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

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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.

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