Question

class Test<T>
{
    T? obj;
}

Error:

The type ‘T’ must be a non-nullable value type in order to use it as
parameter ‘T’ in the generic type or method ‘System.Nullable’

I need to keep a possibly null reference to type T, but T can be either a value type or a reference type, and you can’t have Nullable<T> if T is a reference type. Any solution?

Answer 1

If you need to support both classes and structs inside Test<T>, you could not use the where T : struct constraint and you would not be able to support the T? directly, although you could achieve the desired effect via the semantics of what T? does.

class Test<T>
{
    T obj;
    bool hasValue; 
}

So where you would use obj, you would first check hasValue to see if it is in a usable state (“non-null”).

You could go a step further and encapsulate this to essentially roll your own nullable wrapper (perhaps nest it inside Test so as to not make it part of the publicly visible API) to support both classes and reference types. While it would be redundant, it would bring together the currently separate concepts of a null class and null struct.