[Mono-list] Crazy generic dictionary?

[Robert Jordan]

Andrés G. Aragoneses [ knocte ] wrote:
> Robert Jordan escribió:
>> Andrés G. Aragoneses [ knocte ] wrote:
>>> Robert Jordan escribió:
>>>> Andrés G. Aragoneses [ knocte ] wrote:
>>>>> Today I've come out with this wish:
>>>>>
>>>>> Dictionary<typeof(T), List<T>> InternalCollection;
>>>>>
>>>>>
>>>>> Of course, the compiler tells me an error, but I would want to know if I 
>>>>> can make this kind of collection with C# generics, in order to obtain 
>>>>> syntatic sugar and type safety, I mean:
>>>>>
>>>>> //compilation should succeed only if oMyObj is TypeX:T
>>>>> InternalCollection.Add(typeof(TypeX), oMyObj);
>>>>>
>>>>> Any thoughts?
>>>> class FooDictionary<T> : Dictionary<System.Type, T>
>>>> {
>>>> 	public void Add (T t)
>>>> 	{
>>>> 		this [typeof(T)] = t;
>>>> 	}
>>>> }
>>>>
>>> But that would only allow adding one object:
>>
>> Well, then use that slightly modified Add method:
>>
>>   	public void Add (T t)
>>   	{
>> 		if (t == null)
>> 			throw new ArgumentNullException ("t");
>> 		this [t.GetType ()] = t;
>>   	}
>>
> 
> Ok! But if I modify it to use lists, I get a compiler error:
> 
> class FooDictionary<T> : Dictionary<Type, List<T>>
> {
>      public void Add (List<T> t)
>      {
>          if (t == null)
>          {
>              throw new ArgumentNullException("t");
>          }
>          if (t.Count > 0)
>          {
>              this[t[0].GetType()] = t;
>          }
>      }
> }
> 
> ...
> 
> FooDictionary<mytype> foo = new FooDictionary<mytype>();
> foo.Add(new List<subtypeA>());
> 
> Error	11	Argument '1': cannot convert from 
> 'System.Collections.Generic.List<subtypeA>' to 
> 'System.Collections.Generic.List<mytype>'
> 
> However, it should allow it because subtypeA inherits from mytype, 
> right? I think I've reached a limitation of .NET generics, which I think 
> I read about here[1] :(

This works, at least with MS.NET 2.0 csc:

using System;
using System.Collections.Generic;

class FooDictionary<T> : Dictionary<System.Type, List<T>>
{
	public void Add (List<T> t)
	{
		if (t == null)
			throw new ArgumentNullException ("t");
		// key is the generic argument of List<T>
		this [t.GetType ().GetGenericArguments() [0]] = t;
	}
}

class Test
{
	static void Main ()
	{
		FooDictionary<int> d = new FooDictionary<int> ();
		d.Add (new List<int> ());
	}
}


Robert