Why are Nullable<T> considered a value type?

Question

Have you ever tried to use the Convert.ChangeType() method to convert a value to a Nullable<T> type? Awkwardly, it will throw an InvalidCastException saying "Null object cannot be converted to a value type".

Try running this on your immediate window: ?System.Convert.ChangeType(null, typeof(int?))

For some obscure reason, Nullables are considered value types. For example, typeof(int?).IsValueType returns true.

For me, since Nullable<T> accept null, it's a class type, not a value type. Does anyone know why it would implemented differently?

Answer 1

System.Nullable<T> is technically a structure, so it's a value type (the null value for a Nullable<T> is not exactly the same thing as a null reference. It's a boolean flag that denotes the lack of a value.) However, it's treated specially by the runtime and this makes it an awkward value type. First, it doesn't satisfy where T : struct type constraint (it doesn't satisfy where T : class either, for what it's worth). Second, it exhibits an interesting boxing behavior. Boxing a Nullable<T> will result in:

• A null reference, if the value is null.

• A boxed value of the underlying type if it actually contains a value. That is, in the statements:

  int? x = 2;
  object y = x;
  

  y is not a boxed Nullable<int>. It's simply a boxed int. You can unbox any boxed value of type T to Nullable<T> (and T, of course). Casting a null reference to Nullable<T> results in a null value (as you might expect).

This special treatment by the runtime makes nullable types work more similar to the way null works in reference types.