Creating a List of Lists in C#

Question

I seem to be having some trouble wrapping my head around the idea of a Generic List of Generic Lists in C#. I think the problem stems form the use of the <T> argument, which I have no prior experience playing with. Could someone provide a short example of declaring a class which is a List, that therein contains another List, but where the type of the object contained therein is not immediately known?

I've been reading through the MS documentation on Generics, and I am not immediately sure if I can declare a List<List<T>>, nor how exactly to pass the <T> parameter to the inside list.

Edit: Adding information

Would declaring a List<List<T>> be considered legal here? In case you are wondering, I am building a class that allows me to use a ulong as the indexer, and (hopefully) steps around the nasty 2GB limit of .Net by maintaining a List of Lists.

public class DynamicList64<T>
    {
        private List<List<T>> data = new List<List<T>>();

        private ulong capacity = 0;
        private const int maxnumberOfItemsPerList = Int32.MaxValue;



        public DynamicList64()
        {
            data = new List<List<T>>();
        }

Gromer · Accepted Answer · 2012-09-27 18:55:41Z

135

A quick example:

List<List<string>> myList = new List<List<string>>();
myList.Add(new List<string> { "a", "b" });
myList.Add(new List<string> { "c", "d", "e" });
myList.Add(new List<string> { "qwerty", "asdf", "zxcv" });
myList.Add(new List<string> { "a", "b" });

// To iterate over it.
foreach (List<string> subList in myList)
{
    foreach (string item in subList)
    {
        Console.WriteLine(item);
    }
}

Is that what you were looking for? Or are you trying to create a new class that extends List<T> that has a member that is a `List'?

answered Sep 27, 2012 at 18:55

Gromer

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1 Comment

Craig.Feied Over a year ago

This answer just totally misses the point. OP wants to declare a List of Lists, where the inner lists are generic. Essentially, OP is looking for a List of variant litsts...

RollRoll · Accepted Answer · 2012-09-28 01:49:24Z

34

or this example, just to make it more visible:

public class CustomerListList : List<CustomerList> { }  

public class CustomerList : List<Customer> { }

public class Customer
{
   public int ID { get; set; }
   public string SomethingWithText { get; set; }
}

and you can keep it going. to the infinity and beyond !

edited Sep 28, 2012 at 1:49

answered Sep 27, 2012 at 18:57

RollRoll

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4 Comments

Olivier Jacot-Descombes Over a year ago

+1. Good idea. You can even add customer specific stuff to it like an Add method taking a customer name as parameter, that creates and adds a customer automatically. (You need a where T : new() constraint for this).

Servy Over a year ago

You could just use alias' instead, since the body of the classes are all empty

RollRoll Over a year ago

@Servy, one up, I changed the post to make it as clear as possible to whoever sees it, tks

CDrosos Over a year ago

@ThePoet what lies beyond infinity?

Matthew Layton · Accepted Answer · 2022-05-07 06:52:16Z

A list of lists would essentially represent a tree structure, where each branch would constitute the same type as its parent, and its leaf nodes would represent values.

Implementation

public sealed class TreeList<T> : List<TreeList<T>>
{
    public List<T> Values { get; } = new List<T>();

    public TreeList<T> this[int index]
    {
        get
        {
            while (index > Count - 1)
            {
                Branch();
            }

            return base[index];
        }
    }

    public TreeList<T> Branch()
    {
        TreeList<T> result = new TreeList<T>();
        
        Add(result);

        return result;
    }
}

Example

internal static class Program
{
    public static void Main()
    {
        // Create the root element...
        TreeList<string> treeList = new TreeList<string>();
        
        // You can add branches the old-fashioned way...
        treeList.Add(new TreeList<string>());

        // Add leaf node values to your first branch...
        treeList[0].Values.Add("Hello, World!");
        treeList[0].Values.Add("Goodbye, World!");

        // You can create new branches from any branch like this...
        // Note: You could also chain branch statements; i.e. treeList.Branch().Branch().Branch()
        TreeList<string> branch2 = treeList.Branch();

        // Add leaf node values to your second branch...
        branch2.Values.Add("Alice");
        branch2.Values.Add("Bob");

        // You can also create branches until you reach the desired branch index...
        // The TreeList indexer will loop until the desired index has been created, and then return it.
        treeList[7].Values.Add("Alpha");
        treeList[7].Values.Add("Bravo");
        treeList[7].Values.Add("Charlie");

        // How many branches does the root have?
        Console.WriteLine($"Treelist branches: {treeList.Count}");

        // What's at branch 0's values?
        foreach (string value in treeList[0].Values)
        {
            Console.WriteLine(value);
        }

        // What's at branch 1's values?
        foreach (string value in treeList[1].Values)
        {
            Console.WriteLine(value);
        }

        // What's at branch 7's values?
        foreach (string value in treeList[7].Values)
        {
            Console.WriteLine(value);
        }
    }
}

Now, whether you should implement something like this is another matter. Extending List<T> isn't recommended: Why not inherit from List<T>?

Michael Christensen · Accepted Answer · 2012-09-27 19:01:58Z

3

public class ListOfLists<T> : List<List<T>>
{
}


var myList = new ListOfLists<string>();

answered Sep 27, 2012 at 19:01

Michael Christensen

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1 Comment

piXelicidio Over a year ago

Tested (I'm learning...) and this is the simplest actual answer to OP.

Waldri · Accepted Answer · 2021-12-06 15:26:33Z

1

Look a direct example here:

public partial class Form1 : Form
{
    List<List<Label>> txtList;
    List<List<int>> num;
    public Form1()
    {
        InitializeComponent();
        txtList = new List< List<Label> >() { 
            new List<Label> { label1, label2, label3 }, 
            new List<Label> { label4, label5, label6 }, 
            new List<Label> { label7, label8, label9 } 
        };

        num = new List<List<int>>() { new List<int>() { 1, 2 }, new List<int>() { 3, 4 } };
    }
}

answered Dec 6, 2021 at 15:26

Waldri

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Comments

Usama Tabbassum · Accepted Answer · 2021-10-29 14:56:15Z

0

I have the following code

  public class ChildClass
    {
        public string FieldName { get; set; }
        public string Value { get; set; }
        public string StatusClass { get; set; }
        public string StatusMessage { get; set; }
    }

Creating a list of list obj is as follows

List<List<ChildClass>> obj = new List<List<ChildClass>>();

answered Oct 29, 2021 at 14:56

Usama Tabbassum

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Comments

freakydinde · Accepted Answer · 2015-02-25 16:33:07Z

-3

you should not use Nested List in List.

List<List<T>>

is not legal, even if T were a defined type.

https://msdn.microsoft.com/en-us/library/ms182144.aspx

answered Feb 25, 2015 at 16:33

freakydinde

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1 Comment

Mike Marynowski Over a year ago

What do you mean it's not legal? Of course it is. What you linked to is just a design warning. Such syntax is preferably avoided, particularly in public APIs but there are valid use cases for doing such things.

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