4, a struct instance’s initial default value is all of that struct’s value type fields set to their default value, and all of that struct’s reference type fields set to null.
The default value of a bool variable in C# is false (reference). Therefore, the HasValue property of a default Nullable<T> instance is false; which in turn makes that Nullable<T> instance itself act like null.