TypeScript Never type in C#?

Question

I am wondering is there anyt c# equivalent for TypeScript's never type

example I write this code in TS I will have a build time error.

enum ActionTypes {
    Add,
    Remove
}

type IAdd = {type: ActionTypes.Add};
type IRemove = {type: ActionTypes.Remove};

type IAction = IAdd | IRemove;

const ensureNever = (action: never) => action;

function test(action: IAction) {
    switch (action.type) {
        case ActionTypes.Add:
            break;
        default:
            ensureNever(action);
            break;
    }
}

error is: Argument of type 'IRemove' is not assignable to parameter of type 'never'.

This is very useful when someone change the logic in one file and I want to be sure this new case is handled everywhere.

Is there any way to do this in c#? (I googled around but I did not find anything)

This is what I have so far...

using System;
class Program
{
    private enum ActionTypes
    {
        Add,
        Remove
    }

    interface IAction {
        ActionTypes Type { get; }
    }

    class AddAction : IAction
    {
        public ActionTypes Type
        {
            get {
                return ActionTypes.Add;
            }
        }
    }

    class RemoveAction : IAction
    {
        public ActionTypes Type
        {
            get
            {
                return ActionTypes.Remove;
            }
        }
    }

    static void Test(IAction action)
    {
        switch (action.Type)
        {
            case ActionTypes.Add:
                Console.WriteLine("ActionTypes.Add");
                break;
            default:
                // what should I put here to be sure its never reached?
                Console.WriteLine("default");
                break;
        }
    }

    static void Main(string[] args)
    {
        var action = new RemoveAction();
        Program.Test(action);
    }
}

I want to be sure I have an error at build time not run time.

Answer 1

Unfortunately, I don't think the C# compiler is smart enough to do that. Even if you throw a new exception in the default case in the switch statement on action.Type, no compile-time error will appear regarding the missing ActionTypes.Remove case.

I found this MSDN blog post that says about the never type that it's "unlikely that it will become a feature of mainstream CLR languages."