Detection of nullable types...

[rubikzube*]

Hi.

I'm trying to write a snippet of code to detect if a variable is a
nullable<t> struct, and I can't figure out the right syntax or if such
a thing is even possible... Below are the results that I got when I
attempted to perform some simple tests.

Nullable<int> i = 32;

bool isNullableClass = i is Nullable; // false
bool isNullableStructInt = i is Nullable<int>; //true
bool isValueType = i is ValueType; // true

bool isNullableStruct = i is Nullable<>; // doesn't compile b/c must
provide a struct
bool isNullableStructValueType = i is Nullable<ValueType>; // doesn't
compile because ValueType is not a struct

System.Type iType = i.GetType(); // returns System.Int32

I also tried reflecting through the code and walking through the
documentation, but there doesn't seem to be any method of detecting if
a variable is a nullable struct. Does anyone know how to do this?

 

[rubikzube*]

*Update*

Actually, the following test produces even stranger results and has
prompted me to give up on my quest... although I think this is a major
issue.

int i2 = 32;
bool i2isNullableStructInt = i is Nullable<int>; // true
bool i2isInt = i is int; // also true

 

[Barry Kelly]

I'm trying to write a snippet of code to detect if a variable is a
nullable<t> struct, and I can't figure out the right syntax or if such
a thing is even possible... Below are the results that I got when I
attempted to perform some simple tests.

Either the variable is declared to be of type Nullable<T> (aka T?), or it
is stored in an object reference. The implementation of nullables in the
CLR is such that it is impossible to have a boxed nullable on the heap.
That is:

int? x = 4;
object o = x;
Console.WriteLine(o.GetType().Name);

should result in System.Int32 being printed to the console. The reason is
that when the CLR tries to box an instance of Nullable<T>, it looks at the
value inside the nullable struct. If the value is null, it boxes to a null
reference. If it isn't null, it gets boxed as the inner value of type T.

So, given a reference to a (possibly boxed struct) object of unknown type,

I also tried reflecting through the code and walking through the
documentation, but there doesn't seem to be any method of detecting if
a variable is a nullable struct. Does anyone know how to do this?

By design (and because of a late DCR (design change request)) between .NET
2.0 Beta 2 and .NET RTM, boxed nullables cannot occur.

The rational is that a boxed null is unintuitive. For example, with Beta
2, the following code would print 'True' followed by 'False':

int? x = null;
Console.WriteLine(x == null);
object o = x;
Console.WriteLine(o == null);

This was unintuitive, because the o reference wasn't null, it was pointing
to a boxed Nullable<int> instance whose value was null. This caused too
much confusion, so the CLR was modified to ensure that any attempt to box
a Nullable<T> resulted either in a null reference or a boxed instance of
T.

-- Barry

 

[rubikzube*]

Wow. That was an extremely lucid response. My only follow up would be
that while a boxed null is unintuitive, there are situations where it
would be nice to determine if a generic value type passed into a
function should be considered nullable, just for transitivity in code
if nothing else.

As it stands, there is no way to know.

Thanks, dude.