How to override `is` operator

Question

we know:

int? number = 10;
Console.WriteLine(number is int); // true

but:

NotNull<string> text = "10"; // NotNull<> is my struct
Console.WriteLine(text is string); // false

I want text is string return true, how can I do that?

-------------------------------- edit

here is my NotNull:

public class NotNull<T> where T : class
{
    public T Value { get; }

    private NotNull(T value)
    {
        this.Value = value;
    }

    public static implicit operator T(NotNull<T> source)
    {
        return source.Value;
    }

    public static implicit explicit NotNull<T>(T value)
    {
        if (value == null) throw new ArgumentNullException(nameof(value));
        return new NotNull<T>(value);
    }
}

if a class was declaring like:

public class A
{
    public NotNull<string> B { get; set; }
}

I just hope any serializer can serialize and deserialize it same as:

public class A
{
    public string B { get; set; }
}

-------------------------------- edit 2

I found this is a impossible question:

1. if NotNull<> is class, default(NotNull<>) is null, I do nothing.
2. if NotNull<> is struct, default(NotNull<>).Value is null.

sorry about the question.

Answer 1

On MSDN you have list of overloadable operators: Overloadable Operators (C# Programming Guide)

These operators cannot be overloaded:

=, ., ?:, ??, ->, =>, f(x), as, checked, unchecked, default, delegate, is, new, sizeof, typeof