可空引用类型是否不可空<t> ?</t>

Question

I created a new C#10/.net6 library project in VS2022 using the standard template, NRTs are enabled by default (nullable: enabled). I notice that nullable reference types do not appear to support Nullable properties Value and HasValue

MRE (using #nullable annotation explicitly)

using System;
                    
public class Program
{
    public static void Main()
    {
#nullable enable
        object? o =null;
        if(o.HasValue)
        {}
        Console.WriteLine("Hello World");
    }
}

'object' does not contain a definition for 'HasValue' and no accessible extension method 'HasValue' accepting a first argument of type 'object' could be found

Are nullable reference types not actually Nullable<T> ? o is declared as object? so why does the compiler refer to object rather than object? ?

Answer 1

No, they are not. From the docs :

However, nullable reference types and nullable value types are implemented differently: nullable value types are implemented using System.Nullable<T> , and nullable reference types are implemented by attributes read by the compiler. For example, string? and string are both represented by the same type: System.String . However, int? and int are represented by System.Nullable<System.Int32> and System.Int32 , respectively.

Basically NRT is just compile time information (though you can get access it in runtime via reflection like done here ).

As for the case - just use ordinary null check and that's it.

if(o != null)
{

}

or pattern matching:

if(o is {} notNullO)
{

}