How to check if an object is nullable?

Question

Hi,

How do I check if a given object is nullable in other words how to implement the following method...

bool IsNullableValueType(object o)
{
    ...
}

EDIT: I am looking for nullable value types. I didn't have ref types in mind.

//Note: This is just a sample. The code has been simplified 
//to fit in a post.

public class BoolContainer
{
 bool? myBool = true;
}

var bc = new BoolContainer();

const BindingFlags bindingFlags = BindingFlags.Public
      | BindingFlags.NonPublic
      | BindingFlags.Instance
      ;


object obj;
object o = (object)bc;

foreach (var fieldInfo in o.GetType().GetFields(bindingFlags))
{
 obj = (object)fieldInfo.GetValue(o);
}

obj now refers to an object of type bool (System.Boolean) with value equal to true. What I really wanted was an object of type Nullable<bool>

So now as a work around I decided to check if o is nullable and create a nullable wrapper around obj.

Answer 1

There are two types of nullable - Nullable<T> and reference-type.

Jon has corrected me that it is hard to get type if boxed, but you can with generics: - so how about below. This is actually testing type T, but using the obj parameter purely for generic type inference (to make it easy to call) - it would work almost identically without the obj param, though.

    static bool IsNullable<T>(T obj)
    {
        if (obj == null) return true; // obvious
        Type type = typeof(T);
        if (!type.IsValueType) return true; // ref-type
        if (Nullable.GetUnderlyingType(type) != null) return true; // Nullable<T>
        return false; // value-type
    }

But this won't work so well if you have already boxed the value to an object variable.

Answer 2

Well, you could use:

return !(o is ValueType);

... but an object itself isn't nullable or otherwise - a type is. How were you planning on using this?

Answer 3

Be carefull, when boxing a nullable type (Nullable<int> or int? for instance) :

int? nullValue = null;
object boxedNullValue = (object)nullValue;
Debug.Assert(boxedNullValue == null);

int? value = 10;
object boxedValue = (object)value;
Debug.Assert( boxedValue.GetType() == typeof(int))

It becomes a true reference type, so you lose the fact it was nullable.

Answer 4

bool IsNullableValueType(object o)
{
    Type objType = o.GetType();

    if (objType.FullName.StartsWith(typeof(Nullable<>).FullName))
    {
        return Nullable.GetUnderlyingType(objType).IsValueType;
    }

    return false;
}