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c#null-object-pattern

Generic null object pattern in C#

I'm wondering if there is any approach to implement generic null object pattern in C#. The generic null object is the subclass of all the reference types, just like Nothing in Scala. It seems like

public class Nothing<T> : T where T : class

But it can't compile and I've no idea how to implement the methods of T to provide default behavior or throw an exception. Here are some thinkings:

  1. Use reflection?
  2. Use expression tree when creating Nothing<T>? It maybe looks like Moq. And another question comes: Is it OK to use mock framework/library in product codes?
  3. Use dynamic types?

I KNOW maybe I should implement particular null object for particular type. I'm just curious to know if there is any solution.

Any suggestion? Thanks.

Solution

  • With generics, you can't define inheritance from T. If your intent is to use if(x is Nothing<Foo>), then that just isn't going to work. Not least, you'd need to think about abstract types, sealed types, and non-default constructors. However, you could do something like:

    public class Nothing<T> where T : class, new()
{
    public static readonly T Instance = new T();
}

    However, IMO that fails most of the key features of a null-object; in particular, you could easily end up with someone doing:

    Nothing<Foo>.Instance.SomeProp = "abc";

    (perhaps accidentally, after passing an object 3 levels down)

    Frankly, I think you should just check for null.