Why does Nullable<T> HasValue property not throw NullReferenceException on Nulls?

Question

Consider the following code:

DateTime? ndate = null;
Console.WriteLine(ndate.HasValue);

I would have expected a NullReferenceException, but HasValue will indeed return false. However, since ndate is null, how does the property invocation succeed, since there is no object to invoke the property HasValue on?

Answer 1

Synctactic sugar my friend ... the compiler turns that .HasValue call into code that won't throw an exception :-)

Answer 2

What really happens is that an actual (non-null) Nullable structure gets assigned to the variable, thus the variable is not null but it's Value is.

Answer 3

The Nullable<T> is a struct, basically it can't hold a null value.

Your assignment, is actually compiled into something that looks like this:

Nullable<DateTime> ndate = new Nullable<DateTime>();

Another example, the expression:

int? a = null;

Will generate the following IL:

.locals init (valuetype [mscorlib]System.Nullable`1<int32> V_0)
IL_0000:  ldloca.s   V_0
IL_0002:  initobj    valuetype [mscorlib]System.Nullable`1<int32>

A call to the initobj operation, which initializes each field of the a value type at a specified address to a null reference or a 0 of the appropriate primitive type.

In conclusion, what's happening here is the default struct initialization.

Answer 4

Technically, "ndate" is not null - it is a value type, with it's value specified as being null.

When you write DateTime?, this is just shorthand for Nullable<DateTime>, which is a struct. There is no way for this to technically be null, since it's not a reference type.

Answer 5

I would have expected a NullReferenceException

There is no null reference being dereferenced, so "NullReferenceException" should not be expected. The nullable DateTime is not a null reference. It is a null value.

However, since ndate is null, how does the property invocation succeed, since there is no object to invoke the property HasValue on?

The question presupposes a false premise. There is an object. There's a nullable DateTime representing the null value of a nullable DateTime . That value is an object.

Consider your code:

DateTime? ndate = null; 
Console.WriteLine(ndate.HasValue); 

This is just a short way of writing something that is logically like:

DateTime ndate = default(DateTime); 
bool ndateHasValue = false;
Console.WriteLine(ndateHasValue); 

A nullable value type is simply a pleasant way to write code that means "please attach a Boolean to this thing that keeps track of whether it is logically null or not".