Closest I've got...

Puzzle.cs

namespace Puzzle
{
    // Using the tokens
    // ? null :
    // in that order, create a valid C# program such that
    // the program is also valid when "null" is removed.
    class Puzzle
    {
        enum E { V }

        struct S
        {
            public static implicit operator E(S? k) { return E.V; }
            public static implicit operator S?(E m) { return null; }
            public static implicit operator bool(S? k) { return true; }
        }

        static void Main(string[] args)
        {
            const E m = E.V;
            switch (m)
            {
                case m as S? null : m : break;
            }
        }
    }
}