List, nested

While 2D arrays are possible in VB.NET programs, nested Lists are often a more convenient choice. They can expand and resize themselves more easily.

By creating a List of Lists, we can use each List an independent row. The syntax for creating nested Lists can be cumbersome—the types must be specified all at once.

Example

This program contains 2 subroutines—the Main sub sets up and populates the nested List. And the Display subroutine loops over the List we created, and prints its elements.

Module Module1

    Sub Main()
        ' Part 1: Create a List and place sublists within it with Add.
        Dim list = New List(Of List(Of Integer))
        Dim rand = New Random()
        For i = 0 To 2
            ' Create a row with a random number of integers.
            Dim sublist = New List(Of Integer)
            Dim top = rand.Next(1, 4)
            For v = 0 to top - 1
                sublist.Add(rand.Next(1, 5))
            Next
            list.Add(sublist)
        Next
        ' Call the Display sub.
        Display(list)
    End Sub

    Sub Display(list As List(Of List (Of Integer)))
        ' Part 2: display elements in the List.
        Console.WriteLine("Elements:")
        For Each sublist in list
            For Each value in sublist
                Console.Write(value)
                Console.Write(" ")
            Next
            Console.WriteLine()
        Next
        ' Part 3: display a single element.
        Console.WriteLine("Element at 1, 0:")
        Console.WriteLine(list(1)(0))
        ' Part 4: write list count.
        Dim count = 0
        For Each sublist in list
            count += sublist.Count
        Next
        Console.WriteLine("Count:")
        Console.WriteLine(count)
    End Sub

End Module
Elements:
2 2
2 4 2
4 3
Element at 1, 0:
2
Count:
7

In a sense, there is no such thing as a 2D List in .NET—we just have a List, and we can place separate Lists within it. But as a real-world solution, nested Lists are worth considering.