How to declare an object that is a list of lists of a generic type [closed]

问题

Is it possible to declare an object that is is a list of lists of a generic types. So a little like private List<List<T>> outerList = new List<List<T>>(); but a statement that actually compiles.

I do not want to use dynamics like so: private List<List<dynamic>> list2 = new List<List<dynamic>>(); as I would like type safety.

I'd kind of like to be able to:

foreach(List<T> innerList in outerList)
{
    foreach(T item in innerList)
    {
        doSomethingTo(item);
    }
}

Ideally I'd like to declare a class that inherits from the List<List<T>>

EDIT:

I probably should have been clearer. The inner lists must be able to have different types. so one inner list may be List<string> and another List<int>

FURTHER EDIT:

Following @ErikE's request the question has been reworded here.

回答1:

Sure, you can define that as a generic class:

public class Foo<T> : List<List<T>>
{

}

and then have a generic method that processes one of them:

public void Process<T>(Foo<T> outerList)
{
    foreach(List<T> innerList in outerList)
    {
        foreach(T item in innerList)
        {
            doSomethingTo(item);
        }
    }
}

of course doSomethingTo would then need to be generic as well.

I probably should have been clearer. The inner lists must be able to have different types. so one inner list may be List<string> and another List<int>

Yes, you should have been clearer. There's no inheritance relationship between List<string> and List<int> that would allow you to store them in a single type-safe outer collection. You would have to resort to List<object> or List<dynamic>, or find a different data structure that meets your needs.

回答2:

You can't have a List<List<T>> that its T could be variable in each item. For example if T=string then your list is List<List<string>> and you can not have any List<int> in your top list.

The list you can use is List<List<object>> and the thing that will help you to be typed when working with list, is Cast<T> method like this:

var list = new List<List<object>>();

var intList = new List<int>() { 1, 2, 3 };
var stringList = new List<string>() { "a", "b", "c" };

list.Add(intList.Cast<object>().ToList());
list.Add(stringList.Cast<object>().ToList());

var sum = 0;
list[0].Cast<int>().ToList()
        .ForEach(i =>
        {
            sum += i;
        });

var msg = "";
list[1].Cast<string>().ToList()
        .ForEach(s =>
        {
            msg += s;
        });

Benefits:

We add items typed: (List<int>, List<string>)
We use items typed: (Cast<int>, Cast<string>)

回答3:

If we recall the times of .NET 1.1, there is also a bunch of classes in System.Collections namespace that almost nobody uses nowadays. With that said you could come with something like this:

System.Collections.Generic.List<System.Collections.ArrayList> outerList =
  new System.Collections.Generic.List<System.Collections.ArrayList>();

来源：https://stackoverflow.com/questions/32770783/how-to-declare-an-object-that-is-a-list-of-lists-of-a-generic-type

标签

list

generics

nested-lists