Question

why is typeof int? an Int32

int? x = 1;
Console.WriteLine(x.GetType().Name);

If it is okay then what’s the use of Nullable.GetUnderlyingType?

Answer 1

Calling GetType() boxes your variable. The CLR has a special rule that Nullable<T> gets boxed to T. So x.GetType will return Int32 instead of Nullable<Int32>.

int? x = 1;
x.GetType() //Int32
typeof(int?) //Nullable<Int32>

Since a Nullable containing null will be boxed to null the following will throw an exception:

int? x = null;
x.GetType() //throws NullReferenceException

To quote MSDN on Boxing Nullable Types:

Objects based on nullable types are only boxed if the object is non-null. If HasValue is false, the object reference is assigned to null instead of boxing

If the object is non-null — if HasValue is true — then boxing occurs, but only the underlying type that the nullable object is based on is boxed. Boxing a non-null nullable value type boxes the value type itself, not the System.Nullable<T> that wraps the value type.