Using Lists

[Doogle]

(Newbe Question)

I'm playing around with Lists

I can do:

Dim myList(1) As List(Of String)

I can't do:

Dim myList(1) As New List(Of String)

the IDE returns "Arrays cannot be declared with 'New'"
I circumvent the 'problem' by putting

myList(0) = New List(Of String)
myList(1) = New List(Of String)

in the Form Load event.

The question is, is there another way to instantiate the List array?

[ThomasJohnsen]

The line

Dim mylist() As List(Of String) = {New List(Of String), New List(Of String)}

will do the equivalent of what you expected to happen with Dim mylist(1) As New List(Of String).

#EDIT: Alternatively you could use linq to create an array of <count> new lists of string with:

Dim mylist() As List(Of String) = Enumerable.Range(0, <count>).Select(Function(i) New List(Of String)).ToArray

[Doogle]

Thanks. I can see that's ok for 1 or 2 elements, but if I had, say 100, would I just resort to my 'circumvention' method i.e.

For I = 0 To 99
    myList(I) = New List(Of String)
Next

or is there yet another way ?

EDIT: Ok, thanks again. Just read your Edit.

[ThomasJohnsen]

Just fyi there was a member here, that investigated the performance of some linq. In most (all he had examined iirc) cases the naive (or old-fashioned) method of doing things with loops or similar will be faster or at least as fast as linq. The thing promoting linq is ofc the ability to make short and easy to read/understand code very fast.

[jmcilhinney]

This question has nothing specific to do with Lists. It's the same for any array of reference types, and kinda for value types too. Just a variable is equal to Nothing when you declare it, so each element of an array is Nothing when you create the array. In the case of value types, Nothing is equivalent to the default value for that type, e.g. 0 for Integers, False for Booleans and #1/01/0001# for Dates. If you want to assign a value other than the default to each element of the array then you must do that explicitly. The simplest way to do that for any size array is with a loop:

Dim lists(upperBound) As List(Of String)

For i = 0 To upperBound
    lists(i) = New List(Of String)
Next

Alternatively, you could use LINQ to create the array in the first place:

Dim lists = Enumerable.Range(1, length).
                       Select(Function(n) New List(Of String)).
                       ToArray()

[dunfiddlin]

I'm curious. Why an Array of Lists rather than a List of Lists?

[jmcilhinney]

I'm curious. Why an Array of Lists rather than a List of Lists?

One would assume that the OP wants a fixed number of lists with a variable number of items rather than a variable number of lists with a fixed number of items. Of course, assumption is a good way to get yourself in trouble around here.

[Doogle]

OP is trying to get to grips with VB.Net and 'needed' something like an array of Collections. I was looking at this thread http://www.vbforums.com/showthread.p...85#post4266985 and was trying to work out how I'd store the Questions and Answers. Came to the conclusion that a List for each subject would work. (as would a UDT Array, nD Array and many other things that I haven't discovered yet no doubt) but as I hadn't looked at Lists before thought I'd give it a try.