Consider this following code in C# 2.0

in fact it was complied like this:

please note line 3, when we increased the nullable i, it actually created a new instance of Nullable<int>
     new Nullable<int>(nullable3.GetValueOrDefault() + 1)
and in line 6, when we assign null to a nullable instance, it also created a new instance with null value.
    nullable2 = new Nullable<int>();

Once we created an instance of Nullable<T>, we can NOT modify the inner value any more. When we modify these value, we  actually create a new instance with the new value. Nullable<T> is an immutable type.

It looks like string, right? (though Nullable<T> is a value type)