SwitchOf.cs

using System;
using System.Collections;
using System.Collections.Generic;

class Program {
    public static void Main() {
        var input = "12";

        using (new switchOf(input) {

            (int a) => Console.WriteLine($"int: {a}"),
            (string a) => Console.WriteLine($"string: {a}"),
            
        });
    }
}

class switchOf : IEnumerable, IDisposable {
    Dictionary<Type, Action<object>> _handlers = new Dictionary<Type, Action<object>>();
    object _obj;

    public switchOf(object obj) {
        _obj = obj;
    }

    public void Add<T> (Action<T> action) {
        var type = typeof(T);

        if(_handlers.ContainsKey(type))
            throw new InvalidOperationException();

        _handlers[type] = (object o) => action((T) o);
    }

    public void Dispose() {
        var type = _obj.GetType();

        if (_handlers.TryGetValue(type, out var handler)) {
            handler(_obj);
        }
    }

    IEnumerator IEnumerable.GetEnumerator() => null;    
}