VB.NET: 2-dimensional list is almost 1000x slower than 1-dimensional list?

Question

Consider the following code:

    Dim arr1 As New List(Of Double)
    Dim arr2 As New List(Of Object)

    Dim timeStart As DateTime = Now

    For x As Integer = 0 To 1000000
        arr1.Add(3.14159)
    Next

    Dim timeEnd As DateTime = Now

    MsgBox(((timeEnd - timeStart).Seconds).ToString())

    timeStart = Now

    arr2.Add(New List(Of Double))

    For x As Integer = 0 To 1000000
        arr2(0).add(3.14159)
    Next

    timeEnd = Now

    MsgBox(((timeEnd - timeStart).Seconds).ToString())

It involves 2 lists. The first one is 1-dimensional, the second one is 2-dimensional.

The first routine (which operates on the first list) completes in about .015 seconds. The second routine (which operates on the second list), however, takes almost 10 seconds. The only difference is that the second list is 2-dimensional.

Am I missing something here? Is there a way to speed this up or am I doing something wrong? I have a program that requires several 2-dimensional arrays and it's running extremely slow right now. How can I speed it up so I get the same feedback that I'd get if the lists were 1-dimensional?

Answer 1

The problem lies with one line of code.

Make arr2 strongly typed and it'll be much, much faster...

Dim arr2 As New List(Of List(Of Double))

I ran a quick test and it went from about 7 seconds on my computer to 17 ms with this one change.

As CodeInChaos correctly pointed out, the reason for the slowness has more to do with the .Add() method being dynamically dispatched than anything.

EDIT: Writing VB.NET with Option Strict On is one way to avoid this kind of problem in the future. By doing this, you would've seen a compile-time error that says "Option Strict On disallows late binding."