I need to optimize this snippet to be faster:Dim lst = (From t In DocElet.ChildNodes Select ID = t.item(ID).outerxml).Distinct().ToList()       Parallel.For(0, lst.Count, Sub(i)         Dim P As XmlElement = GetElement(lst(i))       Dim ls = (From t In DocElet.ChildNodes Where t.item(ID).innerText = P.InnerText Select t)         Parallel.ForEach(ls, Sub(D)         Dim verif_date As String = D.Item(DAD).InnerText        Sej.ID = D.Item(ID).InnerText         End Sub)            End Sub)This is the XML structure:<?xml version=1.0 encoding=utf-8?><PatientData><Sejour><ID></ID><DAD></DAD></Sejour>  </PatientData>I'm asking how I can optimize my code because it takes a lot of time (50 sec) in the case where the list contains 20000 elements.
Optimisation XML handling within vb.net application
.net;linq;xml;vb.net
The indentation makes your snippet quite hard to read - the VB.NET syntax for inline delegates is fairly bulky (at least compared to C#), I'd give it more air; being able to read the code is step one.Dim lst = (From t In DocElet.ChildNodes            Select ID = t.item(ID).outerxml).Distinct()                                             .ToList()Parallel.For(0, lst.Count,          Sub(i)             Dim P As XmlElement = GetElement(lst(i))             Dim ls = (From t In DocElet.ChildNodes                        Where t.item(ID).innerText = P.InnerText                        Select t)             Parallel.ForEach(ls,                       Sub(D)                          Dim verif_date As String = D.Item(DAD).InnerText                          Sej.ID = D.Item(ID).InnerText                      End Sub)         End Sub)Ok. So you're running a Parallel.For, and inside that, you're running a Parallel.ForEach. That's quite a lot of overhead just there, it's likely that you're shooting yourself in the foot here, I'd first remove the inner Parallel.ForEach:Parallel.For(0, lst.Count,          Sub(i)             Dim P As XmlElement = GetElement(lst(i))             Dim ls = (From t In DocElet.ChildNodes                        Where t.item(ID).innerText = P.InnerText                        Select t)             For Each foo In ls                 Dim verif_date As String = D.Item(DAD).InnerText                 Sej.ID = D.Item(ID).InnerText             Next         End Sub)What's the purpose of verif_date? If it's not used, remove it:             For Each foo In ls                 Sej.ID = D.Item(ID).InnerText             NextNow the outer loop is making yet another selection:             Dim ls = (From t In DocElet.ChildNodes                        Where t.item(ID).innerText = P.InnerText                        Select t)I'd try to flatten that. I'm not very familiar with the LINQ syntax in VB.NET, but what you want is a SelectMany, where you'd select the child nodes where ID's inner text matches the element's inner text for the node you're iterating - in other words the ls query gets outside the outer loop and into the lst query, so you only need a single loop (which can probably run in parallel).If the XML is massive, it could be worth trying some PLINQ:Dim lst = (From t In DocElet.ChildNodes            Select ID = t.item(ID).outerxml)          .Distinct()          .AsParallel()          .SelectMany(...)          .ToList()Now you can iterate lst with Parallel.ForEach (time it with a normal For Each first) and do your thing.As an aside, I have to say your naming is inconsistent and hard to follow: Sub(i) looks good (Sub(index) would be better), Sub(D) doesn't. I realize VB.NET isn't case-sensitive, but if you're going to have a convention to make function parameters camelCase, by all means stick to it. lst would probably be better off as elements or whatever makes sense - use real, readable words for your identifiers, not just arbitrary abbreviations.