Firstly, why doesn't Dictionary<TKey, TValue> support a single null key?

Secondly, is there an existing dictionary-like collection that does?

I want to store an "empty" or "missing" or "default" System.Type, thought null would work well for this.

More specifically, I've written this class:

class Switch
{
    private Dictionary<Type, Action<object>> _dict;

    public Switch(params KeyValuePair<Type, Action<object>>[] cases)
    {
        _dict = new Dictionary<Type, Action<object>>(cases.Length);
        foreach (var entry in cases)
            _dict.Add(entry.Key, entry.Value);
    }

    public void Execute(object obj)
    {
        var type = obj.GetType();
        if (_dict.ContainsKey(type))
            _dict[type](obj);
    }

    public static void Execute(object obj, params KeyValuePair<Type, Action<object>>[] cases)
    {
        var type = obj.GetType();

        foreach (var entry in cases)
        {
            if (entry.Key == null || type.IsAssignableFrom(entry.Key))
            {
                entry.Value(obj);
                break;
            }
        }
    }

    public static KeyValuePair<Type, Action<object>> Case<T>(Action action)
    {
        return new KeyValuePair<Type, Action<object>>(typeof(T), x => action());
    }

    public static KeyValuePair<Type, Action<object>> Case<T>(Action<T> action)
    {
        return new KeyValuePair<Type, Action<object>>(typeof(T), x => action((T)x));
    }

    public static KeyValuePair<Type, Action<object>> Default(Action action)
    {
        return new KeyValuePair<Type, Action<object>>(null, x => action());
    }
}

For switching on types. There are two ways to use it:

1. Statically. Just call Switch.Execute(yourObject, Switch.Case<YourType>(x => x.Action()))
2. Precompiled. Create a switch, and then use it later with switchInstance.Execute(yourObject)

Works great except when you try to add a default case to the "precompiled" version (null argument exception).