How to convert object to nullable int array in C#?

That will work. Does it now?

Personally, I wouldn't do it quite like that (but close):

int?[] result = null;

var enumerable = propertyValue as IEnumerable; 
// will be null if not IEnumerable.  These checks are always valid

if(enumerable != null && enumerable.All(i=>i is int?) //or is INullable<int>.  they should be the same
  result = enumerable.Cast<int?>().ToArray()

return result;

This will return either a int?[] or null.

Personally I would add checks that throw exceptions so I can identify why I'm getting null. After all, it might be a different object, or a null int?[].

This did not work as "null is int?" always equals false.

Instead, replace with "i=>i==null || i is int"

Here is the full unit test that passes:

[TestMethod]
public void TryPart1()
{

    int?[] value = new int?[10];
    for (int i = 0; i < value.Length; i++)
    {
        value[i] = (i > 4) ? (int?)null : 0;
    }

    object a = value;
    var result = TryPart2(a);

    Assert.IsTrue(result != null);
    Assert.IsTrue(result.Length == 10);

}

public int?[] TryPart2(object a)
{

    int?[] result = null;

    var enumerable = a as IEnumerable;

    if (enumerable != null)
    {
        var objects = enumerable.Cast<object>().ToArray();
        if (objects.All(o => o==null || o is int))
            result = objects.Cast<int?>().ToArray();

    }

    return result;
}