In your first example, the as operator attempts to use the object s as an int?. Since int? isn’t anywhere in the inheritance chain of short?, this operation fails.
In your second example, you’re actually creating a new int? i with the value from short? s. This is a more generous operation, because it doesn’t have to preserve the original s object on the left hand side.
The important point here is that as isn’t allowed to do anything that doesn’t preserve your object’s identity. An explicit cast can.
Here’s what the C# standard says about how the (int?) form works:
6.1.4 Implicit nullable conversions
Predefined implicit conversions that operate on non-nullable value
types can also be used with nullable forms of those types. For each of
the predefined implicit identity and numeric conversions that convert
from a non-nullable value type S to a non-nullable value type T, the
following implicit nullable conversions exist:· An implicit conversion from S? to T?.
· An implicit conversion from S to T?.
Evaluation of an implicit nullable conversion based on an underlying
conversion from S to T proceeds as follows:· If the nullable conversion is from S? to T?:
o If the source value is null (HasValue property is false), the
result is the null value of type T?.o Otherwise, the conversion is evaluated as an unwrapping from S? to
S, followed by the underlying conversion from S to T, followed by a
wrapping (§4.1.10) from T to T?.· If the nullable conversion is from S to T?, the conversion
is evaluated as the underlying conversion from S to T followed by a
wrapping from T to T?.