Replace nested foreach to Increase performance

Question

I have the following code which take longer time to execute. Is there any alternative that I can replace below code with LINQ or any other way to increase the performance?

var usedLists = new HashSet<long>();

foreach (var test in Tests)
{
    var requiredLists = this.GetLists(test.test, selectedTag);

    foreach (var List in requiredLists)
    {   
        if (!usedLists.Contains(List.Id))
        {
            usedLists.Add(List.Id);
            var toRecipients = List.RecepientsTo;
            var ccRecipients = List.RecipientsCC;
            var bccRecipients = List.RecipientsBCC;
            var replyTo = new List<string>() { List.ReplyTo };
            var mailMode = isPreviewMode ? MailMode.Display : MailMode.Drafts;
            OutlookModel.Instance.CreateEmail(toRecipients, ccRecipients, bccRecipients, this.Draft, mailMode, replyTo);
        }
    }
}

Answer 1

What do you actually do? Looking at

foreach (var test in Tests)
{
    var requiredLists = this.GetLists(test.test, selectedTag);
    foreach (var List in requiredLists)
    {   
        if (!usedLists.Contains(List.Id))

it looks to me that you try to get unique "List"s (with all those "var" I cannot tell the actual type). So you actually could replace that with a new function (to be written by you) and do

var uniqueLists = this.GetUniqueLists(Tests, selectedTag);

and finally call

this.Sendmails(uniqueLists);

which would iterate thru that list and send the mails.

If that could speed up the code or not, depends severely on the underlying GetLists / GetUniqueLists functions.

But in any case, it would make a great progress to your code: it becomes readable and testable.

Answer 2

Assuming the result from GetList is large/slow enough in comparison to CreateEmail call, skipping your contains-add hashset comparison will speed things up. Try a call to Distinct([Your ListComparer]) or GroupBy():

var requiredLists = 
    Tests.SelectMany(test => this.GetLists(test.test, selectedTag))
         .Distinct([Your ListComparer]);
// or 
var requiredLists = 
    Tests.SelectMany(test => this.GetLists(test.test, selectedTag))
         .GroupBy(x => x.Id).SelectMany(x => x.First());

foreach (var List in requiredLists)
{   
    // (...)
    OutlookModel.Instance.CreateEmail(toRecipients, ccRecipients, bccRecipients, this.Draft, mailMode, replyTo);
}

You could also try PLINQ to speed up things but I think you might run into trouble with the CreateEmail call, as it's probably a COM object, so multithreading will be a hassle. If GetList is slow enough to be worth the multithread overhead, you may experiment with it in the first call when creating requiredLists.